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I have heard an interesting sound before and after a band opens up - both on the AM broadcast band as well as 20 meters.

I'm using my home brewed K9AY antenna pointed at KFI in Los Angeles before sundown (about 1 1/2 hours before), and I hear a constant tone - which sounds like a carrier, but no audio. I live near San Francisco - so far enough away where we can't listen to LA stations during the day.

And then, at about 1 hour before sundown, the audio emerges, and the tone starts lowering until I hear KFI as if it were local (its a Clear Channel 50KW station).

I have heard the same thing on 20M - especially at night - when the band is closing, the clear audio in the SSB signals starts changing to more of a tone, and then they just drop out altogether. Its much faster on 20M - so I bet it has to do both with the power level as well as the frequency.

On the AM BCB it sure seems like I am witnessing the D layer lift. I imagine that on 20M I am hearing something affected by the F layers recombining?

Has anyone else experienced this?

73,

Rich

[KH6AQ]

I have not heard such a thing. Is this an artifact of your receiver?

[KH6AQ]

For KFI are you referring to the 5 kHz tone that is often heard when AM radios are tuned to a weak signal? I do hear this on my car AM radio.

I believe the 5 kHz tone is due to AM stations mixing together in the receiver and creating mixing products at 450 and 460 kHz. Any stations separated by 450 or 460 kHz will do this. These mixing products then mix with the station you are tuned to which has been mixed to the receiver IF frequency of 455 kHz. When this 455 kHz signal mixes with the 450 and/or 460 kHz mixing products the result is a 5 kHz tone. As the station of interest becomes stronger, and the receiver AGC cuts the gain, the 5 kHz tone becomes weaker until it is no longer audible.

Yes - and your explanation seems right on - and matches my experience. When I hear that tone - there are other signals (all weak) on the same frequency.

Thanks,

Rich

I should have said there are stations on the same frequency - and other stations maybe even as far as 10 khz away - but much stronger. I will try using the crystal filter on the HQ-140X next time - to see if I can notch out the nearby signals - and probably cut out the tone.

The mixing of signals sure does seem to make a lot of sense.

Rich

[KH6AQ]

The HQ-145 below 10 MHz uses single conversion and a 455 kHz IF. This would make it prone to what I describe.

Thanks Dave!