|[AMRadio] Big Vertical on 75|
bbruhns at erols.com
Thu Mar 8 11:08:47 EST 2007
Most short-term fading on HF is caused by polarization rotation of the incoming signal. Because of effects caused by the earth's
magnetic field, the ionosphere twists signal polarization to greatly varying degrees, so the main incoming signal goes through
cross-polarization nulls on the receive antenna at times, and that's when it fades. When that happens, weaker signal paths become
significant, and the phase shift caused by the path-time differences causes severe selective fading.
A circularly-polarized antenna reduces this problem significantly. However, the incoming signal tends to have an elliptical or
circular component, so the correct circular 'sense' is important - and this changes, depending on signal path and time of day.
Even so, simply connecting a vertical and a horizontal antenna together will often result in noticeably reduced fading.
See "CIRCULAR POLARIZATION ON HF"
AM Press/Exchange, Issue #116, December, 1995
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