[amsat-bb] Mast mounted low noise amps

[Jordan Trewitt]

If you really wanted to do it right, you could always make your own with
WA5VJB's MMIC boards. I've used a Qorvo SPF5043Z on them successfully
several times, so the BOM for one is around $5. You'd still have to figure
out sequencers, sensing, switching and what not if you wanted to.
-Jordan
KF5COQ

On Fri, Jan 19, 2018, 6:49 AM Burns Fisher <burns at fisher.cc> wrote:

> Good point, John.  Especially given that I use my preamps mainly for
> satellites, I usually use one antenna for Rx and one for Tx with an SPDT,
> center-off switch to power one , the other, or neither depending on the
> satellite.  So the RF switch is there more in case I screw up than for
> anything else.
>
> For the local repeater (all on the same band, of course), I can turn the
> preamp off entirely.
>
> 73,
> Burns WB1FJ
>
> On Fri, Jan 19, 2018 at 4:53 AM, John <john at amber.org.uk> wrote:
>
> > 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!
> >
> > 73,
> > 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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> _______________________________________________
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