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AM vs. FM: Newsletter Challenge (09/18/07)

Posted September 17, 2007 9:14 AM
Pathfinder Tags: challenge questions

The question as it appears in the 09/18 edition of Specs & Techs from GlobalSpec:

You're driving under a large, concrete bridge and notice that your favorite AM radio station has faded away. Later, while driving under that same bridge, you notice that your faithful FM station still comes in loud and clear. Why the difference?

(Update: Sept 25, 8:34 AM EST) And the Answer is...

Heavy bridges absorb RF energy. AM radio uses lower frequencies. FM radio uses higher frequencies that can also bounce around. In this example, the concrete bridge absorbs so much energy from lower-frequency AM radio that there's too little left for your car's antenna. With the FM station, however, there's enough RF energy left to receive the signal loud and clear.

At higher frequencies, signal bouncing can present a problem. In some cases, you may receive the same signal bounced off multiple objects. With television, this is called a "ghost image".

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#1

Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/17/2007 9:23 AM

AM signal is attenuated by the bridge as you pass under therefore lowering the signal strength as the information on the carrier is stored in the amplitude. The FM carrier has its information stored in the frequency which remain the same even when the signal is attenuated in amplitude

Al

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#2
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/17/2007 10:34 AM

Quite.

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#6
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/17/2007 3:27 PM

While it is a contributing factor, it is not the whole story nor the answer I think they want to this question. However, it is a good thought!

The tunnel is effectively a wave guide and the AM signal frequency is too low or long in wavelength (compared to FM) to transmit through the tunnel's opening. Much like the tiny holes in a microwave's door are too small to permit the microwave RF from "burning" you when you stand next to an operating microwave.

http://ocw.mit.edu/NR/rdonlyres/Electrical-Engineering-and-Computer-Science/6-632Electromagnetic-Wave-TheorySpring2003/F3BB5D1A-6F45-4501-826C-1A5CB6BD5762/0/ps6.pdf

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#7
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 12:12 AM

I would have to go with this one. The wavelength of AM broadcast ranges from about 566 meters to 214 meters, significantly wider than any tunnel. The FM will propagate through the tunnel like a wave-guide since it's wavelength ranges from about 3.4 to 2.8 meters.

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#13
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 4:31 AM

In pure engineering-physics terms, yours would be the best explanation - that the tunnel is effectively a waveguide below cut-off for the long-wavelength AM signals, but the shorter-wavelength FM signals can propagate - including as you do that the FM demodulation is less sensitive to amplitude variations than AM.

However, in the day-to-day world, this would only apply to tunnels that are suitably aligned, and when the signal path was not blocked – as it might be when a pair of counter-moving trucks (UK = lorries) pass each other. In practice, as FM signals are used for traffic information (and hopefully reducing panic when there is a traffic problem in the tunnel), many tunnels have leaky feeders installed* to maintain an acceptable signal even when the tunnel is full of stationary traffic.

* These would usually be coax with a loosely woven screen.

Fyz

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#15
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 4:40 AM

I saw some leaky feeders at a restaurant the other day.

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#16
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 4:47 AM

The filter type ones are more dodgy than the grazers. If stranded on a beech, eat limpets. ( I think )

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#17
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 4:57 AM

These were of the all too common shell-less homo-obesius species. Never tried eating one in a paella but perhaps with some fava beans and a nice Kianti, fhu-fhu-fhu-fhu-fhu-.

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#18
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 5:03 AM

mu..s..t res..is....t , .... must.... r e s is t........

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#27
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 8:02 AM

Folks, the challenge says, "bridge," not "tunnel."

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#31
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 8:18 AM

LOL yeah.. odd how people get off on a tangent.. guess they took the wrong exit.. missing the bridge and wound up in the tunnel.

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#35
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 8:26 AM

And, as the Chunnel dives below the water. . .

It seams for every idea out there that might effect the answer, there is some one willing to change the question in order to make their point relevant.

-A-

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#42
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 8:42 AM

Indeed! Maybe a concrete bridge could still act as a very short tunnel.

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#49
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 10:05 AM

You're right. So leaky feeders are unlikely. But the power attennuation for the low frequency AM signal is still about exp-(4.d/h) where h is the height of the bridge and d is how far you have gone underneath. I suppose that would correspond to about 30...50-dB of attenuation under the middle of a typical freeway bridge for AM, and maybe 10-dB for FM?

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#30
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 8:16 AM

Hey! Um.. where did the tunnel come from again?

Or did the bridge morph into a tunnel?

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#36
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 8:26 AM

The tunnel is figurative. The space under the bridge represents a tunnel or wave guide in this case. Bridges typically do not morph, except in Hollywood where the sci-fi folks dwell.

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#58
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 12:40 PM

Just curious... how does questioning how the thread evolved from a bridge to a tunnel rate an off topic score; when the topic clearly states a bridge?

Maybe the attempt at being humor-ass and suggesting it morphed, could be off topic.. but if anything I was trying to keep it on topic:P

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#59
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 12:57 PM

You appear to be suffering from overthink..it's probably a dog avatar thing...suggest you have a lie down.

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#61
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 2:42 PM

Dogs need more training than cats (there's no point even trying with cats, but I still like tying fire-crackers to their tails) play with that groovy check box for 'off-topic' when you post a comment - it has a default value. Not my fault, I'm always off-topic. Look around and you'll see how confused people get with the 'Rate this post' option. I'm going to give Del one, just cos he's funny and needs one instead of the occasional fish-head or double-entendre.

