[amsat-bb] Mast mounted low noise amps

John john at amber.org.uk
Fri Jan 19 09:53:06 UTC 2018

I'm going to throw something controversial into the mix here: RF sensing is
NEVER a good idea!

I'm running a DG-8 for 2m and a Mutek SLNA for 70cm, both fed with DC up the
coax (the Mutek has been modded to do internal biasing), and my rig switches
the DC off before it transmits, which avoids the risk of the preamp being
inline when you send DC the wrong way up it.

You'll find with most preamps that do RF sensing, their RF Sense max
transmit power is significantly lower than their DC switched rating, and
this is mostly because switching your relays 'hot' is a really bad idea.
Much better to kill the DC, which switches the relays cold, before you send
that RF up the coax and into the circuitry instead of the bypass line.

Depending on the rig you are using, some have built in DC sending up the
coax. Any rig that works like that will 'sequence' its transmit stage; it
will switch off the DC output feed before switching in the RF power
amplifier. If you're using external bias tees to send DC up the coax, I can
thoroughly recommend the PTT-switched kits designed by M1GEO, which will
have a similar effect - the PTT line from your rig will switch out the bias
tee via its PTT input before it actually starts transmitting. Again, this is
sequenced inside most rigs in order to prevent hot-switching of the relays.

As an aside, if you're planning to leave them at the top of the mast
permanently, and not power them from your rig, then I would strongly
recommend a solution that kills the DC when you're out of the shack. We had
a remote station set up that someone left the bias tee connected permanently
for, leaving the preamp constantly in-line for RX only purposes. It got hit
by lightning during a big lightning storm that hit every tower and building
in the area, and because there was power to the system, rather than the
lightning just running down the bypass line, it fried a few components in
the preamp. It's a miracle that's all that it fried!

John (XLX)

-----Original Message-----
From: AMSAT-BB [mailto:amsat-bb-bounces at amsat.org] On Behalf Of
chilesp at verizon.net
Sent: 18 January 2018 21:34
To: amsat-bb at amsat.org
Subject: [amsat-bb] Mast mounted low noise amps

What brand of mast mounted pre-amps are you using and do they sense RF for
switching out of the feed line?

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