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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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#69
In reply to #66
Find in discussion

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

09/18/2007 9:08 PM

It seems like I recall a difference between the capture range vs. the hold range on an FM demodulator and or possibly the AFT or even some interaction between the two since the error signal for the AFT comes from the demodulator. It probably would differ on any given radio depending on the type and quality of the design. It wouldn't apply at all to AM.

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#70
In reply to #66
Find in discussion

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

09/18/2007 9:20 PM

I'll hazard a guess. If you get a lock on a station and loose it when it is dancing at the threshold your radio has a much harder time reacquiring that signal. Hysteresis is the word I am thinking of.

I would expect that would be more of a problem with PLL circuits and it may also be a benefit to prevent excessive flutter when the signal gets that far down in the grass.

I seriously doubt that the challenge answer is simply the FM station's antenna proximity and position is more favorable than the AM station. These challenge questions tend to revolve around some scientific phenomenon that is chosen to be as obscure as possible to make people work for that answer.

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Anonymous Poster
#67

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

09/18/2007 9:06 PM

Surely the bulk of the bridge just creates an AM shadow or dead spot, AM being reflected from above. FM being horizontal travels through the bridge.

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#71
In reply to #67

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

09/18/2007 9:30 PM

Nice thought, but not quite true. You are not the only one to make that statement, but both AM and FM broadcast stations are actually vertically polarized.

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#73

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

09/18/2007 9:37 PM

Sure, dead spots and what not. There are a ton of variables that could be discussed (or cussed?).

For the past 25+ years or longer, FM broadcasting stations have been using antenna configuration that is neither vertical nor horizontal polarization per se, but rather attribute for cross-polarization (doesn't matter what kind of receiving antenna). Automobile receivers have been largely and obviously of vertical polarization, but seemingly many of the newer cars are now integrating the radio antenna within the windshield glass, and that by its layout would be a horizontal polarization basically.

Attempting to analyze the characteristics of a particular bridge or tunnel for the purpose of its "waveguide" or other would be surely moot, nor would it have any impact on any future construction of same.

BTW: AFC (Automatic Frequency Control, FM) or other related reference (earlier entry) has nothing to do with any relevance of explaining to our Guest an appropriate answer.

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Anonymous Poster
#76

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

09/19/2007 12:20 AM

AM, ("amplitude modification") frequency transmission occurrs quite low in the RF spectrum, with characteristically long wavelengths; so long in fact that in general, AM transmissions can only be heard when the receiver is within site of the transmitter, not blocked by an iron bridge. The largest low frequency transmission is accomplished by trailing a 2 1/2 mile longwire antenna from a high altitude jet aircraft. This is how we retain radio contact with submarines while they are submerged.

FM, ("frequency modulation") transmission utilizes a much higher frequency level within the radio frequency spectrum. These much shorter wavelenghts are able to enter the area beneath the bridge, whereas the longer wavelength of AM transmissions simply can't fit under the bridge.

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#79
In reply to #76

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

09/19/2007 4:32 AM

Great thread.

IMHO, posts #4 & #46 are the best & most complete answers, neglecting the minor confusion regarding whether it's a tunnel. In between there are many other good points along with some of the usual confusion.

But Guest #76, "AM, ("amplitude modification")....transmission...so long in fact that in general, AM transmissions can only be heard when the receiver is within site of the transmitter"

uh, it's 'sight', as in 'line-of-sight', and this statement is completely wrong - AM is most certainly not line-of-sight (as highlighted by many of the correct posts which discuss nighttime propagation effects of AM & shortwave transmission).

Continuing, "The largest low frequency transmission is accomplished by trailing a 2 1/2 mile longwire antenna from a high altitude jet aircraft. This is how we retain radio contact with submarines while they are submerged."

I guess that the 'largest...(VLF)...transmission' (which I assume means amplitude or radiated power) probably emanates from ground-based stations (because a whole lot more power is available...).

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#89
In reply to #79

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

09/19/2007 11:08 AM

Good job on correcting the "line of site" error, RF guy.

