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Re: Eggbeaters ?



Dave,

You should read "A Practical Rotator System" by Bob Bruninga, WB4APR:
http://web.usna.navy.mil/~bruninga/rotator.html

In the article Bob analyzes the percentage of time that a Leo satellite
spends at various elevations.  The result of his analysis is that for 67%
of all passes a Leo is between 10-20 degrees; for 84% of the time it is
from 10-30 degrees.  From that I deduce pointing a directional antenna like
a 3-4 element yagi or a corner reflector elevated to 15-degrees will allow
one to cover most passes adequately.  Directional beams of under 10-dB have
a beamwidth of approx 90-degrees (-3dB at 90-deg) so pointing is not that
critical within a couple dB.  In the article Bob recommends elevating
10-degrees, I believe.

BTW for a pass directly overhead a Leo will take maybe 18-20 minutes to
pass horizon to horizon.  How long does it take from 45 thru 90 back to 45?
 ...probably less than five minutes!  This is when the satellite is closest
in range so one does not need as much gain to overcome spaceloss.  If you
loose about 2-dB what does it matter?

Your question about the Arrow (3-elem 145; 6-elem 435) is that the gain
will be down more on UHF but not so much that you cannot operate.  Note the
columns on RNG Gain and ANT gain: as one drops off the other increases
compensating.  In other words one needs about 10-dB more gain near the
horizon than overhead!

I think Bob's simple computer tracking program and system is a natural for
stations wanting a minimal amount of complication in their station.  Yet
probably suffers almost nothing compared to full az-el tracking stations.

Now remember this is for Leos such as AO-51 or ISS, etc.  The situation
changes quite a bit for tracking a Heo like AO-40 or the future P3E.  For
them one will need a better antennas system with narrow beamwidth (high
gain) and good full sky pointing.

I hope this makes sense.
73's Ed - KL7UW

At 08:34 PM 12/3/2005 -0800, Dave Hartzell wrote:
>Thanks all for the input....
>
>Just one question, why is 15-degrees (or so) the magic number here  
>for Yagis?   I have an Arrow, and if I mount that at 15 degrees, what  
>happens when the spacecraft is overhead?  Maybe the beam-width of the  
>Arrow is wide enough...
>
>Dave
>
>On Dec 3, 2005, at 6:07 PM, Edward R. Cole wrote:
>>
>> Instead I would go with a small yagi or the corner reflector  
>> pointed up at
>> 15-degrees (or so) and rotatable in azimuth.  If using linear  
>> polarity it
>> probably matters little if you chose Vertical or Horizontal so pick  
>> what is
>> easy to construct.
>----
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> 
73's,
Ed - KL7UW  
=========================================
http://www.qsl.net/al7eb - BP40iq 
144-EME: FT-847, mgf-1801/1402, 4xM2-xpol-20, 170w
432-EME: FT-847, mgf-1402, 1x21-ele (18.6 dBi), 60w
=========================================
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