Question on FM radio

[Splat]

We are (well me at least) taught that FM radio is line of sight etc etc.

Why is it that I can listen to uninterrupted FM radio in my car going under bridges , between tall buildings etc?


TIA & HNY

S

[FlyingForFun]

Splat,

FM radio waves can penetrate thin walls. Otherwise you'd get no reception on your stereo indoors!

FFF
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[Splat]

Is that all there is to it? Even works is small underpasses, so if that's the case, must be fairly thick walls too.

Just curious.

S

[Don D Cake]

Radio waves are reflected and refracted by objects such as buildings so you can still receive even if you cannot "see" the transmitter. Kind of think of it as not being plunged into total darkness if you do not have direct line of sight of the sun (after all light is just electromagnetic radiation).

[Cardinal]

I'm gonna venture out on a limb here, cause I can't readily support my answer. But aren't our aviation communications radios AM? Which of course does perish under the overpass, at the drive-through, etc.

[luxflyer]

Cardinal,

Radio propagation is mainly a matter of frequency. Modulation (FM/AM) is fairly neglectable here. What we commonly call FM radio, are the freqs. in the 88 - 108 MHz range. Remember that aviation communication takes place in the 118-137 MHz range, and thus will be subject to similar propagation conditions than FM radio, i.e. line of sight. AM broadcast radio takes place in the 0.5 - 1.7 MHz range, therefore propagation is different here. It's just a matter of frequency... basically.

Someone correct me if I'm wrong, but the reason we use AM in aviation ( I believe ) is that you need much less separation between channels. Channel spacing in aviation is currently as low as 8.33 kHz using AM, as opposed to 50 kHz used for FM broadcast.

In a nutshell:

High frequencies ( 65+ MHz ) propagate nearly in "line of sight", FM is used where quality is important (e.g. FM radio broadcast), AM is used where quality is less important, but many channels are needed (e.g. aviation comm, AM radio...).

Hope that was more or less correct,
greetings...

Luxflyer, 5 am local ;)

[jtr]

FM stops at 108.0 mHz where I come from, and A/C VHF starts at 118.0 mHz. So that makes them almost neighbours (I think), and both in the VHF part of the spectrum (I think).

AM is around 500 kHz-1700kHz (0.5-1.7 mHz), which puts it in the MF part.

LF-MF-HF-VHF-UHF-...............-Infra Red-Light

-Edited for another (i think)

[411A]

Amplitude modulation is used with ATC communications because...
if two stations transmit at the same time, it is heard as "garbled", whereas if frequency modulation was used, only the stronger signal would be heard.
In short, 'tis a safety measure.

[quixote]

'Amplitude modulation is used with ATC communications because... '

Not so.
Engineer Edwin Armstrong invented Frequency Modulation (FM) in 1932 but commercial development of FM was strenuously opposed by commercial interests with major investments in AM, including Radio Corporation of America and the British Marconi Company. The corporations knew that public acceptance of and demand for static-free FM broadcasting would destroy their heavy investment in AM transmission equipment and they prevented FM's widespread usage till the '50s. Tired of battling against major corporations for years to establish FM, Armstrong suicided in 1954.
By the time the patent and commercial battles had subsided, Amplitude Modulation (AM) was well established for aviation use and the benefits of of interference- free reception for aviation were not seen to outweigh the cost of equipment conversion to FM.
Fortunately the commercial interests behind internal combustion engines were unable to prevent the development of jet engines.
Don
http://www.vietnamvetsmuseum.org/qxs.gif

[FlyingForFun]

Agree completely with the responses on the difference between frequency bands and modulation types - it's the frequency which determines the propogation of the wave. The question was asking about FM signals in the VHF band, as used by commercial radio stations.

Back to the original question of VHF (not FM) propogation, though... I'd be surprised if reflection is really very significant, although I don't have any numbers to back this up. If VHF waves were reflected off buildings, radio reception (FM radio and aviation comms) would be full of echoes as we pick up a whole load of reflections off any buildings in the vicinity.

Does anyone actually have any numbers on attenuation of VHF signals, both reflected off of common materials such as buildings, and passing through these materials?

FFF
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[Select Zone Five]

The way I understand it is that VHF/UHF radio waves posess relatively low energy (compared to light) and therefore are not absorbed by solid objects but simply pass through them. There is indeed some reflection but not of high enough energy to exist for very long; and anyway, the waves are travelling at the speed of light so any 'echo' would be instantaneous.