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#78
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/19/2007 4:00 AM

there's no point even trying with cats

My now sadly missed, dear departed cat would sit and stay on command. We amazed a number of vets with our obedience routine. And she used to play fetch.

I particularly enjoyed the times we spent in the shower , as did she once she'd accepted she was going to get wet.

No firecrackers were ever involved.

My knowledge of AM vs. FM is at roughly Kris's level, so I'll stick to cat stories this week. Want some kitty treats, Del? <ER tickles Del's ears, having first contected both of them to the same ground plane> Have we all agreed that a bridge is a very short aspect ratio tunnel?

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#134
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/26/2007 11:11 AM

Right on! The wave length of the AM signals is 1722 ft at 570 on your dial and 614 ft at 1600 on your dial. The AM wave is very much attenuated in trying to go through a structure smaller than this diameter. Us techies call it "a wave guide below cutoff".

On FM, the wave length at 88 Mhz is 11 feet and at 108 Mhz it is 9.1 ft. so these waves will go through a much smaller tunnel or bridge.

WS:BSEE, MSEE

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#3

Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/17/2007 11:49 AM

AM broadcast signals range from 530 to 1710 kilohertz, have very long wavelengths, and will propagate along the ground for some distance even beyond the horizon depending on transmitter power. At night I can pick up KMOX in St. Louis, WHO in Iowa, and WLS in Chicago in my house soutwest of Tulsa. The steel girders, beams and rebar in the bridge readily absorb the RF energy at those fequencies and conduct it to ground.

FM broadcast signals are VHF, from 88 to 108 megahertz, and travel mostly line-of-sight. But FM signals can easily bounce off of large storage tanks, metallic light poles or commuication towers. With multiple paths, an FM signal can travel underneath the bridge.

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#4
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/17/2007 1:18 PM

Yes, but the day/night difference you speak about for AM signals has more to do with the ionosphere than simple ground wave propagation. The ionosphere changes in both intensity and altitude throughout the day. That is why 80 meters is great early in the morning and fades rapidly after sunrise.

You are absolutely correct about the reflectivity of radio waves; FM being more favorable to reflection off of solid objects than AM.

Now, here is another kicker and probably the real answer to the challenge. The frequency of the radio wave and the opening size of the tunnel have an important determination of the propagation through the tunnel.

AM signals at 1 MHz have a wavelength of about 1000 feet (rounding up). A 100 MHz signal is just under 10 feet in length.

The tunnel's opening must be at least larger than 1/10th the wavelength for an RF signal to pass through it without severe attenuation to the signal. That would require a tunnel opening bigger than 100 feet to pass AM, but only 1 foot for FM.

Most of the AM signal is either reflected or absorbed at the tunnel's opening.

Table top microwaves are another example of this. The grid or mesh inside the glass of the door is sized such that very little RF energy passes through the glass. Essentially, it's a Faraday cage.

Yes, a lot of the signal gets attenuated by the material surrounding the tunnel, but any signal that is beamed or reflected into the tunnel opening will get attenuated based on its frequency and the tunnel opening diameter.

'73

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#44
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 9:13 AM

What about the fact that the tunnel you mention is actually a bridge? Does that change your answer?

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#60
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 1:05 PM

Good point!

The answer is really, that depends. The length really does not matter. Again, look at your microwave door. The openings are wider than the material is deep. So, no, the wave guide does not need to be of a long length.

Again, this is a challenge question and the information given is not very complete. The question states a large bridge, but does not state what the opening size is nor the length.

Attenuation would be the first thought and an easy answer to the puzzle, but generally these challenge questions have a different answer in mind besides the obvious. The most likely answer is usually something a little more esoteric to make us go ooo and aaah. Since the phenomena I described is both real and a significant contributing factor to RF loss in tunnels, it is my first pick.

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#63
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 4:05 PM

If the attenuation is relying on the effect of the cut-off waveguide, the ratio depth/gap is crucial (i.e. the length really does matter). The (power) attenuation through the guide is simply exp(-4d.g). So you'd better check out your microwave, because one of the following must apply:

You are misinterpreting which is depth (=shortest distance between internal cavity and outside world), and which is gap (=shortest distance between door and body conductors) - both with the doors closed, of course; or
The gap is so small that the system relies on the ineffectiveness of what is in essence a very short antenna (this is not generally recommended for this sort of arrangement, because it requires such a small gap even after the kids have been swinging from the microwave door); or
The microwave is emitting undesirable amounts of radiation

BTW, the mesh in the door (that is used to screen the radiation while still allowing you to see into the microwave) does effectively depend on poor radiation through multiple small holes. Unfortunately, small shallow slits would allow one polarisation to be transmitted.

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#64
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 4:57 PM

This show the propagation in real time.

Presumably a 'micro' wave is somewhat smaller?

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#65
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 6:10 PM

Some possibilities:
As per your link, but at the Blues' (Bishop' Stortford FC)
My hair after de-frizzing
Your fur when ER has finished "stroking" you
What the other ER manages on a bad-hand-day
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/19/2007 2:28 AM

ERII LOL

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#68
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 9:06 PM

You can construct an excellent Faraday cage from chicken wire, which has a very small wire thickness compared to the gap between wires. If you take your AM or even FM radio inside such a cage you would find the signal greatly attenuated.

