HFU HF Underground
Technical Topics => Part 15 AM and FM Station Operation => Topic started by: ThaDood on April 29, 2025, 1546 UTC
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https://www.radioworld.com/columns-and-views/tunnel-radio-how-signals-reach-drivers-underground We don't hear much about Leaky-Cable Broadcasting. Heck, who still makes cables that could even do that, Belden? Still, a permissible way to broadcast license-free. (Thank you Boomer!) Oh BTW, this also works in mines.
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That's a good article, I hadn't seen it yet. Glad you posted it. done a lot of research on that myself. I'm pretty sure I've got newspaper clippings of everyone of those installations he talked about.
Incendently, that first one way back in1939 at George Washingston Bridge, the equipment went into storage after the closing of the 1940 New York Worlds Fair,.. And it was that exact same equipment Halstead used on the bridge that was transferred to the Lincoln Tunnel 11 years later - with exception to the original wire recorder/player that had fed the audio .. by that time technology had improved.
I had always found it peculiar that the FCC found it neccessary to specify that "oil storage tanks are not tunnels" as found in FCC OET BULLETIN NO. 63 (October 1993)
http://transition.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet63/oet63rev.pdf
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Tunnel radio systems
Many tunnels have naturally surrounding earth and/or water that attenuates radio waves. Transmitters that are operated inside these tunnels are not subject to any radiation limits inside the tunnel. Instead, the signals they produce must meet the Part 15 general radiated emmission limits on the outside of the tunnel, including it's openings. They also must comply with the conducted emission limits on the electric power lines outside of the tunnel.
Buildings and other structures that are not surrounded by earth or water (e.g oil storage tanks) are not tunnels. Transmitters that are operated inside such structures are subject to the same standards as transmitters operated in a open area.
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There's another interesting thing found in their chart on page 5 (which I just noticed): Authorization Procedures for Part 15 Transmitters, showing that part 15 transmitters used for tunnel radio, campus stations, other AM carrier-current, perimeter protection systems, and systems at or below 490 kHz , all require "Verification"[/b]
Leaky coaxial cable systems also require only "Verification", however, that only "for operation exclusively in the AM broadcast band". If used in any other frequency then it needs "Certification"
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"All other Part 15 transmitters" require "Certification"[/b[
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We all know about Certification and Compliance of part 15 transmitters, but I cant say I've ever heard of Verified Part 15 transmitters befote.
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Your post had got me thinking about it again and something I've always been puzzled over was the installation cost which had ranged between $150,000 and $200,00 per instalation.. Your talking about a broadcast of a low power AM signal inside an enclosed space -- WHY SO EXPENSIVE??..
Today (or even then) you could remove the crystal out of something like a Rangemaster install it in the middle of the tunnel and you'd have a compliant AM signal that broadcast across the entire band as effectively as those $200,000 leaky cable systems did when they were more common! - no?
But here's something interesting I previously missed.. Evidently there were two companies going by the name "Tunnel Radio of America" - The still existing is interestingly a "Women-Owned Small Business"
So, there was the original one:
● Broadcasting Sep 27 1982 (page 41)
"...Tunnel Radio of America, Hollywood, Fla., whose president, Rodger Skinner, first developed a transmitter which, through a unique filtering and tuning process, can broadcast one signal simultaneously on 24 AM radio frequencies for reception by com- muters traveling through tunnels, is franchisng its technology in major markets around the country and abroad. Skinner developed the transmitter in 1975... The start -up costs on the average range between $150,000 and $200,00.."
And then the existing women-owned one in Oregon:
https://miningdirectory.org/listing/tunnel-radio-of-america.html
● Tunnel Radio of America is an ISO-9001 certified manufacturer and Women-Owned Small Business, headquartered in Corvallis, Oregon. For 35 years, Tunnel Radio has distinguished itself as the trusted designer, manufacturer and installer of specialized radiating-cable antenna systems (“leaky-feeder”) for underground or unreachable areas... ...For more information, visit www.tunnelradio."
