New subject: UHF preamps

28 Dec 2022

      The 70cm band is hamonically related to the 2m band.
2m into 70cm 3rd harmonic interference is more likely than 70cm into 2m.
Happy New Year.
73 - Paul - W2HRO
On Wed, Dec 28, 2022, 10:00 tjschuessler--- via AMSAT-BB amsat-bb@amsat.org
wrote:
...
I do not believe it is RF from the 2 Meter transmit that is leaking into
the antenna side, as I run a duplexer in front of the preamp input.  70 CM
portion of arrow through a duplexer with only the 70 cm signal passing
through that duplexer.  I do the same on the 2 Meter side.  This
arrangement almost totally eliminates any de-sense of the RX on 70 CM from
2 M transmit.
*From:* Paul Andrews w2hro.fn20@gmail.com
*Sent:* Tuesday, December 27, 2022 9:02 AM
*To:* tjschuessler@verizon.net
*Cc:* AMSAT-BB AMSAT-BB@amsat.org
*Subject:* Re: [AMSAT-BB] UHF preamps
Tom,
Are you relying on the transmit RF sensing feature of this 432 MHz
preamp?    RF sensing can be unreliable.  You might use the FT-847's RF amp
enable output to trigger the external preamps.
Another thought.   It is possible that your 2m transmit signal is leaking
into the input of the 70cm preamp.   It's likely that significant coupling
is occurring at the arrow antenna.
You might consider using separate preamps and isolating relays.
73 - Paul - W2HRO
On Mon, Dec 26, 2022 at 11:32 PM tjschuessler--- via AMSAT-BB <
amsat-bb@amsat.org> wrote:
For my portable/temporary satellite antenna rig, I have need of using a
UHF preamp, which I have strapped to the tripod that holds and aims my
Arrow antenna.  I find that my FT847 is just not as sensitive on 70 CM as
it is on 2M.  I have been using an ARR (Advanced Receiver Research )
SP432VDG which is supposed to have a maximum power pass through imitation
of 25watts.  I try to be pretty careful as to how much output I run through
the thing, maybe limiting to 15 watts or so.  Unfortunately, I have blown
the preamp out several times, even with being conscious of the RF level.  I
sent the pre to them, they repaired and sent back.  I recently discovered
that I had blown my unit again.  Symptom is that it adds noise to the
output, but does not amplify any signals coming into it from the antenna.
I have sent the unit in to ARR in the past for repair.  This time I am
contemplating replacing this one with a more robust device that might be
able to handle more RF running through it (RF sensing means you will still
hit the device with an instantaneous burst of RF before the internal relays
kick in.
A friend has confided that he has the higher power version of the ARR
Preamps, but has had similar problems with killing the things.  Thus I am
in the market for maybe another option.  While I oft times, will stroll
over to the antenna tripod, and barrel off the cables going to and from the
preamp to bypass completely from the transmit chain, this is kind of a
pain, even in a portable situation.
I looked at M2s web site and can find a 2 Meter amp, but no 70CM (Might be
missing something.  I have heard a lot of good things on the SSB preamps,
but the only pace I can see them being sold, is Dolstra in Europe.  Don’t
see a US vendor.
Looking for suggestions.  I would be interested in buying pre-owned,  if
someone had a Preamp that would like a new home.
I do have a question for Yaesu FT847 users.  With that radio, you can
power a Preamp via the built in Bia- T power inserter, And assume that when
this mode is turned on, it cuts the inserted DC to the preamp as the
tranciever goes into transmit.  Does this work fast enough to protect, even
RF switched preamps?  That might be a good safety measure as I have
traditionally used external DC to power my preamps.
Wisdom from the cloud here would be much appreciated.
.
Tom Schuessler, N5HYP

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Re: UHF preamps

Paul Andrews

Thomas Schuessler

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