Why does medium wave radio cut out when going through a tunnel?

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Offline chris

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When listening to a radio station that broadcasts on medium wave(AM), why does the signal cut out when going through a tunnel, while FM stations remain perfectly audible?

Chris
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Offline syhprum

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Because many tunnels have a relay system to maintain the VHF signal but not the MF signal.
syhprum

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Offline tommya300

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Can this be possible?

 AM radio is confined to a band from 535 kilohertz to 1,700 kilohertz (kilo meaning "thousands," so 535,000 to 1,700,000 cycles per second).

Amplitude Modulation inherits static interference and at the low frequencies can be absorbed by solid structured matter

All FM radio stations transmit in a band of frequencies between 88 megahertz and 108 megahertz.

Partial advantage Keep in mind planks law
E=hc/(λ) lambda
Higher frequency higher energy

Higher the frequency permits the wave to pass through most solid structured matter, with less or no interference.

Added features...Individual "Radio transmissions" ride on a Center frequency or true frequency, with a given bandwidth.
Higher frequencies have wider bandwidths, this permits FM to run sterio on the upper and lower sidebands, without clipping the audio signal. AM has a lower bandwidth, which is not wide enough to do so. 


« Last Edit: 27/09/2010 16:19:38 by tommya300 »

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Offline chris

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Hi syhprum - so why does the relay system work for FM but not AM? I thought it largely consisted of a "leaky" coax cable running inside the tunnel.

And Tommya - thanks for the thorough answer...
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Offline Geezer

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I would think it would rather depend on what you "shove" (that's a technical term) down the leaky coax. Presumably there has to be some sort of wide bandwidth receiver that picks up the RF signals and drives them into the coax. If it only operates in the VHF spectrum, it's not going to retransmit MW spectrum signals.

There are probably many more reasons than that. I'll bet syhprum has a much better answer!
There ain'ta no sanity clause, and there ain'ta no centrifugal force Šther.

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Offline tommya300

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I would think it would rather depend on what you "shove" (that's a technical term) down the leaky coax. Presumably there has to be some sort of wide bandwidth receiver that picks up the RF signals and drives them into the coax. If it only operates in the VHF spectrum, it's not going to retransmit MW spectrum signals.

There are probably many more reasons than that. I'll bet syhprum has a much better answer!
Maybe there is Geezer, I mentioned Partial advantage, can this be explained that the same happens when going across the open steel Girder bridges.
FM does not depend on amplitude gain AM signals do.
AM carrier wave envelops the audio the component that is susceptible to noise. The path of transmission can add of subtract to the signal's original Amplitude.
 
FM audio component is within the Frequency Phase of modulation, less susceptible to noise. 
« Last Edit: 28/09/2010 03:38:22 by tommya300 »

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Offline tommya300

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More to add...
Chris, I think this will answer your question better...

AM signal amplitude is there half the time

[attachment=13001]


http://en.wikipedia.org/wiki/Wave

AM probagates using a ground plane and the ionisphere, an AM signal enters a tunnel or a sheilded steel frame it just will probagate to surround ground.
FM travel in a straight line, non dependent on ground conductivity, works its best when no obsticals are in its path, but it also reflects off of the ground or objects at some glazing angle.
Just as a light beam does.

So if there is a Leaky Coax type of antenna used, this would explain the AM signal loss.
I guess I never experienced going into a tunnel which was long enough to need a Leaky Coax fit, I never had lost FM reception that way.
« Last Edit: 28/09/2010 05:59:29 by tommya300 »

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Offline Geezer

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I did a bit of poking around, and it seems you have to inject signals into the cable with a transmitter for each station. Combining AM and FM on the same cable does seem to be a bit tricky, but not impossible.

So, it's possible the main reason for not getting MW stations in a tunnel might simply be because no one was willing to pony up the money for the necessary equipment for those stations. Presumably they concluded nobody actually listens to the MW stations while driving  [:D]
There ain'ta no sanity clause, and there ain'ta no centrifugal force Šther.

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Offline tommya300

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I guess so. Since they have cable everything these days and wifi. Buck talks a heck of allot.
Like all things have their limits.

FM will probagate into a tunnel by reflection of some glazing angle, and travel for a distance.

However, it depends on how close the towers are, the power of transmission, the altitude of the Transmission antenna and the path of the transmission, line of sight.
AM is ground conductive dependent, it will look like a short circuit to an AM wave form as the signal attempts to enter the tunnel.
Most likely if you are receiving a strong FM signal as you enter the tunnel, the signal will bounce through the tunnel for a good distance.
For this distance end, I think the FM signal will not fade, it may exhibit something like " Falling Off A Cliff"
 
« Last Edit: 28/09/2010 06:48:52 by tommya300 »

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Offline JP

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Higher the frequency permits the wave to pass through most solid structured matter, with less or no interference.