Looking at my microwave shows a mesh pattern impregnated into the inside glass of the door. The pitch of that mesh is small enough so that the 2.45 GHz RF energy is stopped or reflected by the mesh. One wavelength at that frequency is about 3 cm. The mesh opening is about 1.3 mm. The mesh wire thickness is probably under 0.25 mm.

The point I was making was you don't need a long tunnel to get the effect. A bridge with steel rebar or girders can easily qualify as "chicken wire" to AM signals because of the ratio of the wavelength to the opening size of the "mesh". Once that ratio gets around 1/10 a wavelength or smaller that attenuation is very high.

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#81
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/19/2007 6:29 AM

But, assuming the signal is entering from the end(s) of the tunnel (=sides of the bridge), you do need a hole under the bridge) to get the attenuation. (I'm assuming that the bridge is sufficiently reinforced with metal grid that it might just as well be a continuous sheet of conductor). The key point I'm making refers to the depth of penetration underneath the bridge. That does matter - unless the orifice is filled with reflecting or absorbing materials, the rate of attenuation if the signal is as I previously presented. The same applies to the radiation leakage around the edge of the door of your microwave (though I admit this is more complex than my basic presentation above).

The reason that you can still notice a dead region under a narrow bridge (i.e. equivalent to a very short tunnel) can only be destructive interference - and the extent of an interference zero for AM (that uses wavelengths >200m) can be very much greater than that for FM signal (wavelengths ~ 3-m).

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#112
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/21/2007 7:37 PM

I am not clear on what you ask, but even a chicken wire fence will block radio waves very effectively for the reasons I cited.

Think of the bridge rebar as a giant chicken wire fence. The AM RF wavelength is so long that it will not pass through the "openings", which are RF opaque at those wavelengths. Remember, a 1 MHz AM radio wavelength is about 1000 feet long. So openings of 100 feet and smaller will not pass RF at that frequency.

Try looking up Faraday cage or screen or Faraday shield for an explanation of the theory.

Also, remember that this is a challenge question and they are looking for the "principle" that causes the effect. It is easy to get lost with minutia and every possible scenario that can subvert a principle. So you must ask yourself what factors might "most likely" cause the most RF attenuation for AM and little for FM. Couple that with the knowledge that tis is a challenge question and of all the possibilities, which might be the best answer from that perspective. The union of those two lines of thought will have the highest probability of being the correct answer.

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#113
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/22/2007 6:19 AM

I'm not certain what question I'm supposed to have asked here. But your comment appears to imply that you believe you have encapsulated everything by saying that for AM frequencies* the structure of the bridge can behave as if it was a solid conductor. The problem with this line of argument is that the structure of most bridges is equally opaque to the frequencies used for FM. So you need to consider why the effects of the rather large "holes" (the route by which your vehicle enters and leaves the area) on the signals at the two frequencies. If you reread my meanderings, you will find that this is the issue I am addressing - the effective "opaqueness of chicken wire" being generally taken as a given.

You might also try looking up "waveguides below cutoff" and "interference" for explanations of the parts of the theory that are relevant to the differences between AM-frequency and FM-frequency transmission under bridges or through tunnels (or indeed microwaves through the gaps around the edges of the microwave cooker's door).

*For the avoidance of doubt: abbreviation for "the frequencies typically used for AM transmissions"

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#143
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

10/03/2007 4:22 AM

agree about the steel reinforcing.......but consider that the metal in the bridge acts as a long bipole aerial being driven by the AM radio wave and hence will attenuate the magnitude due to the impedence effect of the oscillating magnetic field driving the bipole......ie result in a decrease in the voltage drop peak to peak induced in adjacent bipole conductors such as in your car.

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#144
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

10/03/2007 7:29 AM

If the dipole is near resonance, re-emission will be in phase quadrature, so the attenuation should be 3-dB. At lower frequencies the effect would be smaller. It actually needs to behave like a continuous conductor for large attenuations. So AM attenuation probably relies to an extent on conduction/capacitance to ground. FMfrequencies screening by the bridge should therefore be more effective than AMfrequencies. The reason for the difference can only be due to differences in the penetration around the screen.

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#62
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 3:01 PM

CQ's answer was what I was basically going to say.

In a word, Propagation.

boB

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#5

Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/17/2007 1:46 PM

FM is a quasi line of site operation. You can receive if you can see the "glow" of the transmitter. The stations are spaced so as too ensure proper reception, and repeaters and reflectors are used to alleviate dead spots. The signal would therefore not be affected by a roof.

AM uses a slower frequency which can be reflected from layers in the atmosphere or stratosphere, stations can be spaced further apart. A roof may effectively block the signals reflected from above.

This scenario however is not always true. In 1986 I stayed in an apartment block on the second floor of a 7 story building. The lockup garages was on t6he ground floor. The building was situated at the foot of a hill with the FM transmitter on top. You had to look at 70cegrees to see the tower.

As you approached the building the signal became fainter and it disappeared on entering the garage. The AM station was about 20 km away. The signal did not disappear in the garage.

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#28
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 8:10 AM

About the signal loss, most antenna's have an optimal radiation pattern which are usually designed to direct the signal horizontally, not vertically. By being close "UNDER" the antenna you are actually leaving the radiation zone. This better explains the signal loss in your garage.

