how far would a 7-watt FM signal go into a long wire?

[fmuser877]

So if you had a 7-watt FM transmitter in a long tunnel how far would it go?
would the best way of doing this be a very long wire?
I notice some times radio 1 goes perfectly in tunnels.
what do they use?

[outis]

So if you had a 7-watt FM transmitter in a long tunnel how far would it go?
would the best way of doing this be a very long wire?
I notice some times radio 1 goes perfectly in tunnels.
what do they use?

If you had the transmitter inside the tunnel and it was straight the signal will go quite far. But if there are turns... it wouldn't. Strong FM stations might go through tunnels depending how the signal arrives at the entrance, how deep down it goes, the supporting structure and turns.

[yellowbeard]

In tunnels and mines they use "leaky feeders" to carry radio stations and walkie talkie RF. You could think of this system as long run of shite coax with a receive antenna and amplifier on one end and a 50 Ohm resistor on the other. A little bit of signal gets out of the coax as it goes along and walkie talkies can get into it. If a tunnel has this system for radio stations then there will be an FM receive antenna somewhere at one end of it, probably a yagi pointed at the nearest big BBC site. This might be useless information - depends what you're REALLY up to...
https://en.wikipedia.org/wiki/Leaky_feeder

[Albert H]

I did a couple of tunnel systems. They can work in two ways:

1. The (notionally) simpler system - a broadband receiving aerial in the vicinity of one of the portals of the tunnel, receiving everything it can "hear". This is then fed to a broadband amplifier, which drives a "replica" of the off-air signal down the tunnel. Care has to be taken with phasing , and to make sure the outdoor aerial can't "hear" the Leaky Feeder, to prevent the whole thing turning into a broadband noisy oscillator!

2. A rack of receivers collect the off-air signals in the vicinity of one of the portals of the tunnel, and the demodulated audio (or in the case of stereo FM, the whole baseband) is fed to a small transmitter. The transmitter feeds the Leaky Feeder through a combiner, that mixes all the signals together. This approach can even be used for DAB, and with the correct synchronisation, drivers using the tunnel won't hear any discontinuity in the signal.

Each system has disadvantages - the "power oscillator" problem of the first option makes it difficult to install and align, and you can carry services that you might not want to convey! The second option requires a lot of hardware, and the RF combiner feeding the Leaky Feeder can be tricky to align, to prevent mixing and cross-modulation problems.

There are further problems within the tunnel, too:

The signal distribution along the feeder won't be even, and if it's a longer tunnel, you might have to put "repeater" amplifiers part way along. There are also distinct nodes and anti-nodes in the field strength - related to the wavelength of each signal, of course. The atmosphere inside a road tunnel is pretty filthy, and corrosive. All the connectors will require regular maintenance, and repeated regular radio surveys need to be done (using calibrated test receivers in a moving vehicle and a chart recorder) to make sure that everything's working as it should.

Finally, there's the issue of tunnel fires. Vehicles have a tendency to spontaneously catch fire (particularly some of those dreadful electric efforts), and the Leaky Feeder is vulnerable to heat damage. After a fire, the Feeder is always replaced!

The structure of the Leaky Feeder cable is just like your regular coax, but there are slots cut into the braid, so that some of the RF can leak out.

The most extensive road tunnel Leaky Feeder systems I've done were in Scandinavia - they have plenty of tunnels, and most of them have comprehensive radio re-broadcast systems.

[SamTheDog]

Leaky feeders are definately the way to go for tunnels and other underground locations. (with the possible exception of caving / cave rescue scenarios).
We did emergency-services kit in london. Blackwall for instance had radio kit in the control room at the north end of the tunnel
and fed the tunnels. At the tunnel ends we had 5-tone radios in waterproof ABS boxes programmed so acknowledge the call from base to they could be "pinged" to see if the system was working.
during emergencies underground ( underground/tube train incidents etc..) one of the two emergency control vehicles would be sent out and the engineer on duty that day would have to run coils of cable down from the control vehicle. We used lossy RG58.
This was for the London-Ambulance VHF radio scheme before they went over to the hopeless trunked system.

[SamTheDog]

does anyone know if university sites are still broadcasting using leaky feeders?
They used to run mediumwave using a low power TX into feeder to cover the university / campus.
often (from memory) on 999kHz or thereabouts.

Not sure if any of this is still running or if any of it ever went over to Band II.

