In a private message, I asked Carl K9LA which locations in
the world might have power coupling nulls toward the USA, EU or JA.
Attached is his response:
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Bill,
Sorry for not responding to your question in a timely manner. NCJ is
taking up a bit of my evening time.
I took a look at short great circle paths from points along the dip equator
spaced every 40 degrees of longitude starting at 0W. I noted the headings
to cover NA, EU, and JA.
dip equator bearings from there to
lat/long NA EU JA
10N/0W 300-320 350-020 030-040
3S/40W 310-330 020-040 000-010
13S/80W 320-010 020-050 310-320
6S/120W 350-060 010-050 300-310
0N/160W 040-060 360-030 310-320
6N/160E 040-060 010-030 320-340
8N/120E 050-070 310-340 020-030
9N/80E 340-030 310-340 050-060
9N/40E 310-340 310-360 050-060
Since we're along the dip equator, the coupling loss is infinite at 090 and
270. At 80, 100, 260, and 280 (+/- 10 degrees) it's around 15dB. At 70,
110, 250, and 290 (+/- 20 degrees), it's around 10dB. At 60, 120, 240, and
300 (+/-30 degrees), it's around 7dB.
If I use 7dB as the "acceptable/not acceptable" boundary, then headings
between 60-120 and 240-300 would be designated "problem headings".
Filtering out those lat/long pairs over water leaves 10N/0W (west coast of
central Africa) to NA and 9N/40E (east coast of central Africa) to JA.
I also took a look at 19N/0W, which is 20 degrees north of the dip equator
for the first line in the table above. The coupling loss is now only about
10dB max at 270, but the headings to NA drop to 290-310. This trade off
has the effect of making the coupling loss about the same as the "right on
the dip equator" case.
I think we agree that this coupling loss issue is not a big problem for the
majority of the ham population - but it can be a problem for those on or
near the dip equator. And it seems to me that how much of a problem you
consider it to be depends on your goals - if you're on or near the dip
equator going for 160m Honor Roll or trying to win a major 160m contest,
it's important. Perhaps a good analogy is me getting my 160m DXCC with an
inverted-L with only 3 radials - that allowed me to achieve my goal, so I
didn't care (hypothetically) about more radials to reduce my losses a
couple more dB or putting up a 4-Square. I think what all this points to
is the concept of station improvement to best overcome whatever nature
throws at us.
Carl
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Here's a reference map which might be useful and helpful to
understand the difference between Geographic Latitude and Magnetic
Latitude. They are not the same because Magnetic North is not at the
Geographic North Pole, but at ~81N and 110W which causes the downward
shift you see for North and South America Magnetic Latitudes. Look
at the right side of the map for Magnetic Latitude lines and the left
side for Geographic Latitude lines. VK6VZ will be interested to see
that his Magnetic latitude is farther away from the Magnetic Equator
at ~44S than W8JI at ~43N, and hence is not in an area significantly
affected by power coupling (also noted in K9LA's March LBM article).
http://www.nwra-az.com/ionoscint/maps/maplats.html
Here's another set of maps which allows you to find your exact Magnetic
latitude...choose your continent, move your mouse pointer to your QTH
and then click for your Geographic Coordinates and Magnetic Latitude.
http://www.spaceweather.com/glossary/magneticlatitude.html
I think K9LA fairly well summarized up this issue in the March
Low Band Monitor (edited with his permission):
"These results indicate that most of us (NA, EU, Asia, VK/ZL,
South Africa and extreme South America) should use vertical polarization
because we're at locations with high dip angles (at least high enough
to make the patterns look omnidirectional with low losses). Those at
low latitudes (the equatorial regions), where the dip angle is low,
should consider a horizontal antenna for East-West propagation (i.e.
generally to contact other equatorial regions)."
Although power coupling is not an issue for most of us, having
a good horizontal antenna is still desirable to complement a vertical
when high takeoff angles predominate (sunrise DX, disturbed conditions,
local ragchewing, etc). Equatorial expeditions may also want to consider
a horizontal antenna for working other equatorial stations, but a good
vertical should be their primary antenna for working NA, EU, JA, VK, etc.
73, Bill W4ZV
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