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Re: Transverters with *70cm* IF's

Hello Dave.

01 Feb 00 08:40, you wrote to Tony Langdon:

 >>  jm> Will the subject be forced into a specific transceiver
 >>  jm> concerns?    Or 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
 DM> Dear Tony,
 DM> Some of us have been thinking about this problem for a long time.  The
 DM> solution is not all that simple.
 DM> For example my station consists of a TR-851A 70cm radio and a homebrew
 DM> 2M receive converter going into a 10M IF (my station HF rig).  This
 DM> arrangement was the least expensive in the late 80's when I assembled
 DM> the station.  It also allows me to use my "best" radio as the
 DM> receiver, the most critical part.
 DM> For receiving 2.4Ghz, I can run my converter into my existing 2M
 DM> converter. That is double conversion to my existing 10M IF.

OK, so far...

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

I think it's worth attempting though.  I have a 2m to 70cm transverter, which
should have the same potential problem.  This is a 20 year old commercial
model, which performs quite well.  However, it is worth noting that the Tx side
of the unit uses double conversion (2m -> 28 MHz -> 70cm).  Perhaps this is to
minimise the possibility of harmonic bleedthrough.

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

This might be worth a look a well. :-)

 DM> Another consideration is the problem of generating a useful amount of
 DM> power on 1.2Ghz.   The only route I have found is to use ERA MMIC amps
 DM> to produce 7mw of drive.  Then use M57787 hybrid amp (7mw in gives
 DM> 1.5W out) to drive a M57763 (1W in gives 18 watts out).  Cost for the

We have a locally available set of kits which will provide a total of 10W SSB
on 1.2 GHz.  Just a matter of when to purchase the kit and start building.

 DM> two amp is $170!   The current plan is too have the converters LO and
 DM> mixer in the shack.  Pipe the RF up to the antenna at low level and
 DM> have a ERA-x buffer stage overcome the loss of the feedline.  The PA
 DM> stage would be at the antenna to reduce loss of the those expensive
 DM> watts at 1.2Ghz.

My converters will be at the antennas too.  The antennas themselves will be
portable affairs.  I hope to procure a tripod or similar to stand them on.

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

I'm following the thread with interest... for obvious reasons. :-)

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

I'm hoping to be operational within 18 months.  As for Mode U/V, it'll depend
on if I can get enough antenna gain out of a portable station for these bands.
:-)  Also need another transverter (10-2m or 10m-70cm) so I can tune the two
bands independently.

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

I have a portable 2m/70cm antenna in planning here, probably get onto that one
during my time off in the next few weeks.  That will be used for SO-35, StenSat
and experimenting with the Fujis (though I hear activity on the Fujis is scarce
down here :( ).


.. otted lab rat!
|Fidonet:  Tony Langdon 3:633/284.18
|Internet: tlang@freeway.apana.org.au
| Standard disclaimer: The views of this user are strictly his own.

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