Issues with FM broadcast interference (6400)

If you or a fellow ham have an old low pass transmitter filter that may tell you a lot about the source of the mixing. Put it in line with ant 1. You may also try RX 1 that may take some circuitry out of the path for testing. Make sure the radio is not in the wide mode that provided RX protection. Use the pan adapter to measure strong signals from 60 khz to 54 MHz. This will identify RF that you don’t expect. Try measuring you interference while changing your amplifier gain. Also if you have an attenuator to put in front of the RX. (Just for evaluation. ) if it’s a simple, first order mixing the interference will change dB for dB. If a 3 rd order mixing it will change several times that amount. It seems the direct sample is simply chopping the FM signal getting into your radio. What is the effect of different antennas, maybe a beam or other of your antennas as an analysis tool. These are just some tools that can be used to isolate interference. 73 Bill W9JJB

Jeff, you say you live 'close' to both those station's transmitters, How close is close?

Chuck,


The issue Jeff is experiencing is nothing out of the ordinary for Flex 6K radios. An external broadcast-band filter was the first thing I needed before I could even try to use s 6500 at my QTH. I found a flat-topped signal in the pan-adapter at AM1490 and images of that signal dispersed throughout the amateur bands. So I bought a DLW filter and it fixed me right up.

I suspect the inadequacy of the preselector filters for mine and Jeff's purposes is related to, either:

1. Depending on the operating band, the filters operate in a low-pass or high-pass mode - thus, allowing the broadcast signals to pass straight through, unimpeded.

Or,

2. The stop-band of the preselector filters provides for only a modest amount of attenuation, say a maximum of 50dB. Whereas, the strong out-of-band signals from a nearby broadcast station may require more than 90dB attenuation to prevent receiver overload.

The situation at Jeff's QTH is similar to that of a repeater site where there is a transmitter and receiver operating simultaneously. At the repeater site you would probably employ a duplexer to provide the necessary isolation (+90dB) between tx/rx.