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RFI? Sound Bite Included


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Yes - the FM Capture effect. Basically, in FM the stronger signal will win. Once the stronger FM modulated signal is about +6dB or better (stronger) than the interfering or weaker signal - you won't notice that there's another signal present, or your ear might be able to discern a slight very slight warble/wobble in the background if the interference is fluctuating right around that 6dB level.

This also gets into the reason why Pre-emphasis/De-emphasis has become the standard for FM radio transmission.

You can read entire books and articles dedicated to the study of FM capture. It was one of the reasons that Armstrong pushed FM as his preferred modulation method. Compared to AM - it certainly did have less static.

Armstrong pushes "staticless FM" technology - Free from interference...

 

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Re-reading your post, it seems you're asking if we can identify the actual noise that you're experiencing, rather than the effect.

I can't hear it well enough to discern what it is. There does seem to be some mixing involved. Are you friends with anyone who has a spectrum analyzer? Are you able to get an RSSI reading of the interfering signal? Does it come and go? or is it always there on your receive frequency whenever your system is idle?

If it's constant, you can be almost certain that it's another transmitter on a totally different frequency that's mixing with something else nearby to  create what you're hearing. Search up "2nd order intermod" to get an idea of what could be going on. Direction Finding could also be useful to pinpoint the area where the signal is coming from.

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So as it turns out there was a signal on 467.625, the repeater input. I could pick it up with a UV-5R and the squelch turned off while near the repeater site. It vanished about 12 hours after I first heard it. That warble/wobble/harmonic vanished with it. As I was checking other near frequencies I found that on 467.5875 was a constant signal again. As I sat there with a radio I could hear what sounded like faint voices and machines. By moving the antenna around and using the terrain I was able to tell it was coming from a particular direction in relation to the repeater site. 

I drove off in that direction and again using terrain I was able to narrow down the signal. As I got close I could hear voices and back up alarms echoing. Sounded like the inside of a warehouse. I was able to narrow the signal down to two city blocks in an industrial area.   I posted about it on FaceBook and found there are these intercoms that use FRS frequencies. These intercoms can be set to "monitor" which leaves one station transmitting for 10-24 hours depending on the model.  It appears that another group of persons had a similar issue in another part of the US. They were able to direction find and found a HoSmart HY777 which was transmitting on 467.625. That is out of spec for their FCC approval and could have been mis-programed.

 

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Intermodulation on RX when using weak front-end radios is best dealt with using cavity filters or preselectors. To test this, add a 10dB attentuator to the radio cable, if the noise is gone, then it is most likely IM, probably caused by very strong signal present. 

G.

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