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Re: Beginer Antenna Selection



Hi Steve

I would echo the "simplicity" vein in Bruce's words.

I recently worked him and W1FC via FO-29 from my location north east of
London, England using a tri-band co-linear at about 30ft, no pre-amps, a
Diamond Tri-Plexer and 45W out of the rig, a TS2000.  Location is 30ft asl.

Not great copy but we made the contact and that's part of the fun!

Regards

David
G8OQW


-----Original Message-----
From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On
Behalf Of Bruce Robertson
Sent: 07 December 2006 16:08
To: sraas@optonline.net
Cc: amsat-bb@amsat.org
Subject: [amsat-bb] Re: Beginer Antenna Selection



Quoting Steve Raas <sraas@optonline.net>:

> Well here is my 1st question.
> 
>  
> 
> In researching what is needed for working sat's , I have found that I
> am
> looking for an antenna that has RHCP, although selectable RHCP & LHCP
> is
> avail.. I just cannot justify the added expense for a 5% probability of
> an
> improved signal. 

Steve:

What an excellent question! My answers and advice below are based on some
experience and on reading this list for the last couple of years. I hope
those who are more knowledgeable will correct any errors.

Antenna selection is partly determined by your rotation scheme. If you are
using azimuth rotation alone, which I and many others would recommend and
you suggest below, the following advice pertains.

I would strongly recommend against *any* circular polarization to start.
Use a linearly polarized antenna of modest length instead. If you do get a
circularly polarized ones for 2m and 70cm, they should have switchable
polarization. This is because different birds use different polarization
for their antenna array, and in LEO they can end up reversing polarization.

However, adding switchable circ. polarization adds enormously to the cost
of the array or, in the case of homebrew, to the risk of muddling the thing
up. 

> 
> That being said.. Are 'Typical' terrestrial SSB Yagis  RHCP?  I am
> looking
> at the following antennas for known performance vs. price:

No, they're not. Yagis can produce circularly polarized signals if two of
them at right angles are fed in proper phase. Thus, a yagi-like antenna
that is circularly polarized will look like a combined vertically-oriented
and horizontally-oriented yagi. 

> 
> 2m -  DPM144-5LVA  (http://www.directivesystems.com/lva.htm)


> 
> 430-440 - DSFO432-15RS (http://www.directivesystems.com/DSFO432-15.htm)

Tuning on this band can be very touchy. If it says it is for 432, I'd give
it a pass: you're going to be up at 435 MHz. 
 
> 
> You may wonder why I'm looking @ such small antennas and I will answer
> that
> My main coaxial run from rig to preamp may be at most 30'.  

These are not small antennas for LEO work. They are, in fact, too large if
you are planning to use an azimuth-only rotation setup, which is really all
you need (esp. with preamps on the mast). I'd go much, much smaller.

As a sort of point of comparison, I'm upgrading from homebrew 8-el on 70cm
and 4 el on 144 with fixed elevation (az-only) only in order to work the
bottom 5 degrees of elevation and DX into W. Europe. That modest setup has
garnered me hundreds of contacts and a world of fun. Like many on this
list, I used the WA5VJB 'Cheap Yagi' design. It works. A recent pdf is at:
http://portal.ucpel.tche.br/py3vhq/home/antenas/uhf-sw.pdf

My guess is that any commercially-built array will be much more than the
cost of a 2m/70cm SWR meter (you can even use a FT-817 as a poor-man's
version!) plus the materials needed for a Cheap Yagi array. You might try
some of the other designs that people have had success with, such as the
Texas Potato Masher II.

All of these are predicated on the fact that a longer boom yagi will make
your (fixed az.) station *less* fun to work with. Greater gain comes at the
cost of a narrower beamwidth, both vertically and horizontally. You'll
therefore find worse nulls as the bird goes overhead, and you'll find the
adjustment of the az. rotor will be unpleasantly touchy. 

There is a long-running debate about the optimal gain pattern for fixed
el., but I'd say my 4-el. 2m antenna has a shade too little directionality
for my liking. Still, I've used it for over a year now. The 70cm antenna is
a champion in my books.

I'm
> probabaly
> going to feed them both with an LMR-400 type cable, possibly LMR-600 on
> UHF
> but with such a short run and loss under 1.0db I don't know nor have
> decided
> if it would be worth it. Also I plan on mounting these ( or similar
> yagis )
> @ a 30deg angle with a simple rotor as an AZ/EL rotor is not an option
> at
> this moment due to cost. So to sup up my initial statement I don't need
> another 3dB of gain to make up for line loss, So in theory I can keep
> the
> antennas smaller.

30 deg. is pretty high, even with the short yagis (brutally so with long
ones). Unless you will not get a signal to the lower elevations, due to
foliage, houses, etc., I would experiment with 10 deg. to 20 deg.

You've probably read that you need to put together the 'best receiving
station you can'. You may also, as I did, fear that people will label you
an alligator if you can't hear very well. In practice, folks on the birds
are very understanding if you use modest power and are beginning. What's
frustrating are the people who have been around for ages, put booming
signals into the bird and don't seem to be able to hear anything but a
signal equally booming. Because one can hear one's own signals, the problem
is quite obvious to others. 

A last thought, which doesn't respond to your questions, but might be as
much fun for you as it was for me: one of my first satellite antennas was a
70cm 1/4w groundplane. I just soldered some house hookup wire to a bnc
connector: two for a groundplane, one for a vertical element. With a 6' run
of RG-58 and a wooden roof, I can hear many birds in CW mode: LO-19
(booming!), the FM of AO-51 (listening in CW mode), GO-32. No rotating, no
nothing. Granted, in my experience such an antenna doesn't quite suffice
for two-way communication. But it reduces the number of variables in
setting up a computer-aided station, if that's the way you want to go, and
AOS prediction in any case.   It is so cool to just listen to the bird come
up, explore the elevation at which it is heard, listen to fading patterns,
and observe doppler. Recreate the OSCAR 1 experience in about 5 minutes of
antenna building!

Later on, when something goes wrong with your outdoor array, you can always
go back to this antenna as a baseline. In fact, when I lost my 70cm antenna
to moisture in the feedline, I used this antenna a) to confirm the problem;
b) to transmit up to VO-52! It's also a great way of testing an unknown
receiver on 70cm: if LO-19 doesn't come in LOUD with the 1/4w groundplane,
 I'd say that receiver is useless for LEO work. 

Similarly, a three-element yagi on 2m will do a great job of picking up
VO-52. I lashed one together, and pointed it by hand within my wood-frame
house. Not  R5 copy, mind you, but great fun and cheap. It will even copy
AO-7.

73, Bruce VE9QRP
----- End forwarded message -----


-- 
Bruce Robertson, 
Dept. of Classics, Mount Allison University
http://heml.mta.ca
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