Topband: Polarity and Phase - fading

[Eric Scace K3NA]

   (At the risk of adding gasoline to the fire...) I wanted to highlight a distinction that Tom W8JI made in an earlier post.

   Tom mentioned that, while using two phase-locked receivers attached to separate antennas many wavelengths apart, he did not
usually observe a significant reduction in long-term fading.  This highlights two families of mechanisms that cause fading:

a) multi-path propagation effects: Here two (or more) copies of the transmitted signal arrive by significantly different paths.
These interfere constructively or destructively at the receive antenna, depending on their relative phase at the instant of arrival.
(On FSK, this situation can result in the mark signal and space signal fading independently, since they are at slightly different
frequencies and the constructive/destructive interference will be different for each signal.)  When the path lengths wobble due to
short-term irregularities in the ionosphere, the combination keeps shifting between constructive and destructive, and short-term
fading occurs.  A separate receiving antenna several wavelengths away from the first will often receive these signals in a different
phase relationship, improving the likelihood that at least one of the two antennas has a copyable signal -- the heart of diversity
reception.  The fading cycle generally is in the order of seconds or tens of seconds.

b) single path propagation effects:  Fades here occur because ionization is not uniform at the small scale in the ionosphere; "small
scale" here means up to a few tens of kilometers.  The small scale structure of the ionosphere includes tall bubbles or clouds of
ionization that drift along magnetic field lines.  At one moment a bubble (or lack of a bubble) may cause a temporary focusing of
rays passing through it towards you -- to the benefit of your receiver.  A few minutes later (typically 3-8 minutes) the bubbles
have shifted locations and rays are no longer focused, or are even de-focused, to the detriment of your receiver.  A second
receiving antenna a few wavelengths away at the receiving location will still be influenced by the same bubble affecting propagation
to the first receiving antenna -- hence, no apparent difference in this longer term fading behavior and no benefit from the second
antenna against this phenomena.  But someone else 20 km away from your site will see a different timing for this
focusing/de-focusing.  This "patchy" behavior is especially noticeable when the band is barely open for the signal you're trying to
copy, and a station will be audible for a bit, then disappear, and then re-appear again a few minutes later; we don't pay much
attention to it when signals are louder.

-- Eric K3NA