Hi Frank (and Rick)
Somewhere I have a map of the lines of geomagnetic latitude superimposed on a
Mercator projection of the world, but I can’t find it right now. Unlike the
ruler-straight lines of conventional latitude, geomagnetic latitude lines
wander across the world like a collection of snake tracks.
As a result of how geomagnetic latitude snakes across the globe, a comparison
can’t be directly made between similar geomagnetic latitudes in the northern
and southern hemispheres – where Tom W8JI lives is probably very different to
me in terms of the closeness of his geomagnetic latitude to the electron
gyro-frequency. As Carl K9LA points out, the geomagnetic latitude relates to
polarization and involves the ordinary and extraordinary waves that propagate
through the ionosphere, and how 160m is affected by being close to the electron
gyro-frequency.
About 10 to 15 years ago, Carl, Nick Hall-Patch VE7DXR and Bob NM7M (SK) (also
a physicist like Carl, as I’m sure you recall) helped Mike VK6HD (SK) and I to
understand why our horizontal cloud-warmers outperformed efficient vertical
antenna systems in SW WA.
You are quite correct, the Fresnel zone where I live (the mostly far field
region where ground gain is developed) has very poor conductivity. And, to
repeat your point as this is not as widely known as it should be, poor Fresnel
Zone conductivity has very little impact on the performance of horizontally
polarized antennas, while having a major impact on vertically polarised ones.
While the Fresnel (far field) zone of my location, is basically rock (granite
and ‘coffee rock’), Mike’s final location beside the Kalgan estuary appeared to
have much better Fresnel zone conductivity, with less rock than me and, in
around half the compass directions, salt water. However, his inverted-L with
an 80’ vertical section over 120 buried quarter-wave radials at Kalgan
performed only marginally better than our previous attempts at vertical antenna
systems did.
On this basis, I came to the conclusion that the dominant problem was likely to
be the geomagnetic latitude issue, rather than poor conductivity in the Fresnel
zone – which it certainly is also an issue here.
To investigate this further, I sought out the opportunity to operate directly
by the sea here with a good vertical antenna. After much paperwork, I managed
to get permission to operation from the Cape Leeuwin lighthouse, which is
40m-plus high and on a narrow finger of land surrounded by sea for over 300
degrees.
In a Stew Perry TBDC in the early 2000s, with the assistance of my friend Phil
VK6PH, we put up a full-sized quarter-wave wire vertical on the most seaward
side of the lighthouse, less than 60 metres from the sea. This was fed against
a quarter wave counterpoise and the feeder decoupled with a large ferrite choke
to stop common mode effects. On the other side of the lighthouse was an
inverted vee half-wave dipole. Both antennas were supported from the lighthouse
balcony (at about 40m!) and detuned when not in use. An Yaesu FT-1000MP was
used, running less than 100W
Unfortunately conditions were poor during our evening time into North America,
but at about three hours before sunrise the 160m band opened into Europe.
Right from this point, the vertical was slightly down on the inverted vee by a
few dB, but I would always call on the vertical first and then switch onto the
inverted vee if I got no response. All the way until just after sunrise, the
inverted vee outperformed the vertical, mostly raising the stations who did not
hear us on the vertical.
The only time this situation was reversed was when 160m started to go out as
the sun started to rise and I had by then switched over to just calling
stations on the inverted vee.
After about five minutes of this, the Europeans I could still hear were not
coming back to me anymore. Out of curiosity, I switched to the vertical – and
found I could still raise a few of them. I recall vividly the last QSO with a
CT1 using the vertical about 20 minutes after sunrise, exchanging 559 reports.
The crazy thing is that the vertical appeared to be doing exactly what a dipole
is known for doing on 160m in the northern hemisphere in some cases – extending
the sunrise opening. However, this was the only time the vertical outperformed
the inverted vee.
As far as I know, Mike VK6HD never experienced this phenomenon when he was
comparing his inverted-L quarter wave antenna against his inverted vee dipole.
However, my vertical antenna was directly adjacent to the sea, surrounded by
sea, which may have helped.
The final event was highly interesting, but did not sway me into repeating the
experiment the following year when I also operated from the lighthouse in the
Stew Perry TBDC.
The fact was the inverted vee had been responsible for 80 to 90 per cent of my
QSOs - can’t remember exactly how many – while the vertical had only accounted
for three or four.
Mike VK6HD, Phil VK6GX and I are not the only ones to have experienced the
“verticals aren’t always best for DX” situation here. About five to ten years
ago, I understand a group of German DXers came here and operated in the CQ WW
CW (I think).
The group operated from the the Northern Corridor superstation VK6ANC/VK6NC,
using a quarter wave vertical on 160m. After disappointing results, one of the
ops (Mar DL3DXX, I think) recalled Mike, Phil and I used inverted vees at 90 to
110’ and suspended a inverted vee dipole as high as they could and changed over
to using this. My understanding is then they found they could work a much
larger amount of DX stations on 160m.
Vy 73
Steve, VK6VZ
From: donovanf@starpower.net
Sent: Friday, November 23, 2018 12:56 AM
To: Topband reflector
Subject: Re: Topband: Vertical antennas aren't always best for DX everywhere -
the facts
Hi Rick and Steve,
Steve's QTH is almost directly north of the south geomagnetic pole.
His latitude is approximately 32 degrees south geographic latitude
and approximately 43 degrees south geomagnetic latitude. His QTH
is at approximately the same geomagnetic latitude as the Georgia
in the northern hemisphere.
http://sdnet.thayer.dartmouth.edu/aacgm/aacgm_calc.php#AACGM
I can't comment on the actual horizontal vs. vertical polarization
experience of topband operators in the Georgia.; however, W8JI lives
in Georgia and his experience with a very high horizontally polarized
dipole was mostly unfavorable compared to his 4-square vertical
array. Georgia probably has reasonably good soil conductivity.
My suspicion is that the soil in the Fresnel zone of Steve's vertical
antennas (the mostly far field region where ground gain is developed)
has very poor conductivity. Poor Fresnel Zone conductivity has very
little impact on the performance of horizontally polarized antennas.
AM broadcast antenna engineers who have worked in VK6 may have
some experience with soil conductivity impacts on the effectiveness
of AM broadcasting antennas in that area.
73
Frank
W3LPL
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