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Re: Advice on antennas for working the LEO's



I was unable to access Bob's rotator site.  Is it just me?

73 de Tim, K4SHF
 

-----Original Message-----
From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On
Behalf Of Bruce Robertson
Sent: Thursday, February 22, 2007 12:11 PM
To: Gary McKelvie
Cc: amsat-bb@amsat.org
Subject: [amsat-bb] Re: Advice on antennas for working the LEO's

Quoting Gary McKelvie <garym@garym.org.uk>:


> Several people have suggested using a vertical antenna such as a 
> colinear. I have actually already tried this and the results are very 
> disappointing, which I put down to my location rather than anything 
> else as where I am is not particularly that good certainly form a 
> VHF/UHF point of view.

Gary:

We look forward to working you on this side of the pond. If I remember the
details of this thread correctly, one of your design goals for this system
is to not require preamps. I would venture to say that most every antenna
design or recommendation pertaining to satellite work assumes low-noise
preamps as close to the antenna as possible. This might explain the
difference between your experience and others' with vertical antennas. 

What I love about this aspect of the hobby is the experimentation. Though my
antennas are down right now, there have been many silent mid-Atlantic passes
of VO-52 where I have amused myself by testing the minimum signal required
for reception, used varying antennas, and switched in and out a preamp or
two. Just me and an orbiting radio laboratory; thank you, ISRO!

Conducting such experiments with my pair of FT-817s and TS-2000 suggests
that a preamp is terribly important, especially for 70cm downlink operation.
In fact, my 70cm preamp is an indoor model, and it *still* makes a crucial
difference. I think this is because the NF of these radios'
preamps is just not devised for small-signal work. To put it more strongly,
I would rather spend an evening doodling around on 70cm with a (indoor)
preamp and a coathanger-and-bnc vertical than I would with my 8 element
rotating outdoor beam and no preamp!

Your high-gain, narrow bandwidth antennas will make up for this, of course.
But other beginners might be interested to know that by using preamps and
shorter, wider bandwidth antennas it is possible to have exceedingly
enjoyable LEO satellite operations with a single, azimuth-only TV-type
rotor. The approach offers some advantages: such short antennas are also
easier to build from scratch materials, easier to put up on in the air; and
the wide beamwidth of the antenna makes it possible to manually control the
rotor without too much fuss. The advantage of your az/el system is that it
will be closer to HEO-ready when P3E and SSETI are launched next year.
However, I venture to say that you really will need preamps then.

I started out using HRD, but like others, I have found that recent versions
do not track SSB/CW correctly, and it seems that Simon's focus is now on the
latest digital Swiss Army Knife. If you have difficulties of this nature,
try the demo of SatPC32 or other dedicated programs.

Again, for others with a different set of resources, there's a great
discussion of why a fixed-elevation rotor system works well at:
http://web.usna.navy.mil/~bruninga/rotator1.htm
I would advise that homebrewers begin avoiding circular polarization and the
mechanical challenges that entails. Many of us have had good luck building
the so-called 'cheap yagis':
http://www.wa5vjb.com/yagi-pdf/cheapyagi.pdf
My 70cm one is 8 elements; I found my 4 element 2m to be a bit under-powered
for receiving AO-7, but I had fun with it for 2 years!


73, Bruce
VE9QRP


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