I am not sure what you meant to say regarding the VLF (or ELF) transmissions, but Navy submarines do utilize them by paying out a long antenna trailing behind for miles in open ocean. Years ago aircraft did so also, perhaps not as low in frequency as today's submarines use. In fact, one kind of wire antenna still used by amateurs today for High and Low frequencies was originally used for airships, also called Zeppelins, or Zepps, after the German count (Von Zeppelin) who built many and popularized the notion of airship travel. The antenna is still popularly called a Zepp antenna!

The Navy has used an ELF system for one-way transmissions to submarines, primarily due to the power required as well as the antenna. This required using the earth itself as an antenna. However, those earth stations were dismantled in 2004. The Navy and Air Force still does use VLF transmissions from aircraft to maintain communications with submarine and other seaborne and airborne stations, as well as ground stations. Google TACAMO for more info. Here is info on the Navy's ELF stations:

"Because of this huge size requirement, and in order to transmit internationally using ELF frequencies, the earth itself must be used as an antenna, with extremely long leads going into the ground. The US maintained two sites, in the Chequamegon-Nicolet National Forest, Wisconsin and the Escanaba State Forest, Michigan (originally named Project Sanguine, then downsized and rechristened Project ELF prior to construction), until dismantling them began in late September 2004. Both sites used long power lines, such called ground dipoles, as leads. These leads were in multiple strands ranging from 22.5 to 45 kilometers long. Because of the inefficiency of this method, considerable amounts of electrical power were required to operate the system."

The primary communications with submarines is done with a conventional antenna, which may be launched from the submerged submarine on a buoy that floats up to or near the surface, remaining undetected by surface RADAR or SONAR if still slightly submerged, but still able to make contact with orbiting communications satellites.

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#82

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

09/19/2007 7:08 AM

FM waves are shorter, therefore penetrate structures easier than AM.

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#84

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

09/19/2007 8:46 AM

FM waves are much higher in frequency, and are correspondingly shorter in wavelength. A typical AM signal at 1000 kHz has a wavelength of 300 meters. AM waves will penetrate large structures, but will be attenuated by them. The steel reinforcing bars in the bridge would also act to block the signal by acting as a partial Faraday cage, greatly attenuating the signal. An FM signal at 100 MHz has a wavelength of 3 meters. This wavelength is comparable in size with structures and will tend to scatter from them (remember ghost images in broadcast TV from multipath signal reception?). The FM signal can travel under the bridge by reflecting from the road and the underside of the bridge.

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#92

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

09/19/2007 12:07 PM

That's easy! FM waves are approximately 3 meters (10 feet) long, whereas AM waves are about 300 meters (1,000 feet) long. A 3m wave will easily fit under that bridge where a 300m wave would not fit without being broken up into many smaller pieces. And when it breaks, it would end up in random size pieces which would be hard to tune in, as they would cover a random spectrum.

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#94
In reply to #92

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

09/19/2007 12:22 PM

Hey Stormin!

A 300m LONG wave will fit easily under a bridge, as long as it is not 300m HIGH!

And if it did break up, I would just use my Super-D-Duper Spread Spectrum Software Defined Receiver in Random mode to pick up the pieces!

ROFL!

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#96
In reply to #94

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

09/19/2007 12:44 PM

Sounds good. My only problem is - would your Super-D-Duper fit in my old Volvo? And would my old brick be able to negotiate the bridge once loaded up?

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#101

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

09/20/2007 11:32 AM

I certainly observe that myself on a regular basis while listening to talk radio. The answer has to do with wavelength and the relative size of the bridge structure. The FM wavelength is only about 3 meters, while the wavelength of AM is 300 meters. Without getting overly technical with waveguides below their cutoff frequency and that sort, it's easy to relate this to the optical analogy. The much larger wavelength is filtered by the relatively small opening created by the roadway and the overpass; while the shorter wavelength passes readily into the opening and onward to your car antenna so that your favorite easy listening channel arrives!

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#110

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

09/21/2007 2:45 PM

Radio has come a long ways since the days of crystal sets that used "cat's whiskers." And, believe it not, crystal radios are making a comeback.

