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2m uplink - protecting downconverter



John Toscano wrote, about methods to avoid accidental transmission into a
downconverter:

Another solution would be to have a pair of coax relays, one on each
side of the downconverter.  Whenever the downconverter is turned on,
it would switch the coax relays into circuit and disable VHF transmit,
(or enable the T/R bypass relay you mention).  And whenever the
downconverter is turned off, the coax relays switch the downconverter
out of circuit and bypass it completely, and then either re-enable
the VHF transmit circuit or bypass the T/R relay.  A little more
complicated arrangement, but somewhat more foolproof.

Wayne replies:

Only ONE coaxial relay is required to switch between the 2m antenna (TX+RX)
and the downconverter output.  That is what I do.  Three things happen when
I flip a switch to the "downconverter" position:
1. 12V is applied to the downconverter
2. 12V is applied to a coaxial relay to select the downconverter instead of
the 2m antenna
3. 12V is applied to a "2m TX inhibit relay" inside my FT-847

Assuming you don't want to swap cables, you need to use either a coax switch
or a coax relay to switch between the 2m antenna and the downconverter.  A
coax relay isn't much more complex than a coax switch, and it gives you the
ability to automatically disable the 2m transmitter when the downconverter
is selected.  Totally foolproof.  It's worked for me when switching from
AO40 to a LEO satellite where I needed to transmit on 2m.  On two occasions
I was unable to get an uplink to a LEO satellite because I forgot to flip
the relay control switch to the "2m" position.  I probably would have blown
up my downconverter if I didn't have TX inhibit slaved to the coaxial relay
control switch.

By a lucky concidence I can operate full-duplex in mode V/S2 without
modifying my station.  My downconverter has a 123 MHz IF, and I already have
an extra cable running from the downconverter to an FT-817 for monitoring
telemetry.  So I can receive the S2 downlink with a 123 Mhz IF using my
FT-817, and transmit on 145.9 MHz with my FT-847.  I'm not sure if I will be
motivated to get up at 2 AM though.  Maybe next Wednesday when the window is
6-7 AM Central Daylight Time.

Wayne Estes W9AE
Mundelein, IL, USA




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