[amsat-bb] Re: Advice on antennas for working the LEO's

[Robert Bruninga]

> Subject: [amsat-bb] Re: Advice on antennas for working the
LEO's
> 
> I was unable to access Bob's rotator site.  Is it just me?
http://web.usna.navy.mil/~bruninga/rotator1.htm

Use the EDU version instead:

http://www.ew.usna.edu/~bruninga/rotator1.htm

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