[amsat-bb] Mast mounted low noise amps

[Burns Fisher]

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?
>
> _______________________________________________
> Sent via AMSAT-BB at amsat.org. AMSAT-NA makes this open forum available to
> all
> interested persons worldwide without requiring membership. Opinions
> expressed are solely those of the author, and do not reflect the official
> views of AMSAT-NA.
> Not an AMSAT-NA member? Join now to support the amateur satellite program!
> Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
>
> _______________________________________________
> Sent via AMSAT-BB at amsat.org. AMSAT-NA makes this open forum available
> to all interested persons worldwide without requiring membership. Opinions
> expressed
> are solely those of the author, and do not reflect the official views of
> AMSAT-NA.
> Not an AMSAT-NA member? Join now to support the amateur satellite program!
> Subscription settings: http://www.amsat.org/mailman/listinfo/amsat-bb
>