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Re: Up and dwnlink polarization of HAMSAT



Hi Guys,

This is an often confused subject by hams.  I offered the screw analog some
time back when this same question was raise.  Polarization is always
referenced from a starting point.  For radio propagation it is easiest to
understand from a transmission viewpoint (looking down the axis of the
antenna in the direction of propagation):  RHCP appears to be a electrical
vector that rotates clockwise as it moves in the direction of propagation;
LHCP appear to be a electrical vector that rotates counter-clockwise.

The perception that the receiver and transmitter are opposite comes from
where you view the process.  Always view it from the origin of transmission.

I think that some may have heard the for Moonbounce the transmitting
antenna is RHCP and receiving antenna is LHCP.  That is the standard but
the reason is that circular polarization reverses when it is reflected at
the Moon's surface.  This is not the case when transmitting directly
between two CP antennas (as is the case for satellites to/from earth).

Figuring out how to wire up a x-yagi for circular pol is more confusing to
most folks.  You have to go back to the rotating electric vector to analyze
it.  Then your vertical elements and horizontal elements must be separated
by 90 degrees of phase (either physically or using phasing lines).  One
must be careful to have the center conductor of the coax going to the part
of the driven element that is up or to the right for proper RHCP.  I'll not
go further on this one.  You only need to know this stuff if putting up a
CP x-yagi.  Most of the mfr's give amply diagrams and instructions.  A
helix is easy to visualize the sense of rotation of CP.

73's Ed - KL7UW

At 02:43 PM 7/1/05 -0500, Charlie Schlieper wrote:
>Thanks Drew.
>Your comments are appreciated.
>
>I thought I had read somewhere that a radio receiver senses a CP wave
>as the opposite CP polarization from the transmitters view.
>
>I am glad we have the expertise of others, sharing their experiences
>through this medium, to keep us straight.
>
>73,
>Charlie, N5TD
>
>
>----- Original Message ----- 
>From: "Andrew Glasbrenner" <glasbrenner@mindspring.com>
>To: "Charlie Schlieper" <n5td@vvm.com>; "Don,w7das" <w7das@cox.net>; 
><amsat-bb@AMSAT.ORG>
>Sent: Friday, July 01, 2005 2:05 PM
>Subject: Re: [amsat-bb] Up and dwnlink polarization of HAMSAT
>
>
>> No...RHCP matches RHCP. Think of it this way. Take two bolts, both with 
>> the same threads. Think of them like helixes. Put a nut on one bolt. Point 
>> them at each other like antennas would be. Move them close enough together 
>> that the ends touch. Notice how the nut (the wavefront) will thread off 
>> the first bolt right onto the second! If a LHCP transmit antenna required 
>> a RHCP receive antenna this would not be true.
>>
>> It's a simplistic analog, but always one that helped me understand 
>> circularity.
>>
>> The part about defining what constitutes RH or LH is true though. Groups 
>> disagree on which way the thumb points for which one, LOL. Been too long 
>> for me to remember who's on what side of that fence.
>>
>> 73, Drew KO4MA
>>
>> -----Original Message-----
>> From: Charlie Schlieper <n5td@vvm.com>
>> Sent: Jul 1, 2005 2:29 PM
>> To: "Don, w7das" <w7das@cox.net>, amsat-bb@AMSAT.ORG
>> Subject: Re: [amsat-bb] Up and dwnlink polarization of HAMSAT
>>
>> By golly, I think you're right Don.
>>
>> You just reminded me not only of that, but the fact that Optical theory
>> denotes the polarization (polarisation) opposite from us radio guys, or
>> as the twist is seen coming into the RECEIVE antenna, instead of the
>> TRANSMIT antenna relationship.
>>
>> Thanks for that refresher!
>>
>> Charlie, N5TD
>>
>> ----- Original Message ----- 
>> From: "Don, w7das" <w7das@cox.net>
>> To: <amsat-bb@amsat.org>
>> Sent: Friday, July 01, 2005 9:18 AM
>> Subject: Re: [amsat-bb] Up and dwnlink polarization of HAMSAT
>>
>>
>>>I may have read incorrectly, but I thought polarisation was determined by
>>>the rotation of the electric field about the axis as the wave moved AWAY
>>>from the source.  If this is true, wouldn't the polarisation be opposite 
>>>as
>>>it arrived on Earth?  i.e. transmitting with an LHCP antenna in space 
>>>would
>>>be best received with an RHCP antenna on Earth?
>>>
>>> 73,
>>> Don
>>>
>>>
>>> ----- Original Message ----- 
>>> From: "William Leijenaar" <pe1rah@yahoo.com>
>>> To: <amsat-bb@amsat.org>
>>> Sent: Friday, July 01, 2005 2:37 AM
>>> Subject: [amsat-bb] Up and dwnlink polarization of HAMSAT
>>>
>>>
>>>> Hi AMSATs,
>>>>
>>>> This morning I received the answer from ISRO about my
>>>> question for the uplink polarization, and why they
>>>> used LHCP on the downlink.
>>>>
>>>> The uplink of HAMSAT is in RHCP, opposite of the 2m
>>>> downlink that is in LHCP.
>>>>
>>>> I was also supprized to hear that the downlink is in
>>>> LHCP, as most amateur sats use RHCP.
>>>> The reason that ISRO used RHCP and LHCP is to avoid
>>>> interaction between the uplink and downlink signals.
>>>>
>>>> By using opposite polarizations the output of the
>>>> satellite would less interact on its 70cm input.
>>>> Personaly I think the antennas are at such close range
>>>> that this will not really help...
>>>> At least we know the right polarizations, and the
>>>> reason why it is done.
>>>>
>>>> Myself I have fixed RHCP antennas, and actually have
>>>> had never any problems with the HAMSAT.
>>>>
>>>> On the other hand the signals are that strong that it
>>>> should be no problem, and most of the hams I hear on
>>>> HAMSAT are in aligator mode and can use some
>>>> polarization attenuation on the uplink :o)
>>>>
>>>> 73
>>>> William
>>>> PE1RAH
>>>> ---
>>>> ----
>>>> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
>>>> Not an AMSAT member? Join now to support the amateur satellite program!
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>>> ----
>>> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
>>> Not an AMSAT member? Join now to support the amateur satellite program!
>>> To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
>> ----
>> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
>> Not an AMSAT member? Join now to support the amateur satellite program!
>> To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
>----
>Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
>Not an AMSAT member? Join now to support the amateur satellite program!
>To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
----
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
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