Another circular polarization question - backscatter

[Johnym]

Hello,
I am confused as to what happens to a circularly polarized wave when it encounters a perfect conductor and is scattered back to the transmitter. I have always just accepted that if the number of "bounces" it undergoes is odd (say it's reflected from a perfectly conducting flat plate or a trihedral corner reflector), the sense of the scattered wave would be opposite to that transmitted and if the transmitting antenna is also the receiving antenna, theoretically, the received power would be zero. On the other hand, if the number of bounces is even, (the reflector is a dihedral for instance) the received power would be that expected based on transmitted power, range, target RCS, etc. There is someting I am not understanding though. Take the single bounce case: if the Tx/Rx antenna is circularly polarized such that there is a 90 deg phase shift in the horizontal component and zero in the vertical and on reflection from a perfect conductor, each component experiences a 180 deg phase shift (I think this is correct) then the H component will have a 180 deg net phase shift relative to the V when combined in the receive process and cancel. If there are two bounces, the net effect is the same - the H compoenent will still lag the V by 180 deg when combined, although each will have undergone 360deg phase shift on the two bounces. What am I doing wrong??
Thanks,
John

 

[VEBill]

RHCP bounces back as LHCP after a single reflection. And vice versa the other way around too.

In practice, for example, a satellite TV LNBF will be designed to accomodate a single reflection. It would actually be LHCP to receive RHCP after bouncing off the dish once. If you use a dish system with two reflections (they exist), then the LNBFs have to be modified to match.

In practice, another example, CP is resistant to multipath because single reflections are rejected.

 

[pstuckey]

For horizontal polarization, I believe the phase shift is ~180 irregardless of grazing angle. For vertical polarization above the Brewster angle, the phase shift is ~0, so the sense changes (RHCP becomes LHCP and vice versa).

Peter

 

[Johnym]

Thanks pstuckey, but I dont at all understand why horizontal and vertical polarization will experience different phase shifts if they are both propagating normal to the refecting surface. Although, assuming the convention that the polarization sense is defined by the direction of rotation as viewed looking in the direction of propagation, if there is a 180 degree phase shift in either the H or V, polarization on reflection, the sense of polarization will change. If there is a phase shift in both the sense will not. So there must be a shift in one, not the other - not sure why.

I guess I still don't understand the basic mechanics behind the polarization change for a single and double bounce.

John