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Co-locating an LPFM at an extensive Ham site

reelyreal

reelyreal

My consulting engineer and I are putting together my LPFM app for this October and I wanted to get some thoughts, opinions, and concerns.

I have 2 site options within a mile of each other. One is an American Tower site. The other, my preferred, is a tower owned by a friend of mine with an extensive ham radio operation with HF, VHF, and UHF antennas. I need to ensure that my operation wouldn't cause interference to his operation.

Has anyone co-located with an amateur operator before? Are there things I need to watch out for regarding harmonics? I'm pretty good with my AM and FM, but when you start talking about 160 meters and yagis the size of my house, I start to get lost.

Thanks for your help!
 
It's highly unlikely that there would be any interference to his operation on 160 through 10 meters which are below the FM band in frequency, and where mast/antenna mounted preamplifiers are not common. If he does any weak signal or satellite work on 2 meters and up and utilizes mast/antenna mounted preamplifiers, that's where you might get in to trouble. Any preamp is susceptible to overload from a strong signal and will create spurs, which could desensitize his receivers. It is possible that even in the absence of a preamp, his 2M and up receivers could be overloaded with the same result. Placement of the LPFM vs the ham antenna will be a factor.

If it is receiver overload, you have a good chance of solving the problem with a filter in the ham antenna transmission line that notches the LPFM frequency. The filter would have to be capable of handling his transmit power as (usually) ham transceiver have a common transmit/receive port, so we're talking a tuned cavity here, not a small receiver filter. It will depend upon his particular installation and equipment. There is always a minimum amount of loss through any filter at other than the notch frequency, in the order of 10ths of a dB that he should not notice, unless weak signal work is his game.

One thing going for you in this case is that the LPFM operating frequency would be at least 40 mHz away from the nearest frequency that he would be using bettering the chances of no interference or easy notching.
 
I know of an extensive ham station which rented space for a FM translator. I operated HF from that site quite a bit & didn't notice any problems. There was also a multiband ham VHF/UHF repeater system at the site; no problems with that either.

Come to think of it, there was a 1kw or so non-commercial FM at the site as well - again, no issues that I recall.

You'll need to follow all the proper practices regarding grounding/shielding/filtering with the LPFM audio gear. Pickup of ham signals on audio gear is fairly likely.
 
I would also suggest a multi-bay .75 wavelength antenna to kill upward and downward rf radiation. Try to keep other antennas at least 3 meters away from your antenna. Putting a single cavity filter in series with the transmitter output will keep your emissions extra pure and has the added benefit of being a great insulator from lightning and antenna problems.
 
At the LPFM power levels you're talking about, 100W ERP max, you might be doing 1/10 of his TPO, never mind ERP. He could put the full legal limit (1.5kW) into a high gain Yagi if he does any 2M SSB work.

I don't think you will have any issues, as long as your antenna is at least a wavelength away from his vertically. With 100W ERP, it's nothing a filter on either his input or your output that won't fix a problem.

You may want to think about putting a band pass filter on your output, just to keep any of his RF from getting IN, which can cause spurs in your transmitter. Been there, done that with high power paging systems. They made spurs that came and went since they weren't transmitting all the time.
 
Very good info, thanks! I've been so concerned about causing interference to his operation on his "home base" that I wasn't thinking about interference to my station. It sounds like with proper grounding and a filter or 2, I can play it safe and keep everyone "clean." There's enough real estate to be at least a full wavelength away from his antennas, and to stay clear from his 2M rig.
 
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