Of note, there was also a company called "Tunnel Radio USA" in the 1970s.. I have to look but I think that may have actually been the same guy as Tunnel Radio USA was (Skinner)
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Roger Skinner??? The same Roger Skinner that was in with the Pat Murphy interview, with Art Bell, in 1998? https://archive.org/details/pat-murphy-roger-skinner-interview-on-art-bell-3-1998-edited If that's so, the German Laugh In soldier says, "Very interesting..." Oh... As part of why the cost of a Tunnel Radio would be so much, I would suspect that the specific 'Leaky Coax Cable' can't be that cheap, due to lack of demand for it, except for that special purpose. I am speculating here, but that is part of it. That, and for a long tunnel, you'd probably would be using several watts, like 10W, or more, and not 100mW. So, an FCC Compliant AM transmitter, with exciter power from the 10W - 20W range, would be perfect for that. Now, could you shave the cost a lot there? Sure, buy used gear. For the Leaky Cables, you could actually roll your own by reversing the Outer Braid and Inner Center Conductor, and still have a terminated resistor at the end. I believe that Ernie Wilson's Carrier Current Techniques book shows that as an option. Anyway... My $0.02 worth, again.
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Yeah, the leaky cable systems generally involves 10- 100w inputs, but what I'm saying is I dont see why a standard 100mw whip and mast type transmitter could acheive the same thing at much less cost and power. And remember there are no field strength limits involved with part 15 inside a tunnel either. It seems to me most any free-radiate part 15 signal could saturate the tunnel, and if not, simply running a long lead lengthwise from the transmitter would do it.
As for Skinner. (I'm going to listen to your link)... I don't know if it's the same guy as was on Art Bell or not, but it would sure be interesting if it were!
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I doubt that a Free Radiator, i.e. antenna Part 15 10ft antenna, would be effective in a tunnel situation. Ever go in a tunnel, or under a bridge, and totally lose an AM station? Same deal, wave length-wise, a tunnel is too small / narrow. It attenuates Medium Wave FREQ's, like a notch filter. Now, if you were in a huge cavern, like Mammoth Caves, then the 100mW / 10ft antenna on the AM Band could work. Actually, in a tunnel, Part 15 FM limits would be more effective, with the 3M wave length could use a tunnel like a wave guide. Now, why Carrier-Current, or even Leaky Cable, can be more effective is that it just needs to radiate like 20-some feet away to the oncoming traffic in each lane. So, a little of RF Theory & Physics involved here. Now, in a long tunnel, UHF would rock, like with 1/2W FRS radios, where UHF would really benefit from the wave guide effect, with such a small wave length. Does any of that make sense to you?
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I know your a lot more knowledgeable and experienced about such technicalities, but I see no reason why a part15AM transmitter signal could not fill the interior of the average roadway tunnel, especially when it's not confined by feild strength and antenna/lead length.
I realize it cant penetrate the tunnel and that's actually part of my point.. Bear with me a moment here. I think it could effectively work with a Ramgemaster, Procaster or a Talking House inside a tunnel.. Hear me out and tell me where I'm wrong..
Install it in the middle of tunnel with a whip (or maybe even not), what really matters is it's ground leads.. come off the ground lug with two leads leading horizontally towards the two exits of the tunnel (there are no part 15 feild strength limits or ground lead lengths to deal with inside a tunnel) -- all thats required for compliance is that the signal strength outside the tunnel ). So with the xmtr installed inside wouldn't the AM signal essentially fill that tunnel expecially with its ground lead radiating the most length of that tunnel too?
Also, since tunnel radio typically broadcast across the entire AM band (or fm), simply remove the crystal from the Rangemaster for the same effect.
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Actually, in a tunnel, Part 15 FM limits would be more effective, with the 3M wave length could use a tunnel like a wave guide.
You know Halstead, inspired by the newly created part 15 rules, invented the leaky cable system back in 1939, he went AM because that was what receivers cars had. but for many years he promoted FM leaky cable use for many applications.. but 30 years later it was his direct involvement that resulted as the laky cable franchise Cinema Radio for drive-ins in 1971 (although similiar systems came out in the mid 1960s)
And you mentioned Mammoth Caves.. which did have a part 15 AM system within it for many years.. I'm pretty sure it was free-radiate, but I need to check to be certain, it might have been cable..