Isn't it usually the other way around?  Waves tend to go through/around things that are smaller than their wavelength without being disturbed.  Higher frequency means shorter wavelength, which means it would pass through less matter undisturbed.  Lower frequency means longer wavelength, which means it can pass through more matter.

This is why you can hear the bass beat of your noisy neighbor's stereo, but not the treble.  It's low frequency so it goes through the wall easily.

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Offline Geezer

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Why does medium wave radio cut out when going through a tunnel?
« Reply #10 on: 28/09/2010 07:16:55 »
Higher the frequency permits the wave to pass through most solid structured matter, with less or no interference.

Isn't it usually the other way around?  Waves tend to go through/around things that are smaller than their wavelength without being disturbed.  Higher frequency means shorter wavelength, which means it would pass through less matter undisturbed.  Lower frequency means longer wavelength, which means it can pass through more matter.

This is why you can hear the bass beat of your noisy neighbor's stereo, but not the treble.  It's low frequency so it goes through the wall easily.

I think that's right JP. Lower radio frequencies are absorbed less by matter.

Not sure if the audio analogy is quite right though. I think the reason you can hear the bass is because it has so much more power than the higher audio frequencies that it causes your walls to act as a secondary speaker rather than actually passing through the wall. 
There ain'ta no sanity clause, and there ain'ta no centrifugal force Šther.

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Offline tommya300

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Why does medium wave radio cut out when going through a tunnel?
« Reply #11 on: 28/09/2010 07:21:58 »
Higher the frequency permits the wave to pass through most solid structured matter, with less or no interference.

Isn't it usually the other way around?  Waves tend to go through/around things that are smaller than their wavelength without being disturbed.  Higher frequency means shorter wavelength, which means it would pass through less matter undisturbed.  Lower frequency means longer wavelength, which means it can pass through more matter.

This is why you can hear the bass beat of your noisy neighbor's stereo, but not the treble.  It's low frequency so it goes through the wall easily.

Am I confusing this with XRAY frequency? Short wave length hi frequency hi energy wave form, passes through most solid matter?

Ah acoustic percussion beats the walls down hear its echo, shiver me timbers, before passing through...
« Last Edit: 28/09/2010 07:36:54 by tommya300 »

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Offline JP

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Why does medium wave radio cut out when going through a tunnel?
« Reply #12 on: 28/09/2010 07:54:31 »
Not sure if the audio analogy is quite right though. I think the reason you can hear the bass is because it has so much more power than the higher audio frequencies that it causes your walls to act as a secondary speaker rather than actually passing through the wall. 

You may be right.  That's the explanation I recall hearing back in my undergraduate days, but I'm not sure it holds up since sound needs a vibrating medium in order to propagate whereas light doesn't.

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Offline tommya300

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Why does medium wave radio cut out when going through a tunnel?
« Reply #13 on: 28/09/2010 08:44:52 »
Resonance is resonance everything has a frequency...
Frequency spectrum's and antenna responces are from the same formula using different values.
λ lambda
F frequency
V velocity can be the speed of sound
                   or speed of light
                   or a user defined for the best response
                   on a power curve, auto exhaust chamber back pressure related to RPM
                                                                                 (tuned Exhaust)

Speaker cabnet dimensions Volume area inside it, cutoff filter speaker protection
Tuned Headers on an auto mobile top end response low end response
Antenna resonant response
Remember the center frequency upper and lower band pass or the full bell curve 6db per octave 10 db per decade
Just because the media is changed V=λF adapts to all full spectrum

I realize of course sound in a vacuum is obvious.
I should not have said sound passes through...
Sound does not pass through it bangs into the medium and the atomic structure of the medium propagates the pressure like a spring bouncing back and forth and also robs some of the energy during the transition.
That is why it is less loud with the windows and doors are closed.

What makes sound proof sponge sound proof? The reduction or lack of this energy transferring pressure the material absorbs the percussion.
What about anti sound. DSP detects and duplicates the sound and inverts it and transmits it The problem with this is from the time of detection, process time to the output propagation is phase shifted.
A known repetitive value need to be stored to provide anti noised.

