[amsat-bb] Preamps and Polarization Switches

Bob WB4SON at gmail.com
Fri Jul 1 23:22:08 UTC 2016

Hey Fred,

Hard to say what "most" means, but "Mast Mounted RF Switched Preamps" do
show up at a lot of stations. To your point, one might only purchase a 2
meter version, as so many birds are mode UV.  I happen to use one on each
antenna (FO-29 being a big driver for that).  In the US, Both SSB
Engineering as well as Advanced Receiver Research preamps seem to be
popular.  Either will hot-switch 100 watts, both can be powered via the
coax (although that is a special order item for ARR).  I use the SSB
Engineering SP-200 and SP-70 preamp. They were installed in 1998 and have
been working flawlessly for 18 years now, surviving a bunch of bad New
England weather.

Perhaps some folks use diplexers, but I sure would be concerned about
port-to-port isolation.  After all, you are pumping perhaps 50 watts out
(+47 dBm), and expecting to receive a signal that is around -120 dBm.
That's a difference of 167 dB.  I don't know of any diplexers that have
that kind of isolation. Without adequate isolation you are going to
experience RF blocking on your downlink.

As for polarity switches, keep in mind that most satellites use linear
antennas, so simply using a circular polarized antenna will help minimize
deep fades most of the time.  But I also know from personal experience that
sometimes switching from RHCP to LHCP can make a world of difference.

In round figures each step you take is about the same cost:  CP Yagi, Mast
Mounted Preamp, Polarization switch.  Roughly $1200 for all three steps.
Of course then you need to multiply that price by two (70cm and 2m sets),
and toss in another $1500 for a Az/El rotor and controller, and maybe
another $1000 in coax and control cables.  So $5000 for a full system at
new prices.  Eek!

Your plan for starting with just the antennas will far exceed your
expectations compared to the Elk.  As long as you don't mind pulling
everything down off the tower, putting those improvements in later will be
just fine.  Some people will skip the elevation rotor and simply mount the
beams at a fixed elevation of 30 degrees

73, Bob, WB4SON

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