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

Quite.

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

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.

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

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

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.

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

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?

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

Or did the bridge morph into a tunnel?

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.

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

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.

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

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

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.

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.

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.

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).

Fyz

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.

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"

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.

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.

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

In a word, Propagation.

boB

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.

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.

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.

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.

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

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.

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...

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!!

There's no static.

The diff. is in the direction of driving.

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

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.

Answer number three has my vote.

blueice

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?

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.

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.

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!

ahhh go on...you made that up!

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

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

Nice write-up STL.

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

"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!

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

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!"

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.

Good job STL

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.