That last comment about the spectrum analyzer is an excellent suggestion. Trying to solve such interference problems with cavities or isolators may involve far more effort and expense than necessary. Here's a case in point:
I was getting some severe interference to my reception of an Amateur repeater on 147.210 MHz, but not on other Ham repeaters. My base radio at the time was a Kenwood TK-760K2, which is a low-tier commercial product. The interference was sometimes voice, and sometimes data. I could almost eliminate the interference by putting a DCI bandpass filter in the feedline, but then I couldn't receive or transmit on the commercial frequencies used by other VHF repeaters I control. So, I pulled out my trusty Hameg digital spectrum analyzer, and set it to sweep the 140-160 MHz band. As soon as the interference began, I had the answer: 3rd order intermodulation with two local paging systems. Two paging transmitters on a hilltop a few miles from my house were each running about 3.5kW ERP- one on 152.480 MHz and the other on 157.740 MHz. The interference only occurred when the two paging transmitters were on the air at the same time as the 147.210 MHz repeater. This was third-order IM because two times 152.480 MHz minus one times 157.740 MHz (three "times" involved) equals 147.220 MHz, close enough to 147.210 MHz to obliterate the latter. The mixing was occurring in the front end of my base radio, AKA receiver IM. One of the simple solutions to receiver IM is the "local-distant" function that is frequently available in the programming software. The local position adds a small amount of attenuation in the front end, which can make a huge difference in IM rejection. In my case, however, the 147.210 MHz repeater is a distant station that I must use the "distant" setting to receive. The interference went away after I upgraded my base radio to a Motorola CDM1550-LS. Sometimes, an interference issue affects only certain designs, models, or brands. I readily acknowledge that my situation might be exceptional, with two very powerful paging transmitters at a nearby site. What if I had a repeater at the same site as the paging transmitters? That would definitely be a problem faced by repeater owners, over and over again! Back to the original premise, that of seeing what other signals are on the air at the same time the interference occurs, is of major benefit to an interference investigation. A spectrum analyzer is the instrument of choice for that task. 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Nate Duehr Sent: Tuesday, July 31, 2007 2:54 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Re: Coax length between added cavity and duplexer Nate wrote: > For the record, the interference appears as an on frequency signal > (leading me to initially suspect intermod of some type) with clear > audio of the dispatcher and officers. At first I thought I heard a > second level of audio as well that I haven't been able to identify it > whether it is background or another transmitter--partly due to > catching it at the right time, and partly because my QTH doesn't > allow me to hear it very well. KC0MLS thought receiver overload may > be the issue so that's why we're trying the Celwave cavity. Hi Nate... Just to be clear (because it helps figure it out), do you hear every transmission, or only parts of transmissions? (Mixing with something else that goes on and off air, would come and go during a longer transmission on the system you're hearing in your repeater. Often you can find the thing you're mixing with if it's on-site by watching TX LED's and taking along a few receivers... "Okay the dispatcher is on-air, and hey there's the interference, and the red TX LED just came on over here on this panel... and then... there it's gone and the TX LED went out..." You get the idea... Also seeing what's really coming down the RX side of things with a Spectrum Analyzer may be enlightening. Nate WY0X