[outis]

does anyone know if university sites are still broadcasting using leaky feeders?
They used to run mediumwave using a low power TX into feeder to cover the university / campus.
often (from memory) on 999kHz or thereabouts.

Not sure if any of this is still running or if any of it ever went over to Band II.

Carrier Current MW stations? I am not sure I have ever heard it for the FM. I do have a schematic for the LW (150KHz)
There is some discussion on a thread I opened: viewtopic.php?t=3532&start=20

[BriansBrain]

My Radio Brian is only 2.6km away but with a hilly terrain before the
entrance of the first tunnel which is 350m long the second is 425m.

The reception is nearly perfect in both.

Brian to Tunnels.JPG

First Tunnel.JPG

Second Tunnel.JPG

[BriansBrain]

Terrain from Brian to the first tunnel.

Terrain.JPG

[fmuser877]

do you get other stations in the tunnel did you put a leaky feeder in?

[BriansBrain]

do you get other stations in the tunnel did you put a leaky feeder in?

Other stations = No Idea

I put nothing in

[Albert H]

There may be a "dumb" system in the tunnels that "Brian" gets into - anything on Band II will get sent down the feeder!

[BriansBrain]

There may be a "dumb" system in the tunnels that "Brian" gets into - anything on Band II will get sent down the feeder!

I will investigate to see if any other stations can be received

[87to108]

I did a couple of tunnel systems. They can work in two ways:

1. The (notionally) simpler system - a broadband receiving aerial in the vicinity of one of the portals of the tunnel, receiving everything it can "hear". This is then fed to a broadband amplifier, which drives a "replica" of the off-air signal down the tunnel. Care has to be taken with phasing , and to make sure the outdoor aerial can't "hear" the Leaky Feeder, to prevent the whole thing turning into a broadband noisy oscillator!

2. A rack of receivers collect the off-air signals in the vicinity of one of the portals of the tunnel, and the demodulated audio (or in the case of stereo FM, the whole baseband) is fed to a small transmitter. The transmitter feeds the Leaky Feeder through a combiner, that mixes all the signals together. This approach can even be used for DAB, and with the correct synchronisation, drivers using the tunnel won't hear any discontinuity in the signal.

The ones i know of downconvert each desired station to IF frequency (probably the former standard 10.7MHz), filters it, and back up to the original frequency. It ends up back on its exact original frequency so no beating effects from any slight difference. Each station would be leveled in strength. There being NO demodulation minimises interference effects when there is a mixture of both direct and tunnel signal after entering and just before exiting .

I'd expect modern DSP receivers ( if used for 2 above) would lead to unpleasant effects due to their inherent (slight) processing delay, if these were receiving and demodulating the signal and re-transmitting on the original frequency, for the duration of a mixture of both signals.

[radionortheast]

does anyone know if university sites are still broadcasting using leaky feeders?
They used to run mediumwave using a low power TX into feeder to cover the university / campus.
often (from memory) on 999kHz or thereabouts.

Not sure if any of this is still running or if any of it ever went over to Band II.

Carrier Current MW stations? I am not sure I have ever heard it for the FM. I do have a schematic for the LW (150KHz)
There is some discussion on a thread I opened: viewtopic.php?t=3532&start=20

Carrier current using the mains as an aerial at fm frequencies doesn’t work, would be nice if it could you wouldn't need an aerial , earth goes round a house, most wiring is live and neutral, the signal will be raidated by the lead that gose into the back of the transmitter, I tried and thats what happened. At medium wave frequencies it dosen’t mind long leads so the signal tends to go out.

[Albert H]

When you consider the capacitance between the wires in your "twin & earth" ring main, it will have much less effect at lower frequencies (like MW and LW), and I've linked stations from their studio to their tower block roof using 140 kHz FM down the mains when we needed to get it done in a hurry! If you want a circuit for a simple ~100 kHz FM mains link, I'll put one up here.

At VHF frequencies, the loss in mains wiring will be extreme. It's not the way to get any useful radiation.

However, it's possible to improvise aerials out of almost anything: One block I installed on had a scaffolding structure down one side, with the tops of the uprights poking up above the roof. Adding a couple of nylon clamps and jubilee clips, I was able to improvise a J-pole that matched pretty well. We used successfully it for a weekend, and took the gear down off the roof (as usual) early on the Monday morning. The next week, the scaffolding had gone, and we had to go back to clamping a groundplane aerial to the top of the communal mast!

A friend of mine loaded a rig into a metal window frame in a wooden hut on top of a hill! It worked reasonably well for a festival station over a long weekend.