OK - Couple different things going on here. I'll take your questions literally first then go from there:
1) Maybe - A good preamp can help radios like FT8x7 because they don't have the best noise figure, compared to purpose built VHF/UHF rigs like 821/910/9100. You would probably get some improvement by adding in a quality preamp (like a ARR or a HSM) without even touching the feedline.
2) No. The AMSAT broadband pre-amp is ok for capturing telemetry, or amplifying signals over a long coax run, but in a short run it doesn't do anything to the already weak signals you're receiving from a bird close to the horizon. If you wanna go the pre-amp route, a high quality purpose built 70cm is the way to do it.
3) Yes and no.. probably no. The best thing you can do with your yagi is to adjust the polarity on the fly, and the best way to do that is by holding it. Get it off the tripod, hold it in your hand, and listen to the signal. Rotate it until the bird sounds like it's right next to you. I've seen guys with 4 elements and a good location (more on this in a second) work FO29 all the way to -1.0° easy copy. If you're not capable of holding everything up, and are using a tripod, make sure it's the kind that you can tweak polarity fast with, by rotating on the longitudinal axis. It's not as good as holding it in your hand, but it'll do. If you're constantly tweaking your linear polarity, no need to do the CP thing.
Now, reading between the lines on some of the questions:
You didn't ask the question, but since you said portable I have to mention this. The absolute best tool you have at your disposal once you've freed yourself from the shackles of a fixed QTH is LOCATION. I'm going to say it 2 more times because it's that important. LOCATION. LOCATION. By choosing where you setup your station, you can get somewhere at a higher altitude, clear of obstacles like trees and buildings, and away from artificial RFI. By doing this not only can you actually get true line of site to the horizon, where the bird will be for those long distance passes, but you'll have the best SNR when you do finally hear it. Large parking lots work pretty good (think Churches or Walmarts) Mountain tops, scenic overlooks, heck even balconies or rooftops. If you're truly portable go somewhere that you can see the horizon and is away from the noises a home. When i'm using my portable setup in my backyard I'm lucky if i can hear most birds below 10°. When I drive 15 miles to the top of a nearby mountain, my portable setup is good to -1.5° and I break distance records. The only thing that changed was the location. Whenever you start talking portable, you're talking location - and location is bar none the most important factor that you have control over when you're talking about improving reception.
Also, use the absolute best feedline you can afford, that is convenient for your setup. I use 8' of Times (genuine) LMR240 for my portable setup. I know some will use the lmr240uf, or rg8x, or even an off-brand of lmr240, but if you want the absolute best, get the good stuff. LMR400 would be better yet, but its kinda stiff and heavy, and starts become troublesome to use while portable, hence why LMR240 seems to be the best middle ground, for me and the other solid /P ops. I'm sure someone will be in to tell me how it's only 0.1db less to use 8X or rg58 and other sorts of random theory, but these people will be wrong. Whatever loss you get by sacrificing feedline quality will impact your ability to hear at low elevations.
Finally cheap yagis are nice. I used them solely for 6 months, and still use them on occasion for guerilla operation... but a commercially built portable antenna like an arrow is a good investment for any portable operator. Lining up the gain of 2 separate bands on the same boom is a difficult task for most garage builders, and you need all the gain you can get for both tx and rx on those <5° passes. Just something to consider.
Shout if you have any other questions.
73,
Dave, KG5CCI
On Fri, Mar 10, 2017 at 11:24 AM, Scott via AMSAT-BB [email protected] wrote:
I've just started getting on the linear transponders in the last month or so after a 30 year absence from sat work, necessarily with a portable setup, and would be eager to hear experienced opinions on what to do next to improve receive.
I'm running Cheap Yagis on V(3 el) and U(6 el), diplexers, about 10 feet of RG8X, and receiving on an FT817 or 897, usually on FO29 and the XW's, and have up to 25 watts available for the uplink.
Q's are:
- with those radios and only about 10 feet of feed line, will a preamp
help much on 432?
If it will, is the 50$ preamp on the AMSAT site a good start? Or? and
With relatively short yagis, will it make a noticeable difference going
to circular polarization?
WRT to 3, I've pretty much exhausted what I can find the internet, but just don't have a pragmatic sense for what it will do in terms of evening out reception over, say, an FO 29 pass - I read that you nominally lose 3 dbs most all the time but may pick up a lot on the deep fades, but still not sure how that translates to the real world.
Or is the answer that the set up is marginal enough I need to do it all of the above if I want reliable communications above about 5 degrees or so?
Thanks for the bandwidth and any input.
Scott ka9p
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