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Re: Transverters with *70cm* IF's



 > From:          tlang@freeway.apana.org.au (Tony Langdon)
 > Date:          01 Feb 00 21:29:02 +1000
 > Subject:       Re: [amsat-bb] Transverters with *70cm* IF's
 > Organization:  Fidonet: Freeway Usenet <=> FTN gateway 
 > To:            amsat-bb@AMSAT.Org

 > 
 > Hello joseph.
 > 
 > 31 Jan 00 13:05, you wrote to ALL:
 > 
 >  jm> Very timely subject.
 >  jm> Will the subject be forced into a specific transceiver concerns?    Or
 >  jm> will it be non-specific
 > 
 > I can see this being a generic issue, as a lot of people will have both 2m and
 > 70cm SSB (a number of multiband rigs came with these bands as standard).
 > 
 > Tony

Dear Tony,

Some of us have been thinking about this problem for a long time.  The solution
is not all that simple.

For example my station consists of a TR-851A 70cm radio and a homebrew
2M receive converter going into a 10M IF (my station HF rig).  This arrangement
was the least expensive in the late 80's when I assembled the station.  It also
allows me to use my "best" radio as the receiver, the most critical part.

For receiving 2.4Ghz, I can run my converter into my existing 2M converter.
That is double conversion to my existing 10M IF.  

Now comes the tricky part, how to get on 1.2Ghz for the uplink.
I could use a converter to mix my 70cm signal up to 1.2Ghz.  However,
I am told that there will be "problems with the third harmonic of 70cm."
That is, the possible third harmonic of my 70cm radio will mix with the 
output of the upconverter mixer producing spurs.  How serious this 
potential problem would be I do not know.  It would take some experimenting
to find out.

Another possible path would be to move my 2.4Ghz converter's IF to a non-ham
band IF such as 350 or 400Mhz.  Then convert that down to 10M.  In affect, building
a double conversion receive converter.   Then I could use my existing 2M radio,
a TR-751A as the IF for a 1.2Ghz up converter.  This would resolve the 70cm
3rd harmonic problem and still allow me to use my "good" receiver for receiving.
I suspect that this will be the path I will use in the end.

As for the 2.4Ghz converter itself,  I plan to use a DEM 2.4Ghz converter that I
purchased originally for AO-13.  I never used it due to serious drift problems
Lately some work has been done and published on improving that converter.
I plan to build a new and hopefully stable LO for the converter and put in all of
the latest updates and mods.  Hopefully it will do the job.  And yes, the new LO
will be mounted in the shack and be left on all the time to minamize drift. 

Another consideration is the problem of generating a useful amount of power on
1.2Ghz.   The only route I have found is to use ERA MMIC amps to produce 7mw
of drive.  Then use M57787 hybrid amp (7mw in gives 1.5W out) to drive a M57763
(1W in gives 18 watts out).  Cost for the two amp is $170!   The current plan is too
have the converters LO and mixer in the shack.  Pipe the RF up to the antenna at
low level and have a ERA-x buffer stage overcome the loss of the feedline.  The
PA stage would be at the antenna to reduce loss of the those expensive watts
at 1.2Ghz.

As for antennas, I hope something very good comes out of the experiments
with helicals that have been posted on the reflector.  I wish there were more
posts like this.

I suspect that it will take me a good two years to build all of the 
stuff I need.  Till then, see you on mode U/V!

BTW, the current project on the bench is a noise figure meter.

--73--  Dave WA0AUQ


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