Circular polarization transmission/reception 1

[Johnym]

I have really been struggling with this one: I have read that in a data link for instance, where there is a transmit antenna at one end of the link and a receive antenna at the other end, if CP is used, you want to employ antennae of the same polarization sense. I came up with a simple conceptual model of a CP antenna - two pyramidal horns oriented at 90 degrees - one transmitting H polarization and the other V polarization. The power is divided between the apertures and a 90 deg phase shifter is placed in the H arm of the Tx antenna. The construction is the same for the RX antenna. So, with same sense antennas facing one another in the link, the H polarizaton will experience two 90 degree lags (for a total of 180), and the V polarization will experience zero lag relative to H, so the fields will cancel upon combining at the Rx. Correct? This is not what my text says; however, they offer no mathematical explanation. There has to be another 180 deg phase shift - where does it come from? John

 

[IRstuff]

Circular polarization is not the sum of a V and H, it's characterized by a time varying orientation of the field vector.

TTFN

 

[Higgler]

The other 180 phase shift is from mechanical orientation of the horizontal antenna.
Suppose your transmit horizontal antenna connector is in LA and it points northward. Your identical receiving antenna in New York pointing back at LA will have the horizontal connector pointing south. There's your other 180 shift north to south.

kchiggins

http://www.toyon.com

 

[Johnym]

So, Higgler, what you are saying is if I put two identical H polarized horns (one Tx and One Rx) on an antenna range facing one another, I could measure the phase difference between the transmit and receive with a network analyzer for instance and use this phase difference as a reference. If I rotated one of the antennas 180 degrees, so its still transmitting horizontal, but antiparallel, polarization, I will see a 180 degree phase shift on the network analyzer relative to the aforementioned reference. Is this correct? I can see if two antennas were co-located and transmitting, and one were flipped to achieve antiparallel polarization, there would be a 180 degree phase difference measured at some reference plane, so I think it is starting to make sense to me.

Thanks,
John

 

[Higgler]

Yes, rotating the antenna 180 degrees will change the phase measurement 180 degrees on your analyzer.
And if your antennas are circularly polarized, say a basic spiral antenna, for every physical degree it's rotated, your analyzer will show a degree of phase change. That's true is a spiral antenna transmits to a linear antenna on the other end too.
That's a phase shifter technique used by some as a phase adjuster.

kchiggins

http://www.toyon.com

 

[VEBill]

Perhaps this example will also help:

You can make CP using two linear antennas arranged at the obvious 90-degree orientation (for example: cross dipoles), and with a 90-degree electrical phase shift in the feed harness. Or alternatively, with one dipole one-quarter lambda further along the boom.

So, provided that you add the required phase shift, then you can make CP from the sum of two linear. And vice versa backwards the other way too of course.

 

[VEBill]

Sorry - disregard. Reading more carefully I see that Higgler has already nailed the issue perfectly.