Hmmmmmm, cat's whiskers, hmmmmm.......

Del! Oh, Del, where are you?

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#111
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/21/2007 3:03 PM

I was asleep....here is a picture of my finest whiskers collected over a lifetime!

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#114
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/24/2007 9:58 PM

Drat! I wanted to pluck one myself!

Maybe Kris will let me have one from Ol' Sooty, if he hasn't been preserved yet.

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#115
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/25/2007 1:54 AM

Project Sooty is still going. I simply have to wait a while - the darn thing seems eternally youthful. You may be out of luck 3Doug, my project will be short of hair if anything. Come the day, a few of Dels spares may be handy to achieve that 'good-as-new' look (depending on the eventual method used). If Del could acquire a nice tan it will help the colour matching. I think ER mentioned having tweezers.

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#116
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/25/2007 4:07 AM

A fine selection, I'm sure you will agree.

Del, De-el, more fish heads fuzzy-cat...tasty fish heads.

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#117
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/25/2007 4:41 AM

he he. Just in case he creeps in during the night;

That reminds me, I have some pencils to sharpen.

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#118

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

09/25/2007 11:20 AM

Okay, I am wrong. The claimed answer is that waveguides from rebar are not a factor. Or are they?

However, the given answer is misleading, I think. It states that lower frequencies are better absorbed by the concrete. This is not the case. The lower the frequency, the less RF absorption by concrete. Ground Penetrating Radar is a case in point. Lower frequencies offer deeper penetration, at a cost of spatial image resolution.

Yes, FM will reflect off of concrete better than AM because AM tends to pass more RF energy through concrete than FM. So how does this explain the effect cited in the original challenge question?

Can CR4 cite some data that describes the physics of the phenomena rather than the "light weight" answer that was provided to us?

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#119
In reply to #118

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

09/25/2007 1:06 PM

I think you are correct in that the given answer it too simplistic. Almost like you would explain something to a child with really giving an understanding of the scientific principles involved. The answer really goes to the lower frequencies having longer wavelengths that cannot penetrate the smaller openings between the metallic conductive elements of the bridge. Plain unreinforced concrete by itself as you noted is not much of a shield against RF, but the rebar and other conductors, mostly steel, in the bridge structure are.

The answer did mention that FM radio has a higher frequency and "bounces" around, or reflect to create multiple paths, many of which do find RF "holes" through which their shorter wavelengths may pass unhindered.

As usual, or at least often the case, the "answer" falls far short of our expectations!

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#120
In reply to #119

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

09/25/2007 3:09 PM

I have to agree, and I think you could with justice have added a word:
As usual, or at least often the case, the "answer" falls far short of our reasonable expectations!

Fyz

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#121

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

09/25/2007 3:48 PM

Although a number of responses have hinted toward this, none have really talked about it directly. AM signals require about a 49:1 Signal:Noise ratio. In other words, the signal has to be at least 49 times larger than the ambient noise. FM on the other hand only requires a 2:1 Signal:Noise ratio. (This is also why digital music is naturally more free from noise than its analog counterpart).

Both signals attenuate when going through the bridge. The actual signal strength of the AM signal might even be higher than the FM signal, but for intelligent information it has to be monstrously higher, which is typically not the case.

The problem is also a combination of the wavelengths used, natural noise rejection, wave propogation, etc. all of which have been covered already by some pretty smart guys on this page.

When it comes to useful communication, however, Signal:Noise is king. Under the bridge, both signals; FM's line of sight & AM's surface wave attenuate. FM can handle the attenuation better for a number of reasons, all cumulative, including it's miserly efficient 2:1 signal to noise requirement.

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#122

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

09/25/2007 4:43 PM

A relatively few tunnels have a wire (not connected to any electronic device, just on standoffs a foot or two from the side of the wall) that runs the entire length and at least a few feet outside the tunnel entrance. If you drive through the tunnel, the radio reception is much better. The wire acts as an antenna to your radio coupling the RF from outside. Obviously, the longer the wire is outside, the better (to a limit).

