Preamps and Polarization Switches
Newbie here and I am trying to put together a station. I have a question about preamps. I am making the following assumptions but please correct me or outline your station feeds if I am incorrect in my assumptions:
Most experienced operators are using moderate to high gain yagi antennas, circular polarized. I am thinking of purchasing the M2 LEO pack. I am assuming that most folks are using RF protected preamps to keep from blowing them up on transmit. It would be easier if one knew that they were always going to transmit on 2 meters (for instance) and one could place the NON RF protected preamp on 70 cms. However, since there is a mix of V/U and U/V birds, it would seem the only way to get preamps up at the antenna where they belong is to use RF protected preamps ( with weather and RF protection, significantly more expensive). Is there a role for diplexers here rather than running expensive RF protected switching? Also, I know from my listening on my Elk Antenna that change in polarization can make a pretty significant difference in adequate reception. So, it would also seem that there is a need with these type of antennas (M2 LEO) to have a polarization switching (cost for both antennas almost matches the original cost of the antennas). Do most operators have these switchable polarization switches. My shack is about 60 feet from the top of the roof, so I will have to deal with those losses in spite of using good cable. I was thinking of just starting with the antennas (no preamps, no polarization switching) and see how things go. Any thoughts or advice would be much appreciated. Best Fred Castello, KF4FC
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
Hi, Fred
I'm using an M2 2M7 2 meter beam, and an M2 420-450-11 70cm beam, with an SSB Electronic preamp on each. Prior to dedicating these antennas to satellite work, they were my "weak signal" terrestrial antennas. I have them mounted on the cross boom at 45* to the boom so that they're 90* to each other. I have a"reverse connected" diplexer ahead of the preamp on the 70cm antenna, and I use a 2 meter band-pass filter at the 2 meter output of the rig. These things have eliminated the 70cm desense I used to have when transmitting on 2 meters.
I ran separate DC power leads to the preamps after I blew out the preamp power switching transistor in my FT-847 by not checking to see if my 2 meter bandpass filter was DC grounded!
The antennas are linear polarized, and work extremely well, although I do occasionally notice a some fade on certain satellites. It's nothing that really bothers me, and it saved me a ton of $$ compared to having switchable CP antennas. Switchable polarization is "nice" to have, but IMO, unless you have some compelling reason that requires 100% copy, it's not worth the significant cost premium.
I'm have a "used" Yeasu G-5400B Az/El rotator set I got from a friend who bought it new, and then never used it. I built a Fox Delta kit that's a clone of the LVB Tracker, so by using SatPC32, I have full rotator control and Doppler correction.
I use Davis RF "Bury Flex" cable, although there are other very good "RG-8 sized" cables available. The DX-400 cable from DX Engineering is another good quality cable I've used at VHF/UHF with good results.
By all means, start with the antennas, and add things as your budget allows.
73, Jim KQ6EA
On 07/01/2016 10:13 PM, Fred Castello wrote:
Newbie here and I am trying to put together a station. I have a question about preamps. I am making the following assumptions but please correct me or outline your station feeds if I am incorrect in my assumptions:
Most experienced operators are using moderate to high gain yagi antennas, circular polarized. I am thinking of purchasing the M2 LEO pack. I am assuming that most folks are using RF protected preamps to keep from blowing them up on transmit. It would be easier if one knew that they were always going to transmit on 2 meters (for instance) and one could place the NON RF protected preamp on 70 cms. However, since there is a mix of V/U and U/V birds, it would seem the only way to get preamps up at the antenna where they belong is to use RF protected preamps ( with weather and RF protection, significantly more expensive). Is there a role for diplexers here rather than running expensive RF protected switching? Also, I know from my listening on my Elk Antenna that change in polarization can make a pretty significant difference in adequate reception. So, it would also seem that there is a need with these type of antennas (M2 LEO) to have a polarization switching (cost for both antennas almost matches the original cost of the antennas). Do most operators have these switchable polarization switches. My shack is about 60 feet from the top of the roof, so I will have to deal with those losses in spite of using good cable. I was thinking of just starting with the antennas (no preamps, no polarization switching) and see how things go. Any thoughts or advice would be much appreciated. Best Fred Castello, KF4FC _______________________________________________ Sent via AMSAT-BB@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
participants (3)
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Bob -
Fred Castello -
Jim Jerzycke