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#43
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 8:45 AM

AM is broadcast with vertical polarity where as FM is broadcast with horizontal polarity. Perhaps the bridge rotates the polarity ninety degrees making the AM polarity less favorable and the FM polarity more favorable to the vertical whip on the car.

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#56
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 11:57 AM

You probably were experiencing the FM radiation pattern. Often the antenna bays are designed to radiate outward as opposed to straight down on the facility. This probably has several reasons. One would be to reduce RF at the facilities for personel safety and to reduce interference with the oftentimes many other transmitting installations on site. Another reason would be to acheive the greatest distance with the radiation pattern, the majority of your listeners are not situated under or near your FM antennas. Google FM antenna patterns to learn more. http://rfry.org/FM%20Pattern%20Analysis/NEC%20Show%20(RJF%20Web%20Site).pdf is just one of the links I found.

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#8

Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 1:02 AM

Interference from things like lightning, structures,tunnels and bridges affect the amplitude of the signal in AM whereas the ampltude stays constant and are not interfeard with the same conditions as above in frequeny modulation, there by maintaining a strong signel

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#9

Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 1:42 AM

AM wavelengths are of the order of hundreds of meters, long enough to be stopped by all those steel rebars embedded in the concrete structure. I effect the bridge acts as a Faraday cage for the AM signals. FM wavelengths are of the order of a few meters, and can "slip" (for lack of a better word) through the small "holes" between rebars and other bigger apertures in a concrete bridge.

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#10

Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 2:47 AM

FM is small and wiggly so it can wiggle under stuff...but I know where it hide so I sit and watch ready to pounce on them when they come out with their wiggly noses ond long thin tail...

Oh no that's mice.... near enough...

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#11
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 3:55 AM

You made me smile Del

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#14
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 4:35 AM

And mice use leaky feeders as well (skirting board, free-standing cupboards...)

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#19
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 5:05 AM

<ER strokes Del vigorously from nose to tail and then puts out a finger for him to sniff...>

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#20
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 5:11 AM

That's too much information !

Where's my pencil......

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#22
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 5:43 AM

ZAP!!! The static is earthed through Del's nose.

What were you thinking Kris?

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#34
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 8:22 AM

Is this that famous British sense of humor?

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#38
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 8:31 AM

No, it's related to this (which I think Del missed!)

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#41
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 8:42 AM

Ah yes the familiar crack on the tips of my ears on the return stroke.. I have written several learned theses on electrostatic discharge on the feline anatomy. 'Fur'ther research is in order.

I have hidden all the slippers with insulating soles! Ha !

Thanks for the clarification ER...I was purring...without full appreciation of your devious plan...crack ouch!

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#24
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 7:20 AM

"love to eat them mousies,

Mousies what I love to eat,

Bite they little heads off,

Nibble on they tiny feet"

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#25
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 7:28 AM
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#29
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 8:15 AM

Cats, I had a dream last night about lions wandering loose in the neighborhood. One of those rare dreams in vivid living color you know! Of course these cats could have your cat for lunch, literally!!

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#12

Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 4:03 AM

There's no static.

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#21

Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 5:33 AM

The diff. is in the direction of driving.

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#32
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 8:20 AM

Direction, I don't think so. Most antennas in the HF and VHF frequencies receive omnidirectionally.

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#23

Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 5:53 AM

The tunnel acts as a waveguide. RF cannot propagate through a waveguide if the wavelength is greater than twice the width. AM frequency wavelength is 600-1800 ft. FM wavelength is about 10 feet.

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#26

Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 7:55 AM

Answer number three has my vote.

blueice

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#33

Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 8:22 AM

Also on this note of radio waves and signal stengths...

I used to live in north central BC... waaaay out in the boonies (Tatlayoko Lake) Rarely during the day could I get a radio signal (if I did it was CBC out of Vancouver)... but at night I could pick up stations from the southern Pacific coastal states. The signal would come in waves almost where it would be strong and then fade in and out.

Anyone know why I was able to pick up these signals SO far away? I was told it was "skip" off of the atmosphere and mountains. Anyone have a better explaination?

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#39
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 8:37 AM

The HF frequencies being longer waves will bounce off the ionosphere. The ionosphere changes through out the day based on the solar radiation. Usually the times when the sun is not up are the better times to get bounce but is not always the case. Ham radio operators use this to effect to communicate with radio operators on the opposite side of the global all the time. My brother who lives in western PA, USA once talked to a fellow on Pitcairn Island in the south Pacific. Talk about reaching out and touching somebody.

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#74
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 9:50 PM

Yes!

There is a daily cycle to the ionosphere and a 11-year cycle, which is controlled by the Sun and its sunspot cycle.

During the course of the day, the ionosphere undergoes significant changes and impacts the various Ham bands in different ways. In the early morning the 160 meter band is very active. As the day progresses, 20 meters gets active. 10 and 11 meters do very well in the afternoons, but there are exceptions to all of that.

Skip is the term used for radio waves that bounce off of the ionosphere and then off of the land or water. When conditions are right, you can use very little power to talk long distances as the signal bounces back and forth between the land and the ionosphere. Under the right conditions you can talk around the world that way and I have done it.

Case in point; I used 1 watt of RF power to talk from Pennsylvania to a station in South America. My signal strength to the South African station was about the same as a "neighbor" a few hundred miles away from me running 500 or so watts.

