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intermod and quarter wave stub


Got a big clue this morning.  I put up a quick 3/4 wave stub using piece of 
Belden 8214 I happened to have sitting on shelf with N connector already on 
it (left over from old oscar antenna). Cut it long and at the 3/4 
wavelength figuring I could always shorten it to 1/4 wave later..

Test was very successful.  I found the worse spur I could (at low end of 
UHF range - 435.4 with doppler) which was at S9 on this strange FT-847 S 
meter. Put the stub on a Tee right at the UEK-3000 input. Dropped the spur 
level to S 4. Most of the others were not detectable using cw but it 
cleared up almost all the passband. With LSB on uplink, I could certainly 
work thru it - no worse than V/U intermod on AO-10. I had to crank the 
power up to 50 watts to find some of the spurs. The Spreadsheet says I 
should be under 15 watts even at apogee.

So I guess the fundamental is the culprit.

Now what does a 1/4 wave stub (at antenna side of downconverter) do to 
noise temperature?.  Is this a solution that I want to fine tune or do I 
need to think in terms of a Notch Filter?

If the stub is viable, what do I need to tune it properly?  Will listen to 
my signal generator in the sky, UO-11 for a few passes this afternoon to 
make sure I can still hear.

Jerry, W6IHG

Dr. Jerry R. Pixton, PIXOS Designs LLC

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