[Geezer]

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.

[chris (OP)]

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

[Geezer]

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/

[mcjhn]

is it simply because the wavelength of FM matches the entrance of the tunnels so it diffracts/bends around corners better?

[tommya300]

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

[tommya300]

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

[tommya300]

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

[mcjhn]

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/ [nofollow]

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

but you get the same effect in multistory car parks (FM better than AM that is)