Hi Tom,
what you say it's a bit pessimistic in numbers but fundamentally true
because any electrical antenna, expecially when shorted, suffers a great
amount of extra losses if close to other objects (proximity) or it's not
normal to the ground or to other things. Knowing it, the best to do is to
minimize inherent antenna losses meanwhile trying to avoid extra losses.
It's amazing to measure, but with large antennas (where it's not possible to
use an anechoic chamber) I'm always a bit skeptical about results validity.
it's very possible to be misleaded mesuring something else than wanted
(couplings, reflections), mixing the fields or doing mesures at useless
angles of radiation.
Nowadays I definitely find the join of logic, experience, long term reports
and computer modeling at least valid as very extensive measures done by
RF professionals (a kind on the way to become extinct).
My direct experience on loaded elevated counterpoises, anyway, does not
match that of W8XO you reported, and I doubt if that installation had
the requirements to benefit of a loaded counterpoise.
On my house roof top (65ft) I've a 38 ft tower with an HF yagi on the top.
The structure has two separate shunt feeds, one for 80 (gamma) and one for
160m (omega) with distinct loadaed counterpoises. When resonating the
counterpoises the ammeter current reading doubles on 80m and the far field
increases proportionally. On 160m, resonating the counterpoise increases 1.4
times the current and it's hard to notice a field increase, but surely it
doesn't decrease. On both cases the counterpoises set the point were they
are connected as the lowest voltage with all the inherent benefits.
Coming back to the Vertical antennas efficiency evaluation, this is not an
easy argument. It's harder when considering the environment and even less
understable when concepts worth for MW broadcast are blindly applied to ham
radio and the low part of SW spectrum.
Basically, in a nominal ground plane the missing half of the antenna
(and consequentely the current return path) is the ground itself.
A better ground conductivity area leads to bigger efficiency, expecially at
very low angles and maximally for ground wave.
This explains well why the location has a fundamental importance on this
type of antenna, and why commercial MW transmitter are preferably placed
in flat high conductivity soil areas rather than on rocky mountain tops.
Any Ham who experienced a vertical antenna close to the sea shore know
very well what's the huge difference in performance of the same vertical in
a different place.
Above an imperfect ground, the way to recover a ground plane antenna
efficiency is to minimize ground losses creating an artificial ground with
an extensive set of radials, but, unless the set of radials extend much over
a practical number and reasonable lenght for most of the people, on a bad
soil there won't be that efficiency at low angles and for ground wave.
When using a vertical radiator over a properly elevated counterpoise, the
situation is quite a bit different than in a real ground plane, approaching
rather that of an half wave vertical dipole over ground.
Again, even if the antenna is (electrically) center feed and apprently does
not utilize the ground as a part of itself (but the effect of image antenna
exist), the soil type and the antenna height from ground (pseudo brewster
and phase shifts for the purists) determine the amount of low angle
radiation and the efficiency at various elevations.
On the contrary of horizontally polarized antennas where up to extra 6 dB
gain are achieved at some angles (at price of nulls), elevating a vertical
antenna reduces low angle radiation although proximity losses decrease.
Now, the question allowing or not an elevated vertical use with shortened
counterpoises is how much ground wave and very low angles are really needed
in a typical ham radio activity.
If it's true that in the high part of the SW spectrum one degree elevation
is not so rare (looking at angles statistics with an ham perspective) and
any horizontal antenna can easily outperform most of the verticals, between
160 and 80 meters an antenna with a good efficiency at very low angles and
for ground wave is anyway useless in the very most of the DX cases.
If possible then, why not elevating a counterpoise were typical electrical
(intended here as opposed to magnetical) antenna external losses are kept
minimized ?
Not necessarily a few unity dB losses are a bad compromise for a vertical
antenna with loaded elevated counterpoise. A full size (worse if loaded)
horizontal dipole but too close to the ground can be 10-30 dB less efficient
at DX elevations on 80 and 160 meters making the "lossy" vertical a much
better performer that's still somehow competitive with the nicest
installations.
Not bad for who hasn't the area for a conventional ground plane.
73,
Mauri I4JMY
-----Messaggio originale-----
Da: Tom Rauch <w8ji@contesting.com>
A: towertalk@contesting.com <towertalk@contesting.com>; R. Sigismonti
<n3rs@cynet.net>; Maurizio Panicara <i4jmy@iol.it>
Data: lunedì 19 luglio 1999 22.16
Oggetto: Re: R: [TowerTalk] PVC & Losses
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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