Topband: March LBM Article on Polarization

[Robert Brown]

On Sun, 5 May 2002, Bill Tippett wrote:

> Hello All!
> 
>         W1FV's recent reference to K9LA's article in the March 2002
> Low Band Monitor caused me to take another look at it.  Carl shows 
> azimuthal plots of vertical antenna patterns (with the Earth's magnetic 
> field taken into account) from VK6VZ, W8JI, IV3PRK and D44 (Cape Verde 
> Islands).  The first 3 are essentially omnidirectional but Figure 4 
> shows a dip of -10 dB for E/W directions from D44.  This would imply 
> that NA would be a poor direction for anyone using a vertical from D44 
> (true bearing 300 degrees to W4 from D44).
> 
>         But here's where I have an "anecdotal" problem...  Do you all
> remember the 160 signals of C56CW/C56DX in March 1996?  These guys had 
> a tremendous signal all across NA using the Titanex vertical.  In fact
> I recall that this was one of the first expeditions to use it and I 
> started asking around to find out what it was because their signal was 
> so remarkable!  C56 is only 900 km East of D44 and I would imagine that
> any polarization effects would be quite similar.


Friends in Radio Land -
 
     I don't see the problem that Bill, W4ZV, cites in making
reference to K9LA's figure for D44 in the March 2002 issue of LBM
and the strong signals from nearby C56CW/C56DX in March '96.
 
     Those two sites differ somewhat geomagnetically, the earth's
field at Cape Verde Islands points 12.5 degs to the west and
down into the ground at an angle of 15.8 degs from the horizontal
direction.  In contrast, for the Gambia, the field points 9.5 degs
to the west and into the ground at an angle of 6.4 degs from the
horizontal direction.  The field strengths were very similar,
32170 nT for the Gambia and 32,777 nT at the Cape Verde Islands.
 
     For vertical antennas at low latitudes, one expects power
coupling losses of about - 3 dB in the northern and southern
directions.  Calculations with those field parameters yield
-2.7 dB and -2.4 dB, respectively, to the north from the Gambia
and Cape Verde Islands; to the south, the results were -2.9 dB
and -3.0 dB, respectively.
 
     The azimuthal patterns for the power coupling show very
sharp, deep "notches" in the western direction (corrected for the
declinations cited above): -12.5 dB for the Gambia (C5) and-10.6
dB for the Cape Verde Islands (D4).  But those directions are
about 50-60 degrees south of the heading for W4ZV's QTH and others
in the USA.  The power coupling losses for directions to the USA
are -4.0 dB and -3.8 dB, respectively, about 1.3-1.4 dB more than
for the most favorable directions, to the north and south.  Such
differences are only a small fraction of a S-unit.
 
     All in all, it would seem that the strong signals from
C56CW/C56DX were the result of using an excellent antenna and did
not suffer due to propagation in the direction of large power
coupling losses.  The polarization effects are quite similar for
C5 and D4, as W4ZV suspected, but there seems little discrepancy
between theory and experiment.
 
73,
 
Bob, NM7M