Yaesu VX-7R - Main + Sub = What??

[KK4DUY]

Hello all!

On my way home today I had my handheld (yaesu vx-7r) tuned on it's two receivers to two different 2-meter repeaters in the local area. I was very confused when I heard what sounded very much like police radio come through on the 'sub' receiver. My thought was "Why would the Police use an Amateur repeater?"

Fiddling with the radio revealed that I could only pick this signal up if I had -both- receivers on! Turning the main off silenced it, and tuning either away also did as well.

Can anyone explain to me what phenomena I was witness to? The repeater frequencies were only a few mHz apart, so I cannot fathom that the difference or multiple between them might be what I was picking up.

The radio manual indicates that, when operating in dual-receive mode, "strong signals may be received on the image frequency" or that the main/sub (or both) sensitivity might be reduced. This tells me they are known to interact, but I'm afraid I have no idea what it means by "image frequency." It could be an explanation of this might make the rest of it clear to me.

Thanks!

[KB1TFH]

Mine does this as well.

All I can figure is that one of the repeater towers also has a police repeater on it and the dual receive allows it to bleed through.

[KD0FIN]

It's common on the VX-7R when running dual RX for image frequency signals to bleed through.

From Wikipedia:

Image frequency (fimage) One major disadvantage to the superheterodyne receiver is the problem of image frequency (http://en.wikipedia.org/wiki/Image_frequency). In heterodyne receivers, an image frequency is an undesired input frequency equal to the station frequency plus twice the intermediate frequency. The image frequency results in two stations being received at the same time, thus producing interference. Image frequencies can be eliminated by sufficient attenuation (http://en.wikipedia.org/wiki/Attenuation) on the incoming signal by the RF amplifier filter of the superheterodyne receiver.