The reverse is true when the ionosphere is not active. You can use thousands of watts and still not get a contact 200 miles away.

Once you get above 10 meters in frequency it is very hard to "skip", but not impossible.

For those not in the know, 160, 80, 40, 20, 15, and 10 meters refer to Ham bands and the numbers represent the wavelength of the RF signal in meters. 160 meters is about 1.8 MHz, where as 10 meters is about 28 MHz. 2 meters (145 MHz VHF FM) is a very popular frequency for Hams to use.

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#75
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 10:30 PM

Check this out ! All the modern military stuff and yet a couple of dudes were on the case. Very cool.

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#86
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/19/2007 10:23 AM

Very interesting article Kris. Thanks much. Something like this would not be as likely to happen in the US though for several reasons:

1. In a real war-time situation, standard Amateur Radio operations are likely to be suspended. That is what happened during World War II.

2. The Brits involved used a secret code to transmit information. By US communications law, Radio Amateurs operating on assigned amateur frequencies are not allowed to transmit ANY secret codes. (Morse Code and standard encoded digital modes are not considered "secret")

3. There is a amateur related radio service, that uses surplus, military, commercial, or modified Amateur Radio equipment to operate on frequencies outside the Amateur bands and support military training and operations. This is the Military Affiliate Radio System, or MARS for short, with separate Army, Air Force, and Navy/Marine Corps organizations. You must be an FCC licensed amateur radio operator to join MARS, however you must be a registered MARS member to participate and use the MARS frequencies. MARS handles routine and emergency health and welfare communications for military personnel, as well as supporting FEMA and other disaster preparedness organizations, in addition to its training and support role with the military. It might be just by sheer luck that a US-licensed MARS station operator would find himself in such a position as the Falklands Islands ham operator found himself in at the time of the Argentinian invasion.

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#88
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/19/2007 10:57 AM

That's very curious, as this sort of application is the very reason that was given (in the UK at least) for continuing to license HAMs and apportion radio frequencies for their use. I'm in no way expert here, but I don't see how you can restrict verbal codes that hide important data in apparently innocuous chatter; I think the formal situation is that HAMs are obliged to share information with the legitimately elected government; I'm not sure whether this is technically "upon request", or some burden to make this judgement is also in place.

From here it looks quite strange that US governments trust practically every citizen to hold arms in their homes, but still do not trust specifically licensed individuals to behave responsibly in times of war.

Maybe one of the UK amateur radio fraternity will give chapter and verse on our situation?

Finally - I'm not quite convinced that the military wouldn't have established a secondary route to monitor this information flow. But I can see why the actual communications had to be left undisturbed.

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#91
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/19/2007 11:53 AM

"I'm in no way expert here, but I don't see how you can restrict verbal codes that hide important data in apparently innocuous chatter; I think the formal situation is that HAMs are obliged to share information with the legitimately elected government; I'm not sure whether this is technically "upon request", or some burden to make this judgement is also in place"

Well, until the passing of "The Patriot Act" a few years ago, we had nothing in this country quite like the British "Official Secrets Act", controlling how the government would regulate its citizenry regarding intelligence knowledge. And I do not really know all that the Patriot Act entails in this regard either. I do know that our largest organization of Amateur Radio operators, the ARRL-American Radio Relay League (in the UK, its equivalent and associated organization in the IARU-International Amateur Radio Union, is the RSGB-Radio Society of Great Britain), has signed several documents supporting the efforts of our Department of Homeland Security. Since Amateur licensing by its very nature is a type of submission to the government, Hams in general (not to be confused with other UN-licensed two-way radio operators, commonly called "CB-ers" or "Free-banders") are likely to co-operate and assist the government in any way we can.

Regarding the use of secret codes, I never said it there was a practical way to prevent their use, only that it was definitely illegal, and if caught, such a user would be subject to significant fines and/or license suspension or forfeiture. The US does in fact have voluntary "radio cops" who assist our FCC by observing and reporting violations. They are known as the Official Observers Corps, or OO's (pronounced "Oh-ohs") and organized as part of the ARRL. Only a ham with extensive years of experience and a stalwart and untarnished reputation may be appointed as an OO.

And regarding firearms, whoever said that the US government trusts "practically every citizen to hold arms in their homes"? Our government, and specifically the Bureau of Alcohol, Tobacco, and Firearms (BATF), would love nothing better than to see every civilian gun turned in or destroyed. Then they could just be the BAT! Unfortunately for them we have a little law called the Second Amendment to our Constitution that guarantees citizens the Right to Keep (presumably in their homes) and Bear Arms. It thought the British Common Law and the English Bill of Rights (1689) also guarantees this, does it not? The only arguments then should surround not whether or not this is allowed, but the manner in which it is allowed (registration, licensing, etc.)

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#95
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/19/2007 12:30 PM

I believe that I can keep as many knives, swords and other historic weapons in the house as I wish. The right to bear these in a public place is, however, somewhat constrained by laws of apparent threat and nuisance. On the other hand, I've never been stopped for carrying a sickle (which I have used on the verge outside my home, and formerly on my allotment); nor have I known of any one being stopped specifically for driving a vehicle equipped with an Allen scythe or similar (except for the odd occasion when unsuitable vehicles wander onto motorways).

