HFU HF Underground
Technical Topics => Part 15 AM and FM Station Operation => Topic started by: tybee on November 03, 2024, 1908 UTC
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With Talking Houses and similar transmitters which acheive ground via the power outlet..
Doesn't that essentially cause it to also act as a carrier-current station? Wouldn't any receiver near a power outlet receive the signals even if the free-radiating signal was obstructed by walls and floors or whatever?
I realize it's not near the power of a carrier-current transmitter, but wouldn't the signal still travel a long way through the wiring?
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Pretty much any Medium Wave transmitter, that uses power from the commercial power lines, will have some bit of coupling to the power lines, even the commercial sites. That said, the way to reduce that is to use actually isolation transformers in the power supplies. The Talking House, and even the Radio Systems version, I Am Radio, does come with a transformered wall wart, and not a switching supply. That helps. If you wanted to be 'Off-Grid', going solar and battery would isolate even more so, and I've heard of some stations doing so. Very long lines and low FREQ interacting with each other certainly isn't new, and will continue for as long as we have a MW Band. Steps can be done to reduce that coupling, but not totally eliminate it, unless you transmit out in the middle of nowhere, with no power lines around. BTW, not just power lines, but coupling has a tendency to occur with other utility lines as well, land-line phone, CATV, etc. That's, where fibre optics is good. Hopefully, I was able to answer that. BTW, using audio isolation transformers is good to do as well. I've had to do that here.
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Well, that wasn't really my angle. I was kind of asking couldn't you simply use a Talking House to provide AM to everyone in an apartment building by the carrier-current it provides. -as opposed to actually using a carrier-current transmitter and couplings etc.
But your response was excellent. I guess I just didnt ask the question right.
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I have a TH transmitter I picked up at Goodwill (actually I have three I think, they were NIB IIRC). I recall that when I tried it, I ran some tests with a portable radio, and was getting a LOT more RF radiating through my house wiring vs the TH's official antenna. This was using the power supply that came with the TH.
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You know that grounding configuration of the Talking House goes all the way back to the Radio Realty transmitters from early 90s, which the TH was based on. But there were at the very leastt 3 other manufacture during the 80s which also used the ground in the same way. All were certified.
There has to be some reasoning behind it but I never could find anything that showed the slightest indication of a concern from the FCC about that grounding method.. They were concerned however about new subdivisions which had previously used "talking houses" (which were not limited to the brand) to sell the new houses, but once people started moving in the neighborhoods, the transmitters got repurposed as mini radio stations for the new communities.. The idea spread big time and that's actually what led to the infamous 1991 200ft Public Notice. (Former FCC Cheif engineer later said that document was "wrought full of errors".
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How were the Alied Knight kit broadcasters grounded?
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I recall going down a bit of a rabbit hole at the time, looking at how the TH was designed, grounded/wired for power, the FCC regs, and some enforcement history. I recall one time where they went after someone using a TH in an apartment building, with predictably insane range due to all the wiring in the building acting as one large antenna. Yes, even though it's certified. But we all know from basic theory that any ground wire connection at these frequencies is going to radiate. As will any power connection.
I'm convinced that the only way to 100% guarantee you're following both the letter and spirit of Part 15 would be to use a battery powered transmitter (I guess solar would be OK also) with no connections to AC wiring, and just the allowed 3 meter antenna. No ground radials. Maybe you could have a connection to Earth if you mounted it at the ground. And I am not 100% sure the ground rod would be OK, even if it was entirely buried. I'm not sure about loading coils either. In other words, there's inherently no practical way to be assured of being Part 15 compliant other than using a very rigid/limited setup. Which would also keep you well within the infamous 200 ft limit. ;D So in effect you're at the mercy of the field engineer's interpretation. In reality, they have bigger fish to fry (FM).
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" I'm convinced that the only way to 100% guarantee you're following both the letter and spirit of Part 15 would be to use a battery powered transmitter ...there's inherently no practical way to be assured of being Part 15 compliant other than using a very rigid/limited setup. Which would also keep you well within the infamous 200 ft limit..."
I follow your points entirely, but a the fictional 200ft rule plays no part in it. In the early 70s the FCCs general stance on it was that there was technically no limitations on range but once you start going beyond 300 to 500 feet you risk infringing upon the licenced operations and what it serves.
The 1991 "200ft" public notice has always been an invalid document. and even if all it's content were accurate it still could not superceded the actual rules. That document was created nd is still used solely as a deterrent to avoid another fiasco like that of the 1970s with the rise of whip and mast outdoor transmitters.
It is not or has ever been what you'd call a legitimate document.
But yeah, your right, there is actually no possible way of adhering 100% to 15.219 except with a battery powered transmitter .
Based on that fact alone that means that every certified part 15 AM transmitter maufactured over the last 50+ years have been illegal to operate. -- But that can not really be the case now can it?
So you see that's what I'm saying, something is missing from this picture. The FCC has never been ignorant of the fact that an ac powered transmitter is going to achieve ground weather it has a ground lead or not. - But the rules do address this issue in regard to 15.219 by saying any radiation occurring in the lines from its path must conform to the 15.209 limits - and I think it is in that area that explains the questions of legality of TH built-in variable length ground lead.