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....wave length-wise, a tunnel is too small / narrow. It attenuates Medium Wave FREQ's, like a notch filter. ..... Now, why Carrier-Current, or even Leaky Cable, can be more effective is that it just needs to radiate like 20-some feet away ...., UHF would really benefit from the wave guide effect, with such a small wave length. Does any of that make sense to you?
I get where your coming from.. basically, but I'm proposing that despite the legistics of AM wavelengths being confined by the height of the tunnels interior, there's golng to be enough near feild strength going on from the whip and leads to saturate a half mile tunnel.
I don't see how it couldn't.
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Your post had got me thinking about it again and something I've always been puzzled over was the installation cost which had ranged between $150,000 and $200,00 per instalation.. Your talking about a broadcast of a low power AM signal inside an enclosed space -- WHY SO EXPENSIVE??..
Today (or even then) you could remove the crystal out of something like a Rangemaster install it in the middle of the tunnel and you'd have a compliant AM signal that broadcast across the entire band as effectively as those $200,000 leaky cable systems did when they were more common! - no?
But here's something interesting I previously missed.. Evidently there were two companies going by the name "Tunnel Radio of America" - The still existing is interestingly a "Women-Owned Small Business"
My guess is the high prices are the combination of a very small market along with those making the purchasing decision spending other peoples' money.
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Chris is thinking where I am in the price of a Tunnel Radio setup. However, to answer Tybee's question upon whether an intentional radiator Part 15 AM System, (100mW & 10ft antenna.), would provide enough field strength to be feasible in a fairly narrow underground traffic tunnel? My honest answer there is, 'Je ne sais pas...' , since I really don't know the real answer to this. Now, by physics, mathematics, and general radio wave length theory, tells me that the answer make me swing to 'NO'. That said, this is an experiment that I have never done myself to see if it's true, but it indeed would be a neat one to try. Boomer, brought up this scenario to me. Putting a compromised 10ft antenna underground, in a narrow tunnel, might be harder to tune to, since the close-by walls of the tunnel may have a capacitive effect with the antenna. As it is, Monday night I retuned the ATU to my AM1620 antenna, since the close-by apple tree now has full foliage, and adds to a capacitive effect of that 10ft radiator. Not a lot, but enough that I do see a difference on that ATU peak meter. So, that's where I am coming from, and Boomer to. So, if you have a means to take a Part 15 100mW & 10ft antenna setup to the middle of an underground tunnel, to see what would happen. By all mean, please let us know what the results are, as it's a test that I suspect that has never been done.
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My guess is the high prices are the combination of a very small market along with those making the purchasing decision spending other peoples' money.
That was specifically the cost estimates of tunnel radio installations. A part 15 AM leaky cable installation along highways was approximately $10,000 per mile (in the 1970s), but it had only cost about $1,500 a mile for "zones" of synchronized free-radiate part 15 AM transmitter chain installations.
] a compromised 10ft antenna underground, in a narrow tunnel, might be harder to tune to, since the close-by walls of the tunnel may have a capacitive effect with the antenna... As it is, Monday night I retuned the ATU to my AM1620 antenna, since the close-by apple tree now has full foliage, and adds to a capacitive effect of that 10ft radiator. Not a lot, but enough that I do see a difference on that ATU peak meter. .... By all mean, please let us know what the results are, as it's a test that I suspect that has never been done.
Harder to tune... Would you even have to tune it? Tunnel radios work across the band, on all frequencies simultaneously.. Without the crystal in it, what is there to tune? The signal just supposed to splatter the interior of the cave. I ain't got no tunnel to try in.
Your comment about broadcasting 1620 from the ATU makes me wonder what range your getting and have you ever used 1700 and how it compares real world to 1620, Roy o er at part15.us moved down to 1620 or 1630 too and seems to indicate the differences in range as nominal...
I'm also curious what you saying about the capacitance to your apple tree in bloom and it increasing your range --- what? Can a nearby tree really have a observable effect?? What's more, I thought tree foliage actually tends to attune your signal thus decreasing its range.. so... I'm puzzled.
