AM Broadcast 600 KHz Interference
Robert Wilson
Begin forwarded message:
This pops up from time to time when AM radio stations are in close proximity to 2 meter and 220mhz repeater stations. With 2 meter repeaters the problem frequency is 600khz, with 220 repeaters the problem is 1600 khz. You might notice that 600 and 1600 khz are also the transmit and receive frequency offsets for those two frequency bands. So, if you are in close proximity to an AM station on 600khz in the case of a 2 meter repeater you have two strong signals, your repeater output and the 600khz signal. If there is anything, and I mean anything that is capable of mixing those two signals in a non-linear junction of any sort, you are going to have a second order mix on your receive frequency. For example, 146.940 - .600 = 146.340 MHz, your receive frequency. It works both ways, plus or minus. It can also happen when you are sharing a site with 2 FM stations that are 600 or 1600 khz spaced, then the math is a little more complex but the problem still exists.
The problem is when both frequencies are strong in particular area the signal can seem to be coming from everywhere. And just about everything in a strong field can mix, rusty tower bolts, chain link fences are exceptionally noisy and on ad infinitum. Even your test equipment, spectrum analyzer or service monitor can generate the mix if you are not careful with input levels.
The easiest fix is to go to a non-standard offset with either your tx or rx. That is the only way you can avoid the mixing product. And make sure that you go far enough so that the modulation sidebands of both your repeater transmitter and the AM broadcast station are out of your repeater passband or you will still get fuzz on weak signals. Or, you could move out of the field of the AM station. In the case of a crowded site, these mixes can also occur with transmitters external to your particular installation.
I saw one situation where the mix was occurring in the repeater transmitter modulator so the repeater was actually transmitting 600 khz subcarriers on both sides of it's center frequency. This is why decoupling and shielding in repeater transmitters, receivers and anything connected to them such as controllers, allstar and echolink nodes be totally shielded so that all of their buzz is contained inside their respective boxes, and anything outside is shorted to ground before it gets to sensitive low level circuitry inside the boxes where interaction like this can occur.
>From a guy who has been there and fixed a lot of these types of problems,
tony dinkel
wb6mie
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