Most tunnels do not have this added; while it is cheap, it is an un-necessary expense (have you priced copper wire recently?) so is not typically provided.

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#123

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

09/25/2007 5:16 PM

I strongly disagree that absorption, alone, causes the signal to fade. As others have pointed out on this thread, the bridge dimensions can serve to exclude wavelengths larger than the bridge openings. FM-station-frequency1 signals have a much shorter wavelength (less than two meters), and so they are not excluded from traveling through spaces in the bridge larger than roughly two meters.

Nor does concrete, in general, absorb AM-station1 wavelengths more readily than FM wavelengths, and they are both partially absorbed and partially reflected by the concrete. If anything, concrete passes AM-station wavelengths better. Generally speaking, the lower the frequency, the more readily radio waves will propagate through a given non-metallic medium. This is the primary reason, in fact, that VLF and ULF radio waves are used to communicate through rock (spelunkers take note) and seawater (submarine communications). FM signals don't stand a chance in these applications.

Concrete bridges and other large structures are seldom built without some form of steel reinforcement, making an even stronger case for the "waveguide" interpretation.

Moral: AM stations fade not because they're absorbed by the bridge, but because the wavelengths are simply too large to enter the bridge spaces. FM signals don't fade because they can enter the bridge spaces.

Note 1: I'm being pedantic here in distinguishing AM and FM signals by the frequencies transmitted by AM and FM radio stations, respectively. I am not making a distinction here between them at all in terms of modulation. Modulation is irrelevant in this context, and I want to point that out just to make sure we're reading from the same page.

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#124
In reply to #123

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

09/25/2007 5:54 PM

I think I agree with your comments on the dominant importance of frequency. But there is a difference in AM vs FM sensitivity as well - though nowhere near the 28-dB indicated by previous correspondents; this would be partly because the noise bandwidth of typical FM demodulators is so much greater than that of AM, and partly because multipath interference under the bridge will dramatically degrade the modulation quality of the FM input signal.

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#125
In reply to #124

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

09/25/2007 6:12 PM

If both our receivers were built with the same sensitivities, equivalent noise bandwidths, and so forth, we level the playing field. FM multipath interference is possible inside the bridge only because the FM signal can enter the bridge spaces. AM cannot (it can, but not more than as an evanescent wave which decays exponentially as we progress inward).

Even if all we heard was FM multipath interference, we would still have an FM signal no matter how unintellegible. But the Newsletter Challenge solution offers only that the signal degradation occurs because the two frequencies are absorbed by differing degrees. No mention of relative receiver sensitivites. No mention of noise bandwidth. No mention of anything else. Just absorption. And in this context, I object to the given solution.

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#131
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/26/2007 9:30 AM

I didn't realise you were addressing the "official" solution. To my mind, that was just sci-babble and totally ignorable. I was addressing the composite question as posed - AM frequencies suffer greater attenuation than FM, but that is not the whole story.

For what it is worth, my experience is that you often get appreciable AM signal under bridges, although the listening quality is usually considerably degraded. FM is somewhat better, as a result both of higher signal levels and of the noise suppression inherent in the modulation system. On top of this, AM sound sound will disappears if you drive through - because the radio's AGC filter cannot significantly attenuate the audio signal, so it doesn't have time to follow the level of the carrier. The AGC and/or limiting in an FM receiver, on the other hand, can be as good as instantaneous.

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#129
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/26/2007 9:15 AM

This is the primary reason, in fact, that VLF and ULF radio waves are used to communicate through rock (spelunkers take note)

Hello Eu (Pronounced like "you" or "yew"?),

Amateur Radio operators (Hams) who belong to Cave Rescue organization do use very low frequency bands for rock penetration. I wonder if Mine Rescue workers do this also, but it could be that all that metal (ore or structural steel and cables) works against longer wavelengths just as the bridge metal does.

STL (pronounced "ess tee ell")

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#130
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/26/2007 9:22 AM

Dang, no wonder I was confused.