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#97
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/19/2007 12:48 PM

"laws of apparent threat and nuisance"

That figures. In Britain it is probably illegal to be impolite! <grin> Can you be fined for giving someone THE FINGER? Wouldn't that be an "apparent threat"? I guess you can be arrested for passing gas in a public place! <ROFL> That would definitely be an "apparent threat and nuisance", poison gas attack as it were! Talk about your concealed weapon!<guffaw>

I wonder what our Aussies friends have to say about gun ownership and use, since their laws, like ours, probably also derive from English Common Law.

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#98
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/20/2007 3:58 AM

It's all quite a change from the days when every man/boy over the age of 13 (I believe) had to attend archery practice on a Sunday afternoon. And in Hereford, the English were obliged to shoot any Welshmen (not sure about Welshwomen) seen in the city on a Sunday.

And this should give some interesting debate on the carrying of swords in a public place and their use in ritual. When a LAW member appears in court, King Arthur Pendragon's (he changed his name by deed poll) sword is uncovered, as they insists that this is an instrument of his faith and so this is the object upon which they swaer their oath.

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#99
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/20/2007 8:50 AM

"insists that this is an instrument of his faith and so this is the object upon which they swaer their oath."

What's next, Islamic Jihadists using an IED to swear in upon as an instrument of his faith? "Geez, Mr. Mohamed, we're all out of Quran's today, how about's you use this nice C4 vest we took off a dead mullah last week?"

ROFL

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#100
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/20/2007 9:26 AM

"obliged to shoot any Welshmen (not sure about Welshwomen) seen in the city on a Sunday"
Round here, it seems to be the Welshmen shooting off (their mouths) - although more on a Saturday-Sunday night than during worshipping hours. Earlier this month we tried redirecting them to the Egham area for 'congenial' company that would not object to their ministrations, but the MinAg turned them back.

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#102
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/20/2007 12:07 PM

If Chuck ever get's a shot at it, he wants to be 'Defender of Faith' (absent 'the'). At least it will keep him busy.

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#103
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/21/2007 3:40 AM

Not if the A of C, Mountain Ash the Welshman, has any say in the matter!

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#104
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/21/2007 3:52 AM

I thought all the Welsh were called Dai ? Once asked a bloke in a field if he was shearing. Not very generous in his reply.

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#105
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/21/2007 11:18 AM

At least you didn't do what my Mom did years ago when our cousins visited from Scotland. My Dad and his cousin's husband decided to go play tennis. My Mom offered to come along and "shag balls" for them!

The Scottish gentleman turned all sorts of shades of red, thinking that he had just been propositioned by relatives!

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#106
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/21/2007 12:03 PM

I find to difficult to make it through a day without hearing things in a certain way. A prim elderly lady nearly killed me off at a dinner party, simply by asking for condiments. Our ensuing 30 second chat mimicked every 'smiley' in the book, and then some.

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#107
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/21/2007 12:08 PM

":A prim elderly lady nearly killed me off at a dinner party, simply by asking for condiments."

Something tells me you were being deliberately obtuse!

<ROFL>

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#108
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/21/2007 12:55 PM
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#109
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/21/2007 2:01 PM

I wonder how she would have reacted if you had said you live in a condominium!

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#40
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 8:38 AM

If you are speaking of AM or low frequency broadcast then there is no better answer since that is the correct one

Low frequency radio waves bounce off the ionosphere. At night the ionosphere is at a higher altitude which decreases the angle of incidence and lengthens the distance of the point of return after the bounce, from the point of broadcast.

FM and TV are not reflected from the ionosphere and continue into outer space. Unless an AM station is on a clear channel they often must lower their power and or change directional patterns at night to prevent interference with distant stations.

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#37

Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 8:27 AM

I think maybe this contributor has caught a lot of people out, the question says "bridge" most of the contributors rate is as a tunnel, I don't know the answer anyway!

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#45
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 9:24 AM

...Well pay attenuation then !

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#46

Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 9:47 AM

OK, as a licensed amateur radio operator, I do know a little about how radio waves propagate. Radio waves can be absorbed or reflected by metallic elements. They are most effectively absorbed by elements which are some multiple (1x, 2x, 3x, etc.) or even fraction (1/2, 1/4, 1/8, etc.) of their wavelength. Also, they do not need to be grounded for this to be effective, as the energy may also be dissipated as heat just traveling through an isolated electrical circuit made of interconnected reinforcement such as wire matting, screen, or mesh, often found in concrete structures

To answer the question directly, the major cause would be as stated earlier, the AM broadcast band uses frequencies with very long wavelengths that are effectively blocked, absorbed or reflected, by the metallic content of the bridge, including rebar and other reinforcement inside the concrete, steel girders, supports, railings etc. The FM broadcast band uses frequencies with much shorter wavelengths, so short that they bounce off of many metallic objects and so flood an urban area (which is more likely to be filled with large amounts of metal structures) with multiple path RF radiation which finds its way though the many gaps in the bridge structure, which the longer wavelength AM frequencies cannot do.

If, historically, FM had been created first and assigned the longer wavelengths (shorter ones at the time being deemed "unuseful" because they did not follow a groundpath as far and were less predictable by "skywave" or ionospheric propagation), then AM had come along later and been assigned to shorter wavelengths, the question likely would have been worded the other way around! Indeed, some AM signals are used at VHF frequencies. Aircraft typically use AM for two-way voice communication in a band just above the FM broadcast band. In this case, these AM signals act no different than their FM counterparts in the public service band slightly higher or the broadcast band, just below them.

