The sum of LHCP and RHCP fields of equal magnitude is a linearly
polarized field. The orientation of that linearly polarized field
depends on the relative phases of the LHCP and RHCP fields.

Roy Lewallen, W7EL

J. Harvey wrote:

FYI:
CP = Circular Polarization
RHCP = Right Hand CP
LHCP = Left Hand CP
FYI = For your information ;-)


New concept...?

If one were to arrange for the emission of a RHCP signal, and the same
- but inverted - LHCP signal, then the resultant composite 'dual-CP'
emission should be more-or-less undetectable by any linear antenna.
This is because a linear antenna in any orientation would respond
equally to both circular senses and, since one sense carries the
inverted signal, they'd cancel out (within the linear antenna) to the
degree that balance and symmetry is maintained.

Only an appropriate CP antenna would be able to extract the 'dual-CP'
signal. Obviously - use a more complex 'dual-CP' antenna system to
extract both senses, suitably de-invert one, and then combine at RF
for purposes of improving S/N by +3dB.

Since the RHCP and LHCP senses are inherently isolated from each
other, there doesn't seem to be any reason why such a 'dual-CP' signal
couldn't be easily generated and extracted. The RF hardware
implementation is fairly obvious (nearly trivial).

Also, that such a 'dual-CP' signal would be more-or-less ignored by a
linear antenna also seems obvious.

The most immediately-obvious application is a 'stealthy' transmission,
at least until someone shows up with a bent whip on their receiver.

Interesting, but not that exciting.

Then the penny drops...

This concept could be applied to satellite broadcasting (for example)
to provide ANOTHER (3rd) downlink polarization. In other words, for
FSS satellite (for example) where they already use two linear
polarizations (H and V) to double their effective bandwidth, this
'dual-CP' transmission technique could provide a '3rd axis' (thereby
increasing the total effective bandwidth by another up to +50%).

The satellite example has the huge practical advantage over the
'stealthy' example in that the receiving station is co-operating (if
his antenna is defective, he'll get it fixed). This co-operation would
help to maintain isolation of the 3rd axis.

The isolation won't be perfect for various reasons (for example,
perhaps Faraday rotation through the ionosphere would upset the CP
balance). But the isolation doesn't have to be perfect for the new
axis to provide valuable RF real estate.

The same sort of concept could also be applied at the baseband level
thereby reducing some of the RF hardware while providing the same
benefit on a transponder-by-transponder, narrower bandwidth, basis.
It's an option to further explore anyway.

Any comments or technical objections?

de VE1BLL (Jeff)

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