[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.
>
>[snip]
>
>
>>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.
>
>Tim,
>    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
>strength.
>


[snip]


We are in agreement.  The point I should have stressed is the
fact that AO-10 is not pointing directly at the earth since the
attitude control is lost.  Since the antenna does not point directly
at earth or the relative station in question, the 3dB gain does
not apply since the realitive polarity of the satellite is constantly
changing relative to the station on the earth.

While the signal emmanating from the satellite is not truly circular
as perceived by the earth station, a circular antenna does not
have to be rotated on its axis like the linear polarized Arrow since
it has the ability to capture multiple ange of incident from the
constantly changing downling signal.  The dancing Arrow as you
mention is done only to change the incident angle at any given
moment which is faster than any human can handle.  With a
circular polarized antenna, one will also try to dance between
LHCP and RHCP.   As the relative polarization changes as the
birds (AO-10) does its unpredicted manuervers relative to earth
through orbit, so does the not-so-circular polarization sense as
received by the earth station.  Meaning the Left Hand Circular
Polarization and Right Hand Circular Polarization needs to be
switched on a circular antenna.

Relativity, orientation of the satellite, polarization of the satellite
antenna, and a host of other factors affect the received polarization
of the signal on the earth.

I worked UO-22 for years with a linear antenna which I documented
years ago as favoring vertical polarization versus horizontal.  The
amount of data received between the two polarities was significant
with a linear polarized antenna.  A circular polarized antenna smoothed
out the rough edges in this case and did not receive much more
data from UO-22 at that time versus the vertical polarized antenna.

Will linear polarized antenna work with AO-10 ?  Yes.  I used the
same vertical polarized antenna on AO-10 as I used on UO-22
and it worked.  It appeared to work as well as my circular polarized
antenna.  There were times it worked better and times it did not
work better, because of the uncontrollable changes.

Do circular polarized antennas work better ?  It depends on what
polarity the satellite is using and if the antenna is pointed toward
earth.

The theory still holds true behind the gain and effects of circular
versus linear polarity,  but one needs to understand that the
polarity is affected by other factors if the attitude of the spacecraft
does not keep the circular polarized antenna pointed at you.  This
is the case with AO-10.  To say that a circular polarized antenna
gives you a 3dB advantage assumes the antenna on the satellite
and the one on earth are pointed directly at each other.  This is
almost never the case. However, the circular polarized antenna
need not be rotated on its axis (not too practical for special rotators
or fast enough) to keep up with the ever changing signal.

Great discussion!



Cheers,

Tim - N8DEU



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



AMSAT Top AMSAT Home