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3373 experiments.....and a little note about an ic706



I'm using an IC-706 (original version) for my IF.
I noticed spinning the dial from 90Mhz thru 129 Mhz, the
noise from the downcoverter dropped off sharply, at about 122 Mhz.
Theorizing that this was the 706 and not the downconverter,
I went off to search the net for "icom 706 is deaf".  Sure enough
I found several pages saying that the 706 does not hear well there.
One author said cutoff is below 120Mhz another said 140mhz.
I did the mod outlined here:
http://www.cc.jyu.fi/~pjka/IC-706/ic-706rx.html
The noise floor was now much flatter, giving me S3 with no
attenuator from 120 to 125mhz.  (previously 122Mhz had S0 noise)

I finally pulled the covers off my 3373 and set up my diode/resistor
multiplier and my IC736 to use as a 50Mhz signal source (50.02 x 48).
I'm using the dipole feed on the 3373.  With the source close by, i
could hear the signal in the noise, but no increase in s-units ...
S3 noise floor... Ok i said, the stub comes out!

I removed the stub and retried... WOW... S9+ hmmmm I think it did
improve things... Ok, so if the stub dumped 2401Mhz 6 S units, lets
see if i can construct a new stub for 2155 (image)  After 3 tries,
I've had some success. I'm using some salvaged solid shield 50ohm
coax thats about 1/8" thick.  The original stub was 1/16" thick.
I'm doing this all by guess and by golly, so bear with me.  I
started nibbling away at the newly installed stub, hoping for
a null in the 2155 signal.  I put 2401(/48) and 2155(/43) in
the vfo A and B to make it easy to compare.  I nibbled and nibbled,
but never really could match the depth of null that the installed
2400 mhz stub did.  Ok, so I decided to keep on cutting... I managed
to null out 2400 mhz again... hmmm..

Can someone check me... 246Mhz is a little more than 10% of 2401, so,
about a 10% longer stub would be required for 2155. That is what I 
attmpempted to do.  I also noticed that the stub that was in the unit
was 1.5" long.  My calculations show me that 2400mhz 1/4 wave * velocity
factor should make the stub in the 0.8" ballpark. Does that little green 
strip that the stub connected to have something to do with it?

I know I improved things quite a bit.  I have yet to put it back into
service on the dish, for the real test.  Should I even mess with a stub
at this point?  I've removed my stub, and verified an equal signal on 
2401 and 2155, so I think I'm back to baseline (minus the stub).

Kevin
N0BEL

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