[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: Has anybody heard AO-10 lately ?

At 07:13 AM 8/16/2000 -0500, Tim Cunningham wrote:

> >AO-10 has a circular polarization that varies.  The Arrow is a linearly
> >polarized antenna.  You loose 3 dB of signal there as well.
> >
>Since there is no attitude control for AO-10, there is no guarantee that
>the circular polarized antennas are pointing toward you on earth.  With
>that in mind the 3dB loss does not apply.  While it may be true that a
>circular polarized antenna will provide a better average overall signal
>than a linear polarized antenna, it is very easy to roll the Arrow antenna
>to change polarity as the incoming signal changes throughout the orbit.


>When working AO-27, UO-14, or SO-35 I find that rolling the Arrow
>antenna to change the polarization during an orbital pass change make
>a big difference.  Relativity plays a part as the satellite tumbles and
>moves through its orbit. I have not used it for AO-10, but now I am curious.

    Your comments and some similar ones by others on the BB lately suggest 
what I believe is a misconception about circular vs. linear 
polarization.  In a linearly polarized signal the "polarity" is defined by 
the direction of the electrical field (vertical, horizontal, in between, 
etc).  The accompanying magnetic field is 90 degs to this.  Rotating an 
Arrow antenna will indeed show dramatic differences in linearly polarized 
signals and when the elements on the antenna line up with the electrical 
field, you'll get maximum signal.  Since satellites with linearly polarized 
downlinks do move, tumble, etc., the axis of the polarization shifts and 
moving the antenna works just as you describe.   The Arrow "dance" during 
AO-27 and UO-14 contacts is well documented :-).

However, with a circularly polarized signal, the axis of polarization is 
rotating essentially at the speed of light.  The wave is "corkscrewing" as 
it propagates.  Unless you can rotate a linearly polarized antenna at the 
speed of light in the correct direction, you shouldn't see any effect on 
the downlink signal related to your linear antenna polarization 
angle.  You'll always be 3 dB down from the maximum you could achieve with 
correct circular polarization.  Likewise, if you switch the polarity from 
right- to left-hand on a circularly polarized antenna receiving a linearly 
polarized signal, you shouldn't seen any difference in signal strength 
either.  Using a linearly polarized antenna to receive a circularly 
polarized signal is not necessarily a bad thing as when the squint angle 
increases, strange things happen to circularly polarized signals and the 
effective polarization can actually reverse from right- to left-hand (or 
vice versa).  Trying to receive a right-hand circularly polarized signal 
with a left-hand polarized antenna creates MUCH MORE than a 3 dB 
loss.  However, these polarization changes will be "transparent" to a 
linear antenna.

All of the above are, I believe, the theoretical facts.  However, if the 
signal is less than perfectly circular; that is if the signal strength 
varies with the polarization angle, then rotating a static antenna might 
show some changes in signal level. I suppose reflections, obstructions, 
refractions, imperfect antennas, and perhaps even atmospheric conditions 
could cause some changes in this regard.

FO-20 & FO-29 seem to have strongly circularly polarized signals and the 
polarization often changes dramatically during a pass.  It would be 
interesting to hear if the Arrow "dance" has any effect on their signal 

  Stacey E. Mills, W4SM    WWW:    http://www.cstone.net/~w4sm/ham1.html
    Charlottesville, VA     PGP key: http://www.cstone.net/~w4sm/key

Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org