Michael Keane K1MK wrote:
>Circular polarization represents one extreme case with specific, and
>highly contrived, initial conditions and propagation path with respect
>to the Earth's magnetic field.
>
>There are other equally contrived initial conditions and propagation
>path for which the wave would be purely linearly polarized when exiting
>the ionosphere.
>
>Independent of the degree of polarization conversion that takes place,
>the plane of polarization (the major axis for elliptical polarization)
>of a wave propagating through the ionosphere will be rotated (Faraday
>rotation). And small variations in the ionosphere conditions along the
>path of prorogation are capable of producing the large rotations in the
>plane of polarization that we observe.
>
The plane of polarization is not only rotated, but under some
ionospheric conditions it can also become de-polarized - in other
words, 'smeared out' - by multipath effects.
With a completely depolarized wave, the energy collected by a linearly
polarized antenna will be the same at any angle; and the energy
collected by either left-hand or right-hand circularly polarized
antennas will also be the same. Any kind of plane or circularly
polarized antenna will be 3dB down from its best.
Up at 432MHz, Faraday rotation is quite small and changes very slowly,
on timescales of several hours. When I had a 432MHz moonbounce antenna
with continuously rotatable linear polarization it was possible to
analyze the Faraday and depolarization effects in some detail. With a
quiet ionosphere, the incoming signals had a very pure plane of
polarization, and the cross-polarization nulls at +/-90deg were both
sharp and very deep. But auroral disturbances could completely smear the
plane of polarization so that signals were almost the same strength at
any angle of the antenna.
Faraday rotation is proportional to the inverse cube of the frequency,
so compared with 432MHz, the rate of change at 14MHz will be about
30,000 times greater! That is why the plane of polarization can be heard
to change within seconds at HF. The same also applies to the
depolarization effect - if present, it will be much, much greater at HF.
[...]
>
>> The ability to switch quickly can be an advantage,
>
>Definitely. And even more an advantage (for reception) if that switching
>can be done instantly and automatically as in diversity reception.
>
A Google search for [ k3 diversity audio OR sound OR recording ] will
bring up some nice sound clips from EI6IZ and N1EU.
--
73 from Ian GM3SEK
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