Ron:
See some interposed comments.
73,
Steve,
AA4AK
At 04:05 PM 3/3/2007, Ron D'Eau Claire wrote:
I've not tried modeling 128 radials, or at that low of an elevation angle
but your results sound good, Stephen.
Remember there are two sources of ground losses in verticals, near field and
far field losses.
**************
NEC takes both into account. In fact, NEC lets you observe both the
near field and far field radiation separately. This turns out to by
highly useful if you're trying to troubleshoot your neighbor's TVI problems.
***************
Near field currents are those produced at the base of the antenna. Even
fence wire is a vastly superior conductor to the wettest soil. The more
radials dividing up that current, the less is left to "warm the earthworms".
That results in more antenna current and more radiation. Most of us Hams
focus on minimizing these losses because we can't do anything about
far-field losses, but they are very significant. The other way we reduce
lossy ground currents is to elevate the antenna and radials.
************
It is important to appreciate that these are two fundamentally
different phenomena. With a radial system on the ground, you're
trying to use the induced ground image as the second half of the
antenna. Resonance of the radials is not critical. The principle is
essentially that the more wire you have on the ground, the lower the
effective ground losses.
In the case of an elevated ground plane, the resonant radials serve
to isolate you from the ground and its losses. The missing half of
the vertical element is the effect of the resonant radials rather
than the lossy ground. To get effective isolation from the ground,
you need either higher elevation or more radials. That is why roof
mounted CB antennas with four resonant radials are so effective; as a
fraction of wavelength, they are high off the ground. On 80 meters,
putting the ground plane at 15 feet elevation and using as few as 6
resonant radials yields surprisingly good results.
As I'm sure you know, you can tell when the vertical antenna is
performing better; the SWR goes up. A lossy vertical will have a low
SWR because the high ground losses are in series with the radiation
resistance and the sum comes out perversely close to 50 Ohms. A low
loss vertical is around 30 Ohms.
************
The ground
currents are induced currents, so doubling the distance between the radials
and the earth reduces the induced currents by 75% assuming the same number
of radials.
Far field losses occur out a distance of wavelengths from a vertical antenna
where currents induced in the lossy earth by the electromagnetic wave
decreases the signal at low elevations. That's why a vertical shows sharply
reduced levels below about 15 degrees above the horizon, under the best of
conditions. A four radial configuration shows the major lobe at about 20
degrees above the horizon. That's a limitation we all have to live with.
***********
Although the major lobe peaks out at 20-25 degrees, there is still
finite energy radiated at 6-10 degrees. On really long haul
communications, it is that weak but finite low angle energy that
propagates long distances. The higher energy starts out stronger, but
makes more hops over a long path, and each ground reflection,
especially on dry land, is extremely lossy.
The lossy ground bounces matter. From Maine, I find that Hawaii on
QRP (half the path is land and half is water) is a chip shot. Alaska,
which is a thousand miles closer but entirely over land is virtually
(but nor completely) impossible to work on QRP.
***********
What varies most as the height is changed with a four-radial configuration
is the overall gain as the induced grounds currents and losses decrease with
height. With the more common 4 radial configuration, a near-the-ground
ground plane antenna with the radials 10 feet up (to clear heads walking
under them) will show just about 0 dBi at a 22 degree elevation above the
horizon.
That's why a horizontal dipole is usually preferred to a vertical if there's
sufficient space to erect it.
***************
That's the gotcha. If you have a horizontal dipole at the same height
as the top of a vertical dipole, in the broadside direction the
horizontal wins hands down, provided you have two supports high
enough to support the dipole. The current loop of the horizontal is
twice as high as the current loop of the vertical.
Of course, the vertical dipole has its current loop much higher than
a ground mounted vertical or the typical elevated ground plane, and
so will be the better performer.
Of course, if you're going to implement a full size vertical dipole
at frequencies below 14 MHz you need a really tall tree.
***************
The change in orientation turns the ground
into a reflector rather than absorbing so much RF current. It's a lossy
reflector to be sure, but it's still effective. Even at a modest 30 foot
height, a 40 meter dipole will show a gain of about 1.3 dBi at a 20 degree
angle above the horizon, roughly the same as the vertical, with the bonus of
a huge high-angle lobe produced by the ground reflection that the vertical
lacks, giving superior short-skip performance.
And, of course, those lucky Hams who can put their horizontal dipole up
about 1/2 wavelength where it works best get a huge advantage. At 20 degrees
it shows nearly 6 dB gain: equivalent to multiplying the transmitter power
by four times!
**********
All true.
**********
But most of us live with Marconi's problem, especially on the lower bands.
Even if Marconi had understood the Hertz (dipole) antenna, for his
transmitters operating near 100 kHz he'd have needed to string up 4,680 feet
of horizontal wire at a height of over 4,900 feet to achieve optimum
results. So he stayed with his tiny (in terms of wavelengths) top-loaded
verticals with the best ground system he could devise and still got out well
enough to prove that "wireless" worked and worked quite well. In the same
way, those Hams who live without space for a decent horizontal radiator use
verticals, some of them quite small, and continue to prove that we can still
get out and work the world when conditions are right.
By the way, I really admire Force 12's various comments about verticals. The
readily agree they are a compromise between size and performance, and they
note that their spectacular DX performance has nearly always been achieved
on a beach at some rare DX site. Being on the edge of salt water reduces the
far-field losses a great deal, and the signal the antenna is radiating is a
rare DX call that attracts anyone who can hear it!
***************
What I especially admire about Force 12 is that they've engineered
their antennas to give real-world results that are very close to the
idealized results predicted by theory.
The enhancements of a seaside location and the "DX effect" are
frosting on the cake.
If they really want record breaking contest results, they should use
a rare DX call, a seaside location and female operators on SSB. The
female voice seems to give you the same effect as another 6 dB in
transmitting power. We have a "go for blood" contesting group here in
Maine that deliberately schedules as many female operators as
possible on Field Day to run up their score; it really works. Of
course this requires YLs who do not mind insects and outdoor
plumbing, and those are few and far between.
***************
It's no wonder that shipboard systems using the old 600 meter (about 400 -
500 kHz) marine band often logged large distances in spite of their tiny
antennas. A shipboard antenna might be 200 feet long, but at 450 kHz that's
hardly bigger than a mobile whip on 40 meters! The advantage they had was
the world's best ground system for both near and far fields surrounding the
ship in the middle of a salt water ocean.
Ron AC7AC
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