I don't think the tuned circuits in the 1208 can be mistuned as far as
21 MHz. They may not be tuned for 50 MHz, though and so be attenuating
the desired signal. I'd suggest a TV type high pass TVI filter that will
pass 50 MHz and notch out 21 Mhz. One way to prevent undesired mixing
products is to reject the unwanted signal before it gets to the mixer.
Once the mixer sees many signals it can't help but produce first order
mixing products which is what you are seeing.
Your 160m antenna isn't helping a bit. Its very effective on 21 Mhz and
probably not so good on 6m. A 6m antenna tuner (link coupled probably)
may help. The high pass filter will help a great deal. Peaking the
receiver of the 1208 on 6m signals will help them and have virtually no
effect on the 21.7 Mhz signal.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson. Reproduction by
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