Other questions were raised however, which deserve a clearer answer. One question was why AM broadcast stations from far away could be heard very clearly at night, but not during the day, or not as well. One answer was partially correct, AM broadcast band skywave propagation is better at night because the D layer of the ionosphere (closer to the ground than the higher E and F layers, which play an important role in skywave propagation) becomes highly charged by solar radiation during the day and absorbs most of the skywave propagation. At night, the energy of this layer dissipates and low frequency RF passes through it, to be reflected mainly off the F layers (F1 and F2) which remain more charged at night, but not so much that they absorb RF, but are better reflectors. Added to this is the fact that, in the US at least, the FCC allows certain broadcast licensees to increase their power significantly at night, sometimes 10 times or more, while other station must reduce their power. Originally, the reasoning was so that in times of emergencies, people could be directed to tune their radios to one of these "Clear Channels" which they could receive well day or night. FM broadcasting has made much of that reasoning obsolete, but the system persists, possibly because of the large amounts of money involved which a "clear channel" broadcaster pays for its class of license.

Another reason that FM stations usually have clearer sound with less fading and less interference from man-made electrical noise, natural electrical noise (from the sun and from close or distant lightning) is the nature of the FM signal itself. As one poster noted the intelligence of the FM signal is encoded in the varying patterns of the frequency shift of the FM "Sideband" signals (sideband frequencies = carrier frequency +/- modulation frequency) and not in the changing amplitude, as in AM. Since noise is additional amplitude without frequency variation, a detector which is decoding frequency variation only would be immune to stray noise energy. FM modulation also has an effect that AM does not. Known as the "capture effect", if two FM stations are broadcasting on the same frequency (or close to it), the one that is received the strongest will be heard completely, "capturing" the frequency, while the weaker station will not be heard at all. Only when the stations are very near to each other in signal strength, such that minor variations in strength cause one or the other to be alternately stronger at different times is there a problem. This can create an unpleasant "wavering" effect, as the two signals struggle for dominance.

On the AM broadcast band, however, you will often hear both a strong station and a weaker station on the same frequency, especially at night when a local station may have to turn down its power and still compete with stronger powered stations coming in from far away. In this case, the human brain can even act as a filter, listening only to the station it wants to hear and rejecting the other information. I noticed this when listening to the local Radio Disney station shortly after sunset, when the Disney station has just reduced its power, while driving in my car with my kids. The kids of course want to keep "their" station on, even though, to me, all I can hear now is the noise of the distant stations, they keep humming along with their favorite songs on Radio Disney!

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#47
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 9:56 AM

less interference from man-made electrical noise, natural electrical noise

Like I said earlier, it's Steely Dan's fault.

Woof woof ! Nice post STL, I'm just jealous.

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#48
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 10:00 AM

ahhh go on...you made that up!

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#50
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 10:07 AM

Oh, you got me, CopperZincPrimate! Yeah, I made the whole thing up! Couldn't pull the (steel) wool over your steely eyes, though!

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#51
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 10:22 AM

This is completely gratuitous, but the mention of diversionary wool..... (Virginian) is too much.

So far, my topical knowledge ends here.

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#52
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 11:00 AM

Just an off topic question out of curiosity...

I see someone has several replies to comments, but there is no body to these comments? Have they been deleted, or is there something I am missing.

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#53
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 11:10 AM

Hit the + symbol at the top right corner to expand those comments.

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#54
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 11:13 AM

AHA!

LOL ok thanks a bunch.. I just figured this out on my own some how :) Handy... to screen out distracting off topic coments:)

I can go home now, I learned something today :D

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#55
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 11:18 AM

Took me ages to figure out lots of small detail as well. Usually asking a question out of the blue will get a result when it come to navigating CR4.

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#57
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 12:33 PM

Thanks for the explanation.

Here in Pittsburgh, we have many tunnels and the effects are usually quite noticeable. However, in the longest (and oldest) tunnel, fm fades out while am stays strong and clear.

Tom

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#72
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 9:32 PM

Nice write-up STL.

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#80
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/19/2007 6:07 AM

Hi STL

That's better than I could ever do; but there's one significant inaccuracy:

You say "since noise is additional amplitude without frequency variation". This is unfortunately wrong. Noise is uncorrelated with the carrier for your signal, and so is effectively (in power terms) 50% phase modulation and 50% amplitude modulation. The reason that additive noise is much less audible on FM than on AM is that the peak FM excursion is 75-kHz, whereas the sensitivity of human hearing drops rapidly above 4-kHz; this corresponds** to a "modulation index" of 17.75 at 4 kHz, and more below that frequency. The process of FM demodulation attenuates the average noise below 4-kHz by more than 26-dB. For more detail, see (for example) "Reference Data For Electrical Engineers".

**To avoid overcomplicating matters, I've omitted technical discussion of pre-ephasis/de-emphasis, but these are used to further attenuate the effects of noise above 3-kHz. I've also omitted discussing stereo, on the grounds that most modern systems simply reduce the stereophonic part of the signal to near zero when the input becomes noisy

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#85
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/19/2007 9:21 AM

"You say "since noise is additional amplitude without frequency variation". This is unfortunately wrong."

