[amsat-bb] Opinions on improving receive for portable ops
dave at druidnetworks.com
Fri Mar 10 19:35:13 UTC 2017
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
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.
On Fri, Mar 10, 2017 at 11:24 AM, Scott via AMSAT-BB <amsat-bb at amsat.org>
> 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
> 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:
> 1) with those radios and only about 10 feet of feed line, will a preamp
> help much on 432?
> 2) If it will, is the 50$ preamp on the AMSAT site a good start? Or? and
> 3) 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
> 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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