Geometric polarisation rotation

Last Updated on September 7, 2024 by John Berry

As I’ve noted on many pages, a wave launched as vertically polarised is best received by an antenna that is likewise vertically polarised. The same applies if horizontally polarised. So, if something happens along the path and the wave has its polarisation distorted by some angle relative to that when launched, there will be a loss of signal received. Whether that loss will be significant or not depends on the margin available.

Uniquely, in EME, polarisation distortion has two principal components – that due to the geometry of the path, and that due to the Earth’s magnetic field (Faraday rotation). This page covers geometric rotation.

There are two key points to remember: reflection from the Moon is semi-specular and hence polarisation is sustained on return from the Moon, and polarisation is always stated relative to the local horizon.

Core theory

In fact, geometric rotation isn’t rotation at all. Relative to the transmitted wave, the polarisation will be maintained following moonbounce. The diagram below shows this annotated.

A pair of polarisation scenarios to illustrate geometric rotation. The grey dashed lines indicate propagation to stations on the side of the Earth facing the Moon.
We are viewing from space toward Earth and on toward the Moon beyond. — *A pair of polarisation scenarios to illustrate geometric rotation. The grey dashed lines indicate propagation to stations on the side of the Earth facing the Moon.*
We are viewing from space toward Earth and on toward the Moon beyond.

The transmitter launches a horizontally polarised wave that may be received horizontally polarised with respect to the receiver. This is Scenario A.

But for a receiver at some other location on the Earth’s surface, the received wave may be somewhat rotated. This gives a polarisation difference, and this is shown in Scenario B. The apparent rotation may cause loss.

This effect can be more extreme as shown in the annotation below. Here anything up to 90 degrees polarisation difference may be experienced.

A pair of polarisation scenarios to illustrate extreme geometric rotation. We are viewing from space toward Earth and on toward the Moon beyond. — *A pair of polarisation scenarios to illustrate extreme geometric rotation.* We are viewing from space toward Earth and on toward the Moon beyond.

If the polarisation rotation approaches 90 degrees, full cross polarisation discrimination will be experienced with a loss of anything up to 30dB. This sort of loss likely renders the communication non-viable.

The exact rotation (and loss) depends on the locations of the two stations on the surface of the Earth.

Assessing loss

Geometric polarisation rotation and Faraday rotation both act on the wave. Their effects may add or may subtract, making loss somewhat greater or somewhat lesser.

Various radio amateurs have attempted to model the effects of the two, calculating the geometry and estimating the Faraday rotation to determine the net loss. Most EME enthusiasts will say that they have experienced rotation. Rotation can be claimed present when a path that should work fails, but it’s hugely difficult to be sure about the effects experienced. Other path phenomena may cause the failure, confusing the conclusion.

Some radio amateurs implement switchable or variable rotation. If they experience an unworkable path, they will switch or rotate their polarisation to overcome the effects. Again, other path phenomena may cause the failure, confusing the conclusion. Others simply suggest that if the path fails, find another station to attempt contact with.