Fyz,

I am not disputing what you say. As a mechanical engineer your discourse goes beyond my understanding of radio theory. I guess I better crack the books if I want to earn my Amateur Extra license! Would it have been better to say "...with very little frequency variation"?

If not, I guess I don't get where the frequency variation would come from that would show up as noise in a demodulated FM signal. Also, I thought since FM is a special case of Phase Modulation, ordinary phase shifts that are not actually frequency shifts would not affect the output of a true FM demodulator.

Please explain in laymens's terms, or, if you can, think of a mechanical analog that might help my understanding.

Thanks!

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#87
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/19/2007 10:43 AM

Hmm. That's quite a challenge - and not one I think I am equal to. But I'll have a go in a somewhat different way*.

I'll try to start with amplitude and phase contributions of noise. Compare an EXCEL (or similar) graph of a 1-Hz signal with unity amplitude (to represent our undisturbed signal), and an interfering signal (to represent noise) with amplitude 0.2 at 1.2 Hz with an undisturbed signal over a period of 5 seconds (it will obviously repeat after that). [The expressions are "=SIN(2*PI()*D1)+0.2*SIN(2*PI()*1.2*D1)", and "=SIN(2*PI()*D1)"]

You will see (as I think you would expect), +/-20% amplitude variation and about +/-64-ms (=+/-11.5-degrees) peak shift in the position of the zero crossing relative to the unmodulated crossings (which would have been at 0.5-second intervals). As single frequency sinusoids produce both amplitude and phase modulation, I think it should be clear that the sums of uncorrelated signals must also produce both effects.

That was the (relatively) easy bit. Now to phase modulation. The thing to consider is what constitutes the useful signal. It is the phase difference between the theoretical unmodulated signal and the modulated signal. Nominal phase modulation would have a peak level modulation distance of 1 radian (~57.3-degrees) [that would give the same performance as 100% AM]. The spectral peak would be at approximately +/- the modulation frequency, with amplitude 0.5 - rather like 100% AM, but with different phases].
But there is no reason in principle that the phase cannot exceed 57.3-degrees*** - or even 360-degrees or more.** So, using phase modulation, we can have a signal that is much larger than the nominal - and thus improve the signal-to-noise performance. The reason for using FM rather than large phase modulation is that we want to make optimum use of a signal that fits through a specific spectral window. If we modulate the signal frequency by +/-75-kHz with modulation frequencies below 5-kHz, the spectrum will fit roughly in a +/-80-kHz window, but will provide an effective signal amplitude is 75/5 = 15 times what we could achieve with 100% AM - and the signal-to-noise ratio is improved accordingly. The improvement increases as the modulation frequency becomes lower, and is reduced at higher frequencies; fortunately, the sensitivity of human hearing means this is a pretty good compromise.

*The problem I have is that it is as-or-more complex to justify the mechanical analogue than to use graphs, which is what I shall do instead.
**Though the simple sinusoidal approximation breaks down a long way before this
*** 57.3-degrees represents a "modulation index" of 1

Well, at least I tried. If it makes sense, you could probably do a considerably better translation for others.

Fyz

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#90
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/19/2007 11:22 AM

Ooooooooo, my brain hurts (not Hertz). I think I will tackle that another day. I better get back to my 3-D CAD modelling of something I could actually reach out and touch if I had a real one in front of me* instead of just these lines on paper or photons emanating from my PC's LCD monitor!

* That reminds me of one of my favorite sayings: "I'd much rather have a full bottle in front of me than a full frontal lobotomy!"

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#93
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/19/2007 12:22 PM

Unfortunately, some things just don't convert directly. Although the windmill analogy (height of tip of sail above ground for AM, and angle between vertical and line from tip of sail to foot of windmill is OK for PM) works OK, it is more trouble to set up than the results are worth.

Then you can think about measuring PM/FM as simply using an angle between a modulated signal and the unmodulated carrier - if you can track the angle, you can in principle make it as large as you wish; if you can go around several times e.g. 360-degrees, 720-degrees..., you have effectively a modulation distance that is 2*pi, 4*pi... times the carrier level. And the only restriction on how far you can go around is the available bandwidth for the signal... This beneficial effect only breaks down when the noise or interfering signals are so large that you can no longer follow the signal around the circumference.

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#83
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/19/2007 8:19 AM

Good job STL

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#66

Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/18/2007 7:31 PM

Sure is a lot of verbage here on this one. Some of the replies are somewhat good but tend to go off on tangents.

Bridge, tunnel, other locale will result may result in loss of signal. That's the bottom line. The most straightforward explanation is that the FM station was more nearby or propagating inline with the tunnel/bridge. That is only incidental. In addressing simplistic queries as this, it's best to stay with the subject at hand and not necessarily extrapolate beyond.

I've been in radio communications for many years and it all has made sense to me; no mystery except one, and I'll air it briefly here since it's a whacky arena at this point anyway: You are listening (don't have to be driving etc, but then happens to us all more frequently) to an AM or FM station and it's getting weaker steadily, but you are still able to do so with some annoyance (noise/fading). You try and flip (digital) or otherwise retune to that channel (also prevails with the old analog tunable radios). Now, you find that the station can not even be re-located or found. Gone. Faded away. Too far away from the broadcasting station. I have a pet theory about this phenomena, but maybe to go into it at another time.

I've got a bad feeling that I've thrown gasoline into the fire.

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