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Re: intermod and quarter wave stub



If you make the stub 1/4-wavelength at 435 MHz it will also create notches
at 1305, 2170 and 2940 MHz. Unless the coax has a high Q at 2400 MHz, the
2170 MHz notch could introduce some attenuation and increase noise
temperature.

A 2400 MHz 1/4-wavelength coaxial resonator seems a better solution. You
just tune for maximum received signal.

73,

John
KD6OZH

----- Original Message -----
From: "Jerry Pixton" <jpixton@shentel.net>
To: <amsat-bb@AMSAT.Org>
Sent: Thursday, 12 July 2001 16:58 UTC
Subject: [amsat-bb] intermod and quarter wave stub


> All,
>
> 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
> http://www.pixos.com/designs/
> jpixton@shentel.net
> ------------------------------------------
>
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