cross modulation at LF from AM broadcast

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  • Updated 5 years ago
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While listening to beacons in the 200 KHz range, I hear distorted voice and music, presumably from AM broadcast stations. The strongest AM broadcast station is -25 dBm. Is this a problem of the receiver, or perhaps of the AM broadcasters?
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Peter Driessen

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Posted 5 years ago

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Stu Phillips - K6TU, Elmer

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Not sure where you are located or the antenna you are using but here in the San Francisco Bay Area, my 160m top loaded 55' vertical has the strongest AM station at -8 dBm... almost light bulb levels!

I don't get any cross modulation on LF reception... I make sure the pre-amp is at 0 dB (you could also try it at -10 dB and see if that makes a difference).

My biggest challenge is the invisible dog fence... it excites the wire loop with a shared wave at about 20 KHz... the harmonics go well up into the LF portion of the band - so when I earwig on LF, I turn it off - much better!

It could be from the radio station itself but my guess would be that you have something close by acting as a mixer - the old rusty bolt problem. I had all kinds of crud locally that I tracked to a rusty hinge on the control pad to our gate!

You can probably imagine the look on my face as I opened the panel and the noise went away... and then came back as I closed it! A shot of WD-40 nailed it.

I used a small tuned loop antenna and then as I got closer, a piece of coax with a few turns of wire as a sniffer... to find the source.

One suggestion... try switching off the power in your home, leaving just the circuit powering the Flex and see if the noise goes away - try this one circuit at a time. Also unplug everything except the minimum you need to run and listen to the Flex... This way you might be able to find whether the source of the mixing/intermod is local to your property.

Good luck hunting this down... I think its very unlikely that it's in the radio itself - this was one of the first things I checked when I got my alpha 6700 back in February...

Stu K6TU

PS: An oxidized coax connector somewhere in the antenna path can also cause this effect... it can be really frustrating until you get to the bottom of the source. I know from personal experience.