Filtering light wave uses different optic medium for a response,  Radio waves tank circuits LRC, Sound use material with shape and dimension and acoustic properties to get specific responses, a Stradivarius for instance
« Last Edit: 28/09/2010 09:45:56 by tommya300 »

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Offline chris

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Why does medium wave radio cut out when going through a tunnel?
« Reply #14 on: 28/09/2010 09:28:26 »
Hi JP

I think that the reason that the bass propagates from next door (or a passing car) is because long wavelengths (like bass sounds) tend to diffract better than short wavelengths, so the sound will find its way into your house more easily. But more importantly, higher frequencies are more rapidly attenuated in air; the higher frequency means that more air is being moved about per wave cycle and hence the losses are greater. This is why foghorns use bass (low pitch) sounds.

Returning to the radio wave question; you say that waves go around things that are smaller than their wavelength - but this isn't true of the grille on the door of a microwave oven is it? The microwaves are trapped within the oven cavity, but the visible light (from the interior lamp) comes out just fine...am I missing something here?

chris
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Offline daveshorts

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Why does medium wave radio cut out when going through a tunnel?
« Reply #15 on: 28/09/2010 10:19:45 »
In a grill the important scale is the length of the wires, which make it up. These are much longer than the microwave wavelength. In the same way that a long wavelength e/m wave doesn't see a small solid object, it also doesn't see a small hole, so the grill is apparently solid to the microwaves.

To the very short wavelength light, the grill looks like a grill, the diffraction effect is of order a micron, so the grill looks like a grill, light goes through the holes but not the wires.

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Offline syhprum

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Why does medium wave radio cut out when going through a tunnel?
« Reply #16 on: 28/09/2010 12:56:29 »
When I worked in central London I used to drive home via the Blackwall tunnel which in those days had no 'leaky coax' system but rather smooth reflective walls.
I found this was a useful place to test the efficiency of various car radio antenna systems.
with my Ford I could get about 200m into the tunnel before I lost the signal but when I upgraded to a BMW it was nearer 400m, I think the reason why MF reception is not provided is because of the high level of interference generated at low frequencies by the fluorescent lighting and often car electrical systems.
 
syhprum

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Offline tommya300

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Why does medium wave radio cut out when going through a tunnel?
« Reply #17 on: 28/09/2010 17:48:25 »
When I worked in central London I used to drive home via the Blackwall tunnel which in those days had no 'leaky coax' system but rather smooth reflective walls.
I found this was a useful place to test the efficiency of various car radio antenna systems.
with my Ford I could get about 200m into the tunnel before I lost the signal but when I upgraded to a BMW it was nearer 400m, I think the reason why MF reception is not provided is because of the high level of interference generated at low frequencies by the fluorescent lighting and often car electrical systems.
 
Yes and some ignition systems not all also contributed to noise.
For you, did the FM signal just cut out or did you experience a fade?
Ford? Gas prices... did you own a gas station or was it a ford semi with a fuel tanker hitched?
 [;D] [:D] [::)] [?]

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Offline Geezer

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Why does medium wave radio cut out when going through a tunnel?
« Reply #18 on: 28/09/2010 18:58:00 »
There's a new topic on the LF sound question here

http://www.thenakedscientists.com/forum/index.php?topic=34209.new
There ain'ta no sanity clause, and there ain'ta no centrifugal force Šther.

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Offline syhprum

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Why does medium wave radio cut out when going through a tunnel?
« Reply #19 on: 28/09/2010 19:29:22 »
The Ford was just a 2 liter not a gas guzzler, the FM held up for about 200m then flutered out.
syhprum

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Offline Geezer

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Why does medium wave radio cut out when going through a tunnel?
« Reply #20 on: 28/09/2010 22:39:40 »
The Ford was just a 2 liter not a gas guzzler, the FM held up for about 200m then flutered out.

That reminds me of a story about a 2 liter Jaguar, but it's not very appropriate.

I did dig up something about MW transmission in some of the tunnels on the East Coast. I'll try to find it again.
There ain'ta no sanity clause, and there ain'ta no centrifugal force Šther.

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Offline chris

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Why does medium wave radio cut out when going through a tunnel?
« Reply #21 on: 28/09/2010 22:40:03 »
In a grill the important scale is the length of the wires, which make it up. These are much longer than the microwave wavelength. In the same way that a long wavelength e/m wave doesn't see a small solid object, it also doesn't see a small hole, so the grill is apparently solid to the microwaves.

To the very short wavelength light, the grill looks like a grill, the diffraction effect is of order a micron, so the grill looks like a grill, light goes through the holes but not the wires.

