Hi Mauri,
> A coil loaded elevated shorter radial system, intended as a counterpoise
> rather than a ground plane, can still offer a reasonable antenna
> efficiency.
Not according to FS measurements I made.
While the results aren't cast in stone, all measurements need to be
verified by other actual measurements, the poorest radial system I
tested was short loaded radials even though I used very high Q
coils and carefully resonated the system.
That also agrees with conventional wisdom on loss mechanisms
near antennas, because short coil loaded radials concentrate the
electric field in a tiny area, where the field gradient is very very high
between the radials and lossy earth below the radials.
The job of radials is to spread the induction fields (induction fields
include both electric and magnetic fields) around and NOT
concentrate them in one area.
Loaded radials would be reasonably effective if loading system
losses were low IF the antenna was far removed from earth. But
they still cause one additional problem.
There is a large phase shift across the loading inductor, and so the
voltage across the inductor can be extremely high. That's true even
if current through the inductor remains equal at each end.
The large voltage appearing at the radial near the inductor will
cause a strong electric field between the radial and the bottom area
of the antenna. That increases current in the bottom of the antenna
and current through the inductor, while decreasing current in the
antenna up away from the radials. The effect of loading radials is
the same as making the antenna shorter, not just the radial.
My friend K8BBI (W8XO now) ran into this on 160. He installed an
elevated loaded counterpoise under his vertical antenna, and saw
base current in the vertical increase. At the same time that
happened, his overall field strength decreased.
I also can create the effect in my mobile, making base resistance
decrease and current increase. Yet when I measure field strength it
goes down.
One has to be very careful about measuring the wrong thing or
measuring the wrong way, because more base current and a lower
base impedance is almost always assumed to mean more field
strength. There are many cases, like this one being discussed,
where it does not.
Unavoidable gap of an efficient antenna of this type is a very
> narrow bandwidth when the counterpoise is short and the loading is big.
> The required conditions to save the efficiency are low losses along
> antenna elements (loading device's Q must increase with bigger loadings,
> so it's better not to exceed in shortening the counterpoise) and a
> minimized ground losses. In the case where coils are used (better one for
> each radial) to resonate a counterpoise, they must be size proportioned to
> the used power, must have an high Q and beeing preferably placed far from
> the antenna highest current points.
True enough.
It's useful to remember that a
> properly elevated counterpoise made of only 2 opposite radials is already
> enough to generate the classic omnidirectional pattern of a vertical
> antenna and with limited losses compared to a classic ground plane
> antenna.
While that is true, the key words often forgotten are "properly
elevated counterpoise".
A "properly elevated counterpoise" would be one at least 1/2 to 1
wavelength above earth. Most people can't do that. The more
radials and the longer radials used, the closer the antenna can be
placed to earth without ill effect.
I measured about 8-10 dB of loss with four 1/8 wl coil loaded
radials compared to 60 1/4 wl radials on a 1/4 wl vertical on 80
meters. In my opinion, that is too much loss.
There is no evidence that, when the antenna is placed close to
earth, a few loaded radials work efficiently. Every bit of measured
data points the other way. I've seen that with "radial-less" verticals,
both tall tower on the BC band through 80 meters, and at upper HF
(antennas like the R7, the Ameritron Patriot antenna, and so on).
If all you can put up is a loaded, sparse, or short radial vertical it
had better be a long distance above earth and other lossy
media...or you better install it over salt water.
73, Tom W8JI
w8ji@contesting.com
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