-----Original Message-----
From: Moon-net-bounces@list-serv.davidv.net
[mailto:Moon-net-bounces@list-serv.davidv.net] On Behalf Of Joe Taylor
Sent: Monday, June 27, 2005 3:26 PM
To: s57uuu@hamradio.si; Moon-net@list-serv.davidv.net
Subject: Re: [Moon-net] CP / LIN

Hi Marko 

You ask some very reasonable questions, and I'm not sure that I have satisfactory answers for all of them.  
I will do my best.

Marko Cebokli wrote:

> Hello Joe,
> what could be the physical mechanism behind the difference in depolarization between LP and CP?

I think a simple "thought experiment" may help.

Suppose that someone had scattered a large number of resonant half-wave dipoles on the lunar surface, 
with random  orientations.  An incident plane wave with linear polarization would excite currents in 
each dipole in proportion to cos(theta), where theta represents the dipole's alignment relative to 
the E-field of the wave.  Each dipole would re-radiate a signal polarized in its own orientation.  
The overall result would be that 75% of the reflected energy would be in the same plane as the incident 
wave, and 25% in the orthogonal plane.

For incident circularly polarized waves of either sense, symmetry shows that the reflected RCP and LCP 
waves will be of equal strength.

> Thinking naively about polarization:
> A LP wave can be seen as a sum of two CP waves, and each of them gets more depolarized upon reflection.
> How is it then possible that when we add them together again to get linear, the resulting sum has more polarization
> than each of the original waves, if the depolarization is truly random?

In the thought-experiment example, let's think of the incoming LP wave as two CP waves of opposite sense.  
Each wave will become "depolarized" and will produce reflections of equal strength in LCP and RCP; but 
the instantaneous E-fields in these wave components will be significantly correlated.  The correlation 
is proportional to the remaining degree of linear polarization.

> If there is a non-random component of depolarization (causing correlation between the two component polarizations) can it
> be orientation independent (same for all linear pols?)
> 73, Marko S57UUU

Yes, I believe it can.  The example with randomly oriented dipoles would show just this behavior.

One might reasonably ask, where are the "dipoles" in the case of the real lunar surface?  I can only 
suppose that there must be objects (or irregular portions of surface) comparable in size to the 
wavelength, with edges or overall shapes that force induced currents to flow in (locally) preferred 
directions.  Over a large area, such as the central region that produces most of the quasi-specular 
reflection that we use for EME communication, the preferred direction is fully randomized.

With best wishes,
			-- 73, Joe, K1JT

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