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OK... I'll see if I can explain this one. What I mean by tuning is the circuit to tune the compromise of a very short antenna's, very high, impedance. That impedance is not a low one either, but like somewhere around 200k-Ohms, and not 50, or 75 Ohms. As a result, the RF voltage on such a compromised antenna is much, much higher. Going into the Antenna Tuning Unit, (ATU), the RF volts RMS is like around 1V, but RF OUT of the ATU the RF voltage is like over 40V RMS. A low impedance antenna, at MW FREQ's may not see reflections, or absorption, from close-by trees, but having a high impedance antenna does. My close-by apple tree, with full foliage, has something in those leaves, water. So, there's a capacitive effect between my AM1620 antenna, mounted at the apex of a garage, and a close-by apple tree. What do those leaves do? Slightly, lower the impedance of that compromised short antenna. So, I can tweak a sliding inductor, in my ATU, and compensate for the tree's added, near-by, capacitance effect. In late October, I retune the antenna's ATU again, with no leaves on the tree. BTW, when you get any antenna, that comes with matching SWR lowering instructions, what do the directions almost always recommend? Tune your antenna in a wide, open, space, away from objects, buildings, and trees. If you don't, most likely, the antenna will detune in a more open space. Especially, tuning antennas on mobile installations. Now, do I notice a difference in range? Out in my fringes, yes. Not a lot, by any means, but at Part 15 levels, I want to permissibly squeak out as many microwatts of ERP as possible. So, can you imagine the capacitive effects of putting such an antenna setup in a narrow tunnel would do? Now, why am I using 1620kHz, and not 1700kHz? That would actually, make Part 15 MW broadcasting a bit more efficient, but 1620kHz is a compromise between older radios and newer ones. A boombox, portable, table radio, etc., made before 1990 likely can tune 1620kHz, but not higher than that. There's still a lot of older AM radios out there. So, yet another compromise, between reception range and radios' accessibility. That, and 1620kHz is a free FREQ here in the daytime. Was so, in 2010, when I've decided upon that, and it still is. Oh BTW, having a well matched antenna system has this benefit, lowering spurs and harmonic emissions.
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Check this out, what a coincidence, wasn't even looking and just happened to see it..
TUNNEL RADIO MODEL TR-PS-28A WIRLESS RADIO, VAC AND BATTERY OPERATED, NO MIC
https://www.ebay.com/itm/224330410702
$135.96
What strikes me is that he calls it "wireless", not sure where he got that from
(https://i.ebayimg.com/images/g/0v0AAOSwdhJgEZ5C/s-l1200.webp)
There plenty more pictures but neither
them nor the description tells me nothing other than the battery works and I found no other info on it elsewhere. Any idea exactly what this is?:
The inside just shows volt and amp meters and connectors to load and battery. The encloser label offers a clue that it's not terribly old because they have a website, I'm guessing 1990s maybe:
(https://i.ebayimg.com/images/g/-SMAAOSwDYJgEZ5C/s-l1200.webp)
Hoffmen Enclosures Inc.
2100 Hoffman Way Anoke, MN
A Pentair Company
www.hofimenonline.com
Enclosure Type 12,18
Made in Mexico
"TUNNEL RADIO MODEL TR-PS-28A WIRLESS RADIO, VAC AND BATTERY OPERATED, BATTERY WORKS, NO MIC , USED
***Item may be dusty, and have minor scratches from being in storage.***"
(https://i.ebayimg.com/images/g/5vwAAOSwZiFgEZ5C/s-l1200.webp)
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Oh! and one more thing.. or two.. An interesting thing I came across while searching for the above transmitter is that Tunnel Radio AM/FM appears to be alive and well live in Japan!
https://www.jrc.co.jp/en/product/tunnel
(https://www.jrc.co.jp/hs-fs/hubfs/jrc-corp/assets/images/en/product/img-product-tunnel01.gif?width=750&height=525&name=img-product-tunnel01.gif)
and here closer to home is another tunnel radio supplier that I'm not sure I've seen before, but they only do FM, but it's nice to see additional distributers:
https://onair.com.tr/en/page/1033/tunnel-radio-broadcast-and-announcement-system
There's actually -perhaps a thousand - Tunnel Radio systems currently in operation, but only a tiny percentage of those are for the public reception, and those mostly broadcast FM but there are still a few broadcasting AM today.
Oh.. Almost forgot, it never occurred to me until today.. It's when I saw mention that the US/CANADA border Windsor Tunnel had been the first International tunnel ever built..
