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Re: Thanks for help and ... a theoretical question



Message text written by "Brockwell, Stephen E. CECOM SEC FSSE ILEX"
>
A THEORETICAL QUESTION (or more than one)

How feasible would it be to build a receiver for 2.4 GHz using the R2 KK7B
modules (single signal direct conversion receiver) along with a 2.4ghz to
2m
downconverter and a transmitter for 1.2 ghz using the T2 module for a very
compact AO40 system?  I realize that the transmitter would need at least a
10?, 20? 30? watt amplifier but then the whole system (radio and amp) could
conceiveable fit into a briefcase.  With these requiring little power it
may
be possible to power the system using a gel-cel of 7ah or so and then with
the sheet metal portable dish receive antenna and a small loop yagi for the
transmitter have a very portable system.  The R2 and T2 modules are
available from Kanga US.  <
-----------

Steve:
        Feasible? Sure is! In 1993, I wrote a paper for the AMSAT Symposium
on Development of a Portable Mode S Groundstation that used the R2 as a
2meter receiver behind a 2400-144 converter. Very small; worked
satisfactorily. Biggest shortcoming of this arrangement is the lack of
anything resembling AGC; the R2 will absolutely blast the phones off your
head if it gets into a strong signal. (Yeh, I know that i should have
removed the final audio amp or built an AGC, but I didn't.)The unpublished
part of it was that I built a version that used the R1 (single mixer) as a
direct conversion receiver directly from 2400 MHz to audio. The internal
change was that the R1 was built with a surface mount mixer capable of
operation on 2.4 GHz. It also worked, though it is difficult to eliminate
drift in the tuneable 2400 MHz LO. The LO was larger than the R1 itself.
This would have worked on the R2 also with suitable mixers in the R2 and an
LO splitter and 90 degree phase delay line (both easy to implement on
2.4G). I also built T2's for both 70cm and 23cm transmitting. The final was
an M57762 running 15W. Once again, stability of the tuneable LO on 1269 was
a headache. These were complicated analog LO's; modern digital technology
would make this a non-issue. But it did work. I never built a portable
version for 70cm, but did use a breadboarded version crawling across the
bench as an uplink exciter on AO13 a number of times.
        I would encourage you to try this; the R2 is a lot of fun to work
with. Rick Campbell KK7B has designed a versatile and interesting device.
        Incidentally, a copy of my original paper is in "Mode S: The Book".
The paper details a take-apart portable stressed dish antenna for 2400
downlink. Not much detail is included on the R2/T2 arrangement beyond block
diagrams and some discussion.

Good luck!

Ed Krome K9EK
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