[amsat-bb] Re: ARRL dual moxon turnstyle project questions
broberts at mta.ca
Tue Aug 7 07:10:52 PDT 2007
Quoting scott wilson <s.wilson at yahoo.com>:
> Thanks very much for the feedback everyone.
> >From what I was reading in the articles, I was under the impression
> a circularly polarized antenna was a step above linear polarized setups
> such as the Arrow (or any small yagi) and verticals. In practice, I'm
> having somewhat better luck with the linear setup. Until Bruce
> the short yagi/tv rotator setup, I hadn't even considered my own
> setup as an option.
> Here are my experiences so far:
> My first attempt was with a 2m/440 mobile whip with a radial base. The
> antenna was tilted over about 30 degrees to be slightly more
> perpendicular to the predicted path of AO-51. From that pass, I could
> definitely tell that I was tracking a satellite, but couldn't copy
> anything that I was hearing. This was fed into my IC-7000 with about 15
> feet of RG-8X.
I'm delighted to hear that these experiments are meeting with some success.
I think one of the great things about AMSAT work is that it puts the fun
into *listening* to the radio: it's amazing, the signals are coming from a
box in space!
Some thoughts on the above:
0) What you describe above is what I think most of us would expect from the
equipment you describe. I find something like this a very useful baseline
from which to assess later experiments in antenna building.
1) when listening to AO-51's FM signal with a sub-optimal system sometimes
it is interesting to switch your radio to the CW or SSB mode. You will hear
the warbling note of the FM signal much better, though you won't be able to
make out what people are saying.
2) Similarly, with a system such as you describe above, LO-19 might be a
good bird to track. Its CW beacon is quite loud. You can either listen for
it manually (start tuning several kHz above the nominal frequency) and
observe the doppler effect or use it to test out a computer tracking setup.
> Second attempt was receiving with just the 440 section of the dual
> antenna system. The closest I got with that was perhaps a little less
> noise, but definitely no copy. Obviously, that's not inheriently a
> problem with the antenna, but likely a problem with my implementation
> which - which I can't really test. My antenna analyser only goes to
> Bruce's email saying that a TV rotator and linear yagi's would be
> than a full blown system for a beginner contradicted what I had
> convinced myself of - that I would be way too far down in the noise
> the fact that I was linear polarized and had no elevation control, and
> with about 75' of less than stellar RG-58.
> In fact, I have something usable right now on my roof: a cushcraft
> 2m/440 3 el vertical yagi on a tv rotator.
> So far this has the best results to date. I've been able to copy at
> least parts of call signs on the latest AO-51 pass, and managed to get
> pretty clear copy on a couple of CW beacon passes.
> Now I'm considering incremental improvements.
> I have a length of RG-213 that's been waiting to replace the radioshack
> RG-58 feed to the yagi, so that should give me about 2dB.
Replacing the RG-58 will make a *huge* difference. According to this online
calculator http://www.ocarc.ca/coax.htm, your matched loss on 435 MHz is
7.2 dB, meaning that you've raised your noise floor by that much, too.
That's brutal! Matched loss of the '213 will be 3.7 dB, still brutal if it
is in front of a typical receiver. However, if you put the low noise preamp
on the mast, the effects of the '213 will be mitigated. The math is
interesting, and can be followed in M. Davidoff's book, which you really
should get your hands on.
The fact that you could hear better with the yagi suggests that this is the
way for you to go: it's apparently fighting better, even with both hands
tied behind its back :-) You might want to permanently raise the elevation
of the yagi about 20 deg. so as to fill in the pattern overhead at the cost
of some gain at the horizon. Odds are, you'll still make local terrestrial
signals (especially if you make the improvements described below). I would
even consider getting someone to buy you a 75 foot run of LMR-400 for an
If the yagi has one input for both bands, I would, for now, get a 70cm
preamp up there and do without on 2m, where one is somewhat less important.
Have you tried using your yagi to listen to VO-52 or AO-7? Given the
results you describe with AO-51, the former should be quite audible because
it is closer to you, there's less of a penalty in 'path loss' on that
frequency, and your feedline is not tearing up your noise figure quite as
Do you have a second radio with which you can listen as you are
transmitting? On FM birds, a crummy old HT will do fine for both purposes.
Since birds hear better than they transmit, you could use your vertical
with an HT to transmit 5w and the improved yagi set-up to listen. If you
want to go all-mode and are on a limited budget, consider the FT-817 as a
second radio. It takes some fiddling, but SATPC-32 does let you control two
radios at once, one for up-link and one for down-.
Thanks for sharing your experiences with us, Scott. I, for one, am enjoying
hearing about your progress. If you haven't yet had the opportunity, I
strongly suggest joining AMSAT-NA. It's nothing short of amazing that you
can be part of a club that is designing a satellite as cool as 'Eagle',
helping university students with their low-cost satellite experiments and
even ask really simple space physics and communications questions that will
be answered by bona fide experts. Besides its support for all this, it
produces a great journal, and building some of the projects in there has
saved me hundreds of dollars and been great fun.
73, Bruce VE9QRP
More information about the AMSAT-BB