The First International Part 15 AM Station in history! (maybe the only one)
Well, Windsor Tunnell is also the location of the first (and maybe the only) International Part 15 AM Tunnel Radio Station in history!
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Seems it may be more complicated than a simple transmitter rebroadcasting outside programming to a leaky coax...
Found this patent. Since there would be emergency equipment needed to alert drivers that disrupt usual programming
I can see how it has a high cost of instillation. The need to have repeaters along the length of a tunnel and the end of
tunnel transmitting equipment.
https://patents.google.com/patent/US20070182585
As for transmitting various programming across the AM/FM broadcast band I'd imagine that tunnel radio box is some
sort of distribution transformer for the leaky coax.
Would be interesting to find out exactly how it's setup but I can't find much more information online about it.
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From where did you get the patent number? .. I dont see it in the description nor pictures. Maybe you contacted him and asked??,
I had contacted him yesterday and asked what made him call it a "Wireless transmitter", but havent heard back yet.
I notice some of his auctions appear to have additional parts and accessories to this system.
I wonder what tunnel it had come from?
pl
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From where did you get the patent number? .. I dont see it in the description nor pictures. Maybe you contacted him and asked??,
I had contacted him yesterday and asked what made him call it a "Wireless transmitter", but havent heard back yet.
I notice some of his auctions appear to have additional parts and accessories to this system.
I wonder what tunnel it had come from?
pl
It's a random one I found using a search engine to find any patents similar to the topic. No idea if it's the same thing but
reads similar in the description of it.
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Check this out, what a coincidence, wasn't even looking and just happened to see it..
TUNNEL RADIO MODEL TR-PS-28A WIRLESS RADIO, VAC AND BATTERY OPERATED, NO MIC
https://www.ebay.com/itm/224330410702
$135.96
What strikes me is that he calls it "wireless", not sure where he got that from
There plenty more pictures but neither
them nor the description tells me nothing other than the battery works and I found no other info on it elsewhere. Any idea exactly what this is?:
This looks to be just a power supply and a battery. I imagine the transmitter was in a separate box….which is where the left hand cable went.
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Well, as long as we're kickin' this subject around, Boomer came across this. https://www.radioworld.com/columns-and-views/readers-forum/letter-below-ground-antenna-powered-lax-tis
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Well, as long as we're kickin' this subject around, Boomer came across this. https://www.radioworld.com/columns-and-views/readers-forum/letter-below-ground-antenna-powered-lax-tis
Lol.. Yes he did, it's his post that started this thread.
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This is a really cool find! (imo)
Ths video is a year old but in it they talk about a 10 mile long tunnel radio system project in the late 1990s that involved 117 different frequencies for underground communications. Nothing like this had been attempted before, big companies like Honeywell and Nada(?) tried, but the ones that came up with a working solution was these two guys of CanAm Technologies a Canadian company; "leading specialists in communication systems for tunnels and other enclosed environments". The two are interviewed by the host here.
Now two of those frequencies involved the broadcast band, and that's where the part 15 AM and FM comes in.
In the late 90s they took on a project for the ten mile long tunnel undet the city of Boston on Intetstate 90 I think. Part of the objective included communication for the drivers. So they set it up the entire 10 mile stretch of tunnel for drivers going Eastbound would tune into the AM broadcast, while those going West tuned to FM (or visa versa), so the information provided would correspond. He also goes into some detail about the leaky cable design and so forth.
It's a long video, I guess the most interesting is between about the 12 minute mark until about 30 min. But I'm going to have to watch it again cause I feel like I may have missed something.
https://youtu.be/WCm0mTjP0Bo?si=coFmuPA8H4IxX4hy
In this riveting seventh episode of Das Talk Podcast, host Peter sits down with Victor Bermudez and Michael Martinez from CanAm Technologies, leading specialists in communication systems for tunnels and other enclosed environments. The conversation delves deep into the nuances and challenges of creating robust communication infrastructures where conventional methods fall short.
Key Takeaways:
Innovative Use of AM and FM Signals: CanAm Technologies leverages both AM and FM radio signals uniquely to ensure that emergency communications reach drivers within tunnels...."