Unless the tunnel is particularly long/deep so the thicker the wall the more the energy is absorbed and therefore we get radio blackspots in some tunnels.

[phnuff]

This is an interesting debate and one that seems to open a few questions. Thinking in terms of 'non' aviation radio If I drive my car through the Hatfield Tunnel on the A1 which has a bend in the middle so you cant see right through, I can pick up am radio stations all the way through for example radio 5 on 909 Mhz. FM stations however (93.5 Mhz) do not work right through . . . but mobile phones, however which in this country are FM operating at either 900 of 1800 Mhz do work right through the tunnel. From this, I guess the conclusions would be that AM radio is more penetrating than low frequency fm, however as the frequency of FM gets higher it becomes more penetrating (or possibly better reflected?).

Ok, this has little to do with aviation (unless you want to try flying a Cessna through the Hatfield Tunnel), but if anyone can fully explain my observation in ther context of this debate, I would be interested to hear it.

[RadarContact]

Mobiles have a higher signal energy than car-radios (at the receiver's end, that is)

[error_401]

Even simpler.

Reflection and Refraction from buildings are near the receiver and therefore do not necessarily be shifted in time so much that they cause echos.

In aviation sometimes we have to deal with reflections from further away obstacles - then the time difference becomes noticeable. Especially at low bands. AM or FM doesn't matter at all.

Radio waves do not penetrate too far into buildings or the ground. Especially not in concrete buildings. In wooden houses a station may be received throughout.

Tunnels usually shield radio waves. But - some are reflected from the road surface than from the walls - that's why you can receive a station at the entrance even on high frequencies. Low band reflects better and might be received throughout the tunnel.
But consider this. Many tunnels are equipped with sending antennas - first for convenience - then for safety transmissions!
The same is valid for cell phones!

[Don D Cake]

Reflection of radio waves is significant and can cause reception problems known as "multipath", although it can also enable reception when out of "line of sight" as I mentioned earlier. Most materials will reflect fairly well but large, flat metallic objects are the best (the ground is very good too!). Trees are not very good at all and will absorb most of the signal.

When a RF signal is reflected its polarity is inverted, it is also attenuated by at least 3dB ie half. The amount of attenuation depends on the reflecting material's composition and surface. When the direct and reflected signals arrive at the receiver they may be in phase and add, be out of phase and subtract or be somewhere inbetween. If the receiver is mobile, this may cause multipath fading if the direct signal is weak. This is usually only a problem with AM receivers as FM receivers will "capture" the strongest signal. Multipath reception is the cause of "ghosting" on television receivers as the vision signal is AM.

A great example of multipath interfence has been caused by the building of the towers at Canary Wharf in London. The glass of the buldings is metallised to make it look a pretty silver colour. However the buildings acts as perfect reflectors for the television broadcast signals from Crystal Palalce making televsion reception for some people in the South East impossible.

An example....

Take a PAL televison that has a "ghost" image one fifth of the screen width along from the main image.

There are 625 lines at a frame rate of 25 per second. That gives a line rate of 15.625kHz. That means one line is transmitted in 64uS hence a fifth of a line in 1.28uS. So the "ghost" signal arrives 1.28uS later than the main signal. RF travels at 300,000,000m/S so the ghost signal has travelled 3.84km further than the main signal.

[phnuff]

Just to complicate things, I have looked on the Internet and discovered that mobile phones use a system known as TDMA (Time division multiple Access) which splits a 30khz channel into up to 8 time slices with each user working the channel being allocated slots as required. Now at this point I think my scenario about the Hatfield Tunnel and the ability to use a mobile right through is even more complex and at this point I am going to retire to somewhere that sells headache pills !!

[411A]

Quixote,
Your history is more or less correct, but your theory is most definately wrong.
Suggest you find a licensed radio engineer and he might explain it to you in terms that possibly you can understand...;)

[Don D Cake]

In a nutshell....

FM - good signal to noise at a penalty of high bandwith

AM - low bandwidth at a penalty of poor signal to noise

You pays your money....

By the way, even a lousy performance narrow band FM system using 5kHz of deviation and a peak audio frequency of 3kHz still needs a bandwidth of 17.5kHz - no good for 8.33kHz spacing.