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I've got a week-old TYT TH-9800, a quad-band (10/6/2/70cm) mobile, Chinese knock-off of a Yaesu HT-8900. So far, I love it -- several times the power of my HT, and a better antenna actually outside the car, I can stay on a single repeater for my whole commute (does get a little scratchy in the bottoms of draws at 40+ miles out, of course).

I've noticed a couple times, however, that the "right half" of the radio will receive while I'm transmitting, either on the right half as well or on the left half. When it's done it while the left half was transmitting, the left half was on 6m -- and it's only ever done it on one frequency, right half on a repeater setting, 145.470 (standard offset, don't recall the tone but PL enabled).

The first time I heard this, my own voice was intelligible from the speaker while transmitting on the right half, and the left half tuned to, as I recall, a 6m repeater. The other time, I heard pretty garbled signal exactly when I keyed the mic while tranmitting on 6m on the other half of the radio.

I can switch the right half to another nearby channel, 145.250, and no more cross-talk.

Is this just a consequence of a cheap radio, or is it something that just happens sometimes?

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It's something that happens sometimes. Perfect isolation is hard, and harmonics are a thing, so you're most likely to see an issue while transmitting on a lower-frequency band and receiving on a higher-frequency band — but any band combination is possible, due to images.

The difficulty of doing this right is why full-duplex radios are somewhat rare — most dual-receive radios, even if you could mod them to leave the receiver on, wouldn't have enough isolation between the two sides for the receiver to be useful anyway. So it's not entirely surprising that sometimes there are minor issues even on a radio that is specced for full-duplex.

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