Recently had to spend a day baby-sitting an A.T. & T crew and contractor at a combined FM/cell site. 220 ft. guyed tower, with 4 bay FM non-com & several cell services at the top of the tower. Carriers in the 700~1900~2100~2300 bands plus other equipment operating in 800 mhz. Seems the FM signal was combining with a 700 mhz. carrier to produce wideband noise around 830 mhz-- interfering with two broadband receivers, one at 827, the other at 831. I was there because my company takes care of the college-owned non-com, the college also owns the site.
What I learned from the tower crew may explain some of the problems reported about FM (and even AM) "interfering" with cell.
The new LTE equipment has most of the electronics up at the top of the tower. Data runs up to each transmitter by fiber, while each transmitter is powered from the shelter by multi-pair cables carrying -48 volts (three hot leads, three returns). In this installation, several 6 conductor 8 gauge cables with an external shield. Now each cable's shield is carefully "grounded" to a buss bar in the shelter, but not so up top. The cables up at the top of the tower go into a junction box they call a "squid." Probably because it looks like one. From there, separate hot leads and returns are fed to each transmitter by short lead-in wires. Twelve total transmitters in this installation--three for each band arranged around the top. Four shielded 6 conductor cables going into the "squid." Care to guess what is happening?
The first time the crew was there, they were convinced that the problem was caused by re-radiation, so the tower crew went up top, banging various junctions, looking for something to change. I had my doubts..for one thing, on their spectrum analyzer (really a wide-band receiver), dropping transmitter power from 4500 down to around 500 watts made no difference to the level of this wide-band noise. Killing the transmitter caused the noise to disappear--and the frequency relationship clearly indicated that the FM signal was mixing with the 700 MHZ. transmitter. My guess was that the FM RF was getting into low-level stages of the 700 mhz transmitter, creating that wideband mess up around 830 MHZ. The signal transmitted by these LTE transmitters occupies 5 mhz, but it is made up o a bunch of subcarriers coming on and of with traffic. Which is what the errant signal looked like--kind of--. Most likely entry point for the RF? The (unshielded) DC lead-in wires to each transmitter.
So the next week I brought along a little 500 watt SWR transmitter antenna for show & tell. This is a ring stub antenna with a gamma match. With an ohm meter there is a DC short from center conductor of the N-connector input to the rest of the antenna--of course, at FM, this looks like a normal load. My point was that what looks like a ground can work very well as an antenna. Or the "hot" lead--if a short length is exposed to a high RF field--Like maybe 20 feet away from an FM bay?
So the tower crew went back up--taking some short straps up with them. They found several places where the shield was not grounded at the top end of those multi-pair cables, as well as poorly connected grounds where they were connected. Tower climber told me that he's seen a number of installations where the installers never connected the shield of these multipair cables. After a long day of grounding, cleaning and tightening, they got a fair amount of this wideband signal they they'd been chasing beat down into the noise floor.
Something to consider when the cell company comes knocking, complaining that your FM station is destroying the cell service.
FM does it with frequency!
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