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@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@amsat.org] On Behalf Of chilesp@verizon.net Sent: 18 January 2018 21:34 To: amsat-bb@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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