Last Updated on March 16, 2025 by John Berry
Faraday rotation can cause loss in the EME link because the transmitted wave can suffer polarisation distortion. It’s caused by the Faraday effect – polarisation change as a wave propagates an ionised medium while under the influence of a magnetic field. The effect is path geometry and frequency dependent. It’s also dependent on the strength of the Earth’s magnetic field and the ionosphere’s electron density over the paths.
It’s fascinating to watch Faraday rotation at work: a QSO that typically reports -12dB one way and -6dB the other degraded to -12dB and -20dB respectively. The ‘go’ path is unaffected, but the return is decimated. And the difference in events? In the degraded case, it was moonrise at the distant station. Read on for why.
Polarisation
Polarisation is set by the electric field vector of the wave.
I discuss polarisation distortion on an adjacent page. It’s best that a wave launched as horizontally polarised is received by a horizontally polarised antenna. If the wave starts as horizontally polarised, and suffers polarisation change along the path, loss will occur. That loss is anything from a few dB to over 30dB. The loss depends on the characteristics of the antennas and the angle of polarisation difference.
Polarisation in the non-ionised normal troposphere will be maintained. A wave launched there by a horizontally polarised antenna will propagate as a horizontally polarised wave. Hence it will be optimally received by a horizontally polarised antenna. It’s in ionised media that distortion occurs.
Polarisation in ionised media
Polarisation of a wave transiting ionised media may not necessary be maintained. An example is propagation in the ionosphere. Polarisation distortion in the ionosphere is an advantage at HF. There, hams randomly use vertically and horizontally polarised antennas and rely on hectic polarisation distortion. The aggregated received signal over many paths contains both – even many – polarisations so something is always received.
Polarisation may be distorted when a wave propagates through other ionised media such as at VHF through meteor trails and auroral columns. And polarisation may be distorted as a wave heads for the Moon during EME transmission. In all cases, whether the distortion is significant depends on the path geometry – and the strength of the Earth’s magnetic field where the propagation occurs.
Faraday rotation and EME
The diagram below shows a model of the scenario when a wave is launched vertically polarised from a station on Earth, heading into the page towards the Moon (behind).
The electric field vector of the transmitted wave is shown vertical as it leaves the surface of the Earth. As the wave passes through the ionosphere, it is acted on by the Earth’s magnetic field. The Earth’s magnetic field effectively acts to bring the wave in line. The longer the wave spends in the ionosphere, the nearer the polarisation distortion angle comes to ninety degrees.
This last point is significant. Faraday rotation diminishes as the geometry departs from that shown. If the launch angle is higher, the time spent by the wave in the ionosphere is correspondingly lower, and hence the effect is low. For higher launch angles, the distortion angle is then low, and in decibels, the losses are smaller.
Faraday rotation is greatest when the direction of propagation is in line with the Earth’s magnetic field – that is to say from south to north.
Return path
The reflections from the Moon are described as specular or semi-specular and polarisation is maintained when aggregated. So, the polarisation of the wave heading to the receiver back on Earth is the same as that incident on the Moon’s surface.
If the wave returns to about the same location on the surface of the Earth, passing through the same ionosphere, the wave’s and Earth’s magnetic fields will be co-linear. There will therefore be no further distortion.
On the other hand, a wave launched vertical, may be rotated to horizontal and returned horizontal. Not good for fixed polarisation EME communications.
Whether the wave is vertical, horizontal or something else when returned elsewhere on the Earth’s surface depends also on basic geometry. I discuss this elsewhere.
Dependencies
Faraday rotation varies with frequency – worst on 144MHz, moderate on 432MHz and least on 1296MHz and above. I’ll post more here once I conclude more on that. It also varies with the ionosphere’s electron density, and hence varies with sunspot activity and propagation anomalies.