So, a very small piece of grille wouldn't work, even against a focused microwave beam, because the length of the wires would be short?

Chris
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Offline Geezer

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Why does medium wave radio cut out when going through a tunnel?
« Reply #22 on: 28/09/2010 23:48:37 »
Here is the bit about AM/FM. I'm afraid it's all a bit anecdotal.

"They also wanted to put in AM/FM radio over the airways and rebroadcast in the tunnels, so we had to provide for all four of those," Chen said. "Any time you have four different RF sources, the chances of having co-channel interference is higher."

To avert problems, the carrier created specially made cross-band couplers to eliminate interference. But the challenges did not end there. Chen said AT&T also had to adhere to power limitations. If it put more power into the system than Bell Atlantic or the Port Authority did, it would create interference."

From: http://connectedplanetonline.com/wireless/mag/wireless_trouble_tunnels/

There ain'ta no sanity clause, and there ain'ta no centrifugal force Šther.

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Offline mcjhn

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Why does medium wave radio cut out when going through a tunnel?
« Reply #23 on: 29/09/2010 00:41:00 »
is it simply because the wavelength of FM matches the entrance of the tunnels so it diffracts/bends around corners better?

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Offline tommya300

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Why does medium wave radio cut out when going through a tunnel?
« Reply #24 on: 29/09/2010 05:28:17 »
In a grill the important scale is the length of the wires, which make it up. These are much longer than the microwave wavelength. In the same way that a long wavelength e/m wave doesn't see a small solid object, it also doesn't see a small hole, so the grill is apparently solid to the microwaves.

To the very short wavelength light, the grill looks like a grill, the diffraction effect is of order a micron, so the grill looks like a grill, light goes through the holes but not the wires.

The screen on a microwave door is not desigened to be frequency dependent...

The concept of a Faraday cage is logically attributed to Michael Faraday, an 19th Century pioneer in the field of electromagnetic energy.
A microwave oven's door has a screen which prevents electromagnetic energy from escaping into the room.

http://www.wisegeek.com/what-is-a-faraday-cage.htm

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Offline tommya300

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Why does medium wave radio cut out when going through a tunnel?
« Reply #25 on: 29/09/2010 05:37:21 »
is it simply because the wavelength of FM matches the entrance of the tunnels so it diffracts/bends around corners better?
I am having a bit of difficulty trying to verify this.
Can you help me find any creditable information to back this up?
Please display a URL that will lead me to this, highly curious
I thought I seen something about 5 meters referring to what you are saying, but I can not seem to find it again
« Last Edit: 29/09/2010 05:41:31 by tommya300 »

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Offline tommya300

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Why does medium wave radio cut out when going through a tunnel?
« Reply #26 on: 29/09/2010 06:07:51 »
Here is the bit about AM/FM. I'm afraid it's all a bit anecdotal.

"They also wanted to put in AM/FM radio over the airways and rebroadcast in the tunnels, so we had to provide for all four of those," Chen said. "Any time you have four different RF sources, the chances of having co-channel interference is higher."

To avert problems, the carrier created specially made cross-band couplers to eliminate interference. But the challenges did not end there. Chen said AT&T also had to adhere to power limitations. If it put more power into the system than Bell Atlantic or the Port Authority did, it would create interference."

From: http://connectedplanetonline.com/wireless/mag/wireless_trouble_tunnels/

The artical covers real issues in multiple close communication systems.
 RF Cross talk, maintenance, population of equipment, real estate.

Leaky Coax usage attempts on Mining tunnels safety regulations

Here is some backup for you Geezer...
http://www.cdc.gov/NIOSH/Mining/pubs/pdfs/j0199048.pdf
« Last Edit: 29/09/2010 06:15:32 by tommya300 »

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Offline mcjhn

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Why does medium wave radio cut out when going through a tunnel?
« Reply #27 on: 29/09/2010 19:08:21 »
is it simply because the wavelength of FM matches the entrance of the tunnels so it diffracts/bends around corners better?
I am having a bit of difficulty trying to verify this.
Can you help me find any creditable information to back this up?
Please display a URL that will lead me to this, highly curious
I thought I seen something about 5 meters referring to what you are saying, but I can not seem to find it again

sorry no data to back it up, it was a v.short answer (with no more explanation than that) from Dr Karl on a Australian science phone in program to the same question
http://www.drkarl.com/home/

(N.B. not Dr Karl Kennedy from neighbours)

but you get the same effect in multistory car parks (FM better than AM that is)
« Last Edit: 29/09/2010 19:09:59 by mcjhn »