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Re: Intermod

There are two possible sources of interference -- the 5th or 6th harmonic of
the 70 cm transmitter getting into the S-band downconverter or the
downconverter being overloaded by the 435 MHz signal because of inadequate
selectivity in the RF stages in the converter.

Transmitter harmonics can be attenuated with a low-pass filter. The 70 cm/23
cm diplxer units available from HRO and AES have a 70 cm low pass filter so
you could attach one of those at the transmitter output.

If the problem is fundamental overload then a filter could be inserted at
the input to the converter or LNA to attenuate the transmitter fundamental
frequency. This could be either a bandpass or highpass filter for 2400 MHz
or a notch filter tuned to 436 MHz. The ARRL UHF/VHF Projects manual has
construction information for a 2304 MHz bandpass filter. If the center
conductor is shortened to 0.82" it should work well at 2400 MHz.

The type of receiving antenna feed used will also affect pickup of 435 and
1270 MHz from the transmitter. A waveguide feed would be ideal as it
naturally acts as a high-pass filter and has very low loss at the design
frequency. A short helix with a cup may also have this property to some

Your problem could be transmitter harmonics. The fifth harmonic would fall
on the image frquency when using a 144 MHz IF and could cause IMD in the
2-meter receiver. Try a low pass filter on the transmitter first. It has the
least effect on received signal level.



----- Original Message -----
From: "Jerry Pixton" <jpixton@shentel.net>
To: <amsat-bb@AMSAT.Org>
Sent: Wednesday, 11 July 2001 20:13 UTC
Subject: [amsat-bb] Intermod

> All,
> Today I put the UEK-3000 on the back of the elevation rotor and moved the
> dish way over on the VHF side (almost to the elements). Increased the
> spacing from 44 inches to 89 inches. It did reduce the interference some
> but not much. Then I ran a test with the UHF transmitter feeding a little
> element yagi fixed on the horizon towards our local UHF repeater. This
> is about 20 feef below the oscar setup. Still had intermod. So even big
> separation does not help. ?????
> Second set of tests was to step up the UHF band in 1 meg increments. Steps
> 430.5 to 434.5, 436.5 to 441.5, and 443.5 to 449.5 gave no interference.
> But steps 435.5 Mhz and 442.5 Mhz gave interference. Of course right in
> satellite area.
> Third test was to get some finer grain detail.  So I checked across the
> AO-40 passband.  Between 435.495 and 435.55 and from 435.625 to 435.700 I
> get interference.  But there are two strips from 435.55 to 435.625 and
> 435.700 to 435.800 that have no interference. The S band frequency was
> being tuned automatically as if I were using the transponder
> The interference looks like a set of spurs (looking with ao40rcv) about
> 1200 to 1500 hz apart (I can see several in the passband) when keying cw.
> I pulled the cover off the UEK-3000 and see that the chassis is soldered
> shut. So I don't know what room is available for a notch filter.
> I did also just run the FT-847 into dummy load just to make sure. When I
> turn off the power to the downconverter, the interference goes away. And
> when I turn the power back on, the interference drifts for a few seconds
> the LO stabilizes.
> At least there is a segment where voice could be operated and at the low
> end cw. The main group of spurs is around the beacon frequency anyway
> I don't want to transmit.
> And here I was going to play today building some helix prototypes. HI
> Any thoughts.
> Jerry, W6IHG
> ------------------------------------------
> Dr. Jerry R. Pixton, PIXOS Designs LLC
> http://www.pixos.com/designs/
> jpixton@shentel.net
> ------------------------------------------
> ----
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