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

09/26/2007 9:35 AM

Oh, no! I picked my screen name precisely because I though it had nothing to do with software engineering and everything to do with my geographic location and community identification.

Now you tell me it has an IT significance as well? RATS!

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#135
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/26/2007 12:25 PM

I guess you missed my post in Kris's Drawing thread where I mentioned .STL (stereolithography) files used by prototyping machines.

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#136
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/26/2007 2:12 PM

PMSL ! Are you saying that our loveable St Louis guy is a copy !! I thought I was bad for winding people up, but I now doff my Bowler/Deby/Coke hat to you !

Americas finest arch is silver, not them two golden things. Go on guys, scrap ! let me LOL from the sidelines. Is Ken on holiday ? You guys do banter that people would pay good money for.

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#137
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/26/2007 5:04 PM

You guys do banter that people would pay good money for.

We aim to please!

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

09/27/2007 11:20 AM

We aim to please!

-----

You aim too, please.

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#138
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/26/2007 5:07 PM

Yeah, I missed that one, must have gotten busy doing my own Drawings that week.

Solidworks is what I have been using the last two years, and AutoCAD before that.

So "3Doug", I get the "3D" part, but what does "oug" mean?

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#139
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Re: AM vs. FM: Newsletter Challenge (09/18/07)

09/26/2007 7:37 PM

"Old Ugly Gentleman"

Wait, that's what KA5OUG says! I've got to think of something else.

AHA! "Onery, Underhanded, Gassy"

Well, you had to ask, didn't you?

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

09/26/2007 9:35 AM

This surely depends on the mine - coal (mainly graphite) is rather conductive, so you can often forget about VLF penetration. SFAIK,the earliest leaky feeders were developed for coal mines for just this reason.

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#126

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

09/25/2007 11:00 PM

I am disappointed in the official answer also. It implies that concrete absorbs the signal. For concrete to absorb the signal, it would have to be a conductor and not an insulator. As an insulator, concrete blocks the RF energy, and whatever does manage to reach the steel rebar will absorbed and conducted to ground. Any exposed steel in the bridge will act the same as the rebar. If concrete has any conductivity, it is definitely minimal and has little bearing on RF absorption. The wording of the official answer is a bit misleading.

However, concrete might be a good choice for Project Sooty.

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#128
In reply to #126

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

09/26/2007 5:21 AM

Project Sooty : It might be quite fitting - Sooty gets in the way of everything else, so mangled radio reception would be apt. I may need more cats to make a sufficiently large block.

Oh, De-el.....c'mon now, c'mon....

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#127

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

09/26/2007 4:20 AM

The audio-amplitude of an AM-receiver is proportional to the RF-signal amplitude. The automatic gain control tries to keep the RF-level at the AM-demodulator only relative constant. If the signal strength falls very low, so does the audio signal.

The audio-amplitude of a FM-receiver is - ideally - only proportional to the RF-frequency shift, but not proportional to its amplitude or signal strength ! PLL-demodulators are insensitive to RF-amplitude variations. Even large RF-signal strength variations will not have any impact on the audio signal amplitude.

Heinz Lenk

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#141
In reply to #127

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

09/27/2007 11:28 AM

I have a mind to try driving under a few bridges around here where this is a problem and watch the various AM, FM, cellular-phone signals, and general RF trash on a portable spectrum analyzer. That way we dispense with consideration of various kinds of receivers and look at the issue using a consistent, broadband measurement technique.

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Anonymous Poster
#142
In reply to #127

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

09/27/2007 1:03 PM

You must have been using some really poorly designed AM radios. Half-way decent AGC should be capable of maintaining the mean signal level of AM until noise and/or interference become intolerable.
What you say is true if the signal levels are changing very rapidly: if you don't limit the response speed of the change in Amplitude Gain, that change will start to attenuate the Amplitude Modulation (the signal) rather than just the carrier. So there will always be limits to how rapidly the AGC can track. PM and FM really have an advantage in this situation - except that with PM you often have multipath interference so the phase can be all over the place; in this situation the high modulation depth of FM provides additional benefits.

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