[NLRS] Question on FT897 TxH for EME use

[Chris Elmquist]

It seems very similar to a method that holds the radio's AGC at a voltage
level that results in no RF until the sequencing has completed.  This has
been a popular method for a long time.  Your TxH signal seems to be just
a digital form of AGC-- all or nothing-- rather than an analog level.

And I too have not done such with this kind of rig but the theory seems
valid to me.

W0GHZ and I have done modifications to the age-old FT736 to hold off it's
transmit because it does not have any sort of external input like TxH.
But our mod is to cut the TxEnable line that ultimately gates the RF on
and off and either delay that with a simple timer circuit or actually
feed the logic level output from the sequencer to this signal--  So,
we are enabling Tx when ready rather than disabling until ready but
otherwise very similar.

Chris NØJCF

On Thursday (12/17/2020 at 12:40PM -0600), David Palm wrote:
> I haven't done it myself, but according to W1GHZ what you're describing
> using that specific input to hold off RF is the safest possible way to
> protect the preamp.  Everything else is a step down in security, according
> to Paul.
> 
> On Thu, Dec 17, 2020 at 11:07 AM Jon Platt via NLRS <nlrs at mailman.qth.net>
> wrote:
> 
> > Over the last several months I've been piecing together a (very) small
> > 1296 EME station.  I thought I would use essentially what I have to see if
> > I could first actually hear anyone off the moon.  Last Fall I was
> > successful in seeing (and hearing a little bit) the ON0EME 1296 EME beacon
> > as well as decode a few stations.  My conclusion was that I can hear some
> > of the big guns.  So now I am working to add transmit capability.  Towards
> > that end, I now have every thing I need (I think) and have been working on
> > designing the sequencing.  I plan on using my Yaesu FT897 with an older 50w
> > DEMI 1296 transverter.  I do have a 402 series DowKey latching relay for
> > T/R that has 80 dB of isolation and can handle 250 watts at this
> > frequency.  This relay also has the "optional indicators" pins that can
> > provide a signal that the relay has actually switched.   I have a W6PQL
> > latching relay driver and his sequencer board.  My thought is tie my FT897
> > TxH (transmit inhibit) pin to +12v and to use the 402 relays indicator pin
> > to pull TxH low (when the relay actually switches).  In theory, the FT897
> > will not produce RF output if TxH is held high, thus in this design my
> > FT897 will not transmit until the T/R relay is actually switched to
> > transmit and thus protect my preamp.  I do plan to also use appropriate
> > sequencing, however using the FT897 TxH in such a way seems like good
> > insurance.  My question for the group is does this make sense and has
> > anyone used their FT817/FT857/FT897 TxH capability in such a way?
> > 73, JonW0ZQ
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-- 
Chris Elmquist