[amsat-bb] Re: ARRL dual moxon turnstyle project questions

[Bruce Robertson]

Quoting "Reicher, James" <JReicher at hrblock.com>:

> Scott, 
> 
> 1. I would recommend using standard connectors and teeing them rather
> than soldering to a lug.  You'll find that if you want to move the
> array
> for portable work, it will be easier to set up and take down.
> 
> 2.  Although I'm no expert, I wouldn't think you'd even need the same
> length feedline, although it couldn't hurt.
> 
> 3.  LB gives the dimensions in inches, but try converting them to
> millimeters.  Just multiply by 25.4 millimeters to the inch.  I'd round
> to the nearest 1/2 mm and cut as close as possible.  Then, that ".49
> inches" dimension becomes 12.5 mm.  
> 
> 4.  You could always use duct tape... they use it on the shuttle and
> ISS!
> 
> 
> BTW, the 70cm version calls for #12 AWG wire, not #14.  
> 
> 
> 73 de N8AU, Jim in Raymore, MO
>  
> Light travels faster than sound...  This is why some people appear
> bright until you hear them speak.
> 
> 
> Date: Tue, 31 Jul 2007 07:08:16 -0700 (PDT)
> From: Scott Wilson <s.wilson at yahoo.com>
> Subject: [amsat-bb]  ARRL dual moxon turnstyle project questions
> To: amsat-bb at amsat.org
> Message-ID: <862339.97515.qm at web31810.mail.mud.yahoo.com>
> Content-Type: text/plain; charset=iso-8859-1
> 
> I'm working on building the ARRL dual moxon antennas, but the article
> isn't very detailed where I'd like it to be and a little over detailed
> in
> some places I wish it weren't.
> 
> If you want to look, this is the URL
> (http://www.arrl.org/members-only/tis/info/pdf/0108038.pdf)

Scott:

L.B. is a constant and reliable source of wisdom when it comes to antennas.
I've built many of his designs, most recently a 7 element version of the
OWA yagi for 2m. However, I have read a cogent criticism of these moxons,
and I feel I should share it here. 

For a fixed satellite antenna, the ideal pattern should provide diminishing
gain as elevation rises. It should not null out as quickly as a 1/4 wave
vertical, but certainly should have less gain overhead, where the bird's
signal will be many dB greater due to lower 'path losses' and where you
will rarely encounter the bird anyway. 

Note that these moxons do not fulfill this requirement: in fact, the
greatest gain is right overhead. For this reason, I would not build these
antennas, despite the imprimatur of QST. Tony's lindenblad published in
August 2007 QST would be a *much* better choice. (A 435MHz sister antenna
to it can be found from the same source in the 2006 AMSAT symposium
publication.) I'll guess that even a 1/4 wave might make a more practical
antenna.

You joke that a $1000 rotor/yagi setup starts to seem like a good idea. I
know how you feel, but I think it is important that all newcomers to this
field understand that such commercial solutions are really, really
unnecessary, especially for today's LEOs. A cheap tv-duty rotor can
effectively turn a small pair of yagis, say 4 elements on 2m and 6-8 on 435
MHz. The beamwidth of these is large enough that the rotor can be touched
up every three minutes or so. Many, including myself, have found Kent
Britain's 'Cheap Yagis' design to work well in this application. Total
budget: $90, I'd say.

In fact, such a set-up is *better* for the newcomer than a full-up pair of
long yagis with az/el rotor. Why? Well, if your yagis are not pointed dead
at the bird, you'll have worse performance than with shorter ones. So
you've got to keep making sure that the az/el rotors are well-aligned, etc.
This is difficult and not particularly fun.  

Secondly, with so much gain on uplink, you run the risk of swamping the
receiver of a linear bird such as VO-52 or AO-7. This creates a sort of
arms-race situation, where everyone else has to use equal effective power,
or their signals don't get heard, and some QRP stations just fade away. We
all make mistakes from time to time, but in my experience, the operators
who consistently run many s-units over the beacon are also frequently in
possession of long antennas, and perhaps simply can't turn down their
transmitters sufficiently.

You might wonder why many people here, including myself, talk about our
more elaborate stations. Well, I'm struggling to add effective elevation
control (on a budget) and longer antennas because I'm attempting slowly to
upgrade to a station that could operate the HEOs when they arrive. Others
still have HEO-grade stations from those days and use them for the current
LEOs. I've enjoyed the technical challenge of upgrading, but truthfully I
worked way more satellite DX with the simple station described above than
with my much-tweaked more elaborate one.

Finally, I'm going to run off and spend half of that $1000 you mentioned.
The best use of it is on brand-name mast-mounted preamps. ARR or SSBUSA.
Don't even think of receiving 435MHz without one. On 435MHz, I'd rather
have a paperclip as an antenna and a preamp than the biggest yagi without a
preamp. In fact, my first satellite antenna was a 435MHz groundplane
soldered on a bnc connector. Hooked up to a good preamp, it hears LO-19
wonderfully, even while indoors! 

73, Bruce 
VE9QRP