Hi Carl,
> Tom...Rauch that is...I'm getting confused...again.
> Are you saying that a vertical with X number of elevated radials is 6dB
> below the same radiator with X number of on ground radials?
With four carefully pruned resonant radials elevated six feet
(measured on 80 meters), the field strength was about 4 or so dB down
from 64 radials, either elevated or on the ground. That was MEASURED
at one site with one antenna and one meter, not modeled by NEC. The
only change was in the radials themselves, and it was read on a
meter.... not a computer.
> Or lets try it this way....a ground mounted radial system of say 64
> radials would need how many elevated radials at 20' elevation to have the
> same efficiency?
According to my measurements, if the height was 12 feet on 160, it
would take about 20 or 30 elevated radials to equal 64 on the
ground. That's a good savings, but not quite the magic of being able
to use only four.
> If I understand you correctly the work of Chrisman...with top university
> credentials...is not to be believed??
Hmmm, we better defer than to Kenneth Starr. I believe he has data
that compares top university credentials to straight speaking.
Seriously, Chrisman's published articles were all models NOT
measurements (although he uses the word "measured" in the text, if
you read the opening text it plainly states it is all based on a
model). I have no reason to doubt the computer said exactly what he
wrote.
Now ask me if I trust the computer, and I'll say not completely.
Hagan and Barker made VERY detailed measurements of low dipoles by
using probes hung from helicopters. When the very same dipoles are
modeled on NEC-2, and if the height is low, the program is off about
5 dB. IMO, if the computer can't handle a low dipole, it sure can't
handle a low radial.
> My own experience is that the initial full size radiator here with 4
> elevated radials 20' high measured right at 37 or so Ohms on a known
> accuracy noise bridge...I cant measure in 1 Ohm steps but it was midway
> between 35 and 40 Ohms. Based upon K1ZM suggestions I added more radials
> and stopped at 12....I could not see a measurable difference on the
> bridge.....nor in apparent performance.
It's a common but false perception that FS stops changing when base
impedance stabilizes or if it reaches "X" ohms Carl, especially
when the radials are part of a resonant system.
Let me give two examples.
Dave (W8XO) was puzzeled why he lost FS when he installed and
resonated a two radial system to augment his ground system, since
base impedance dropped and base current in the radiator increased.
I built a ten meter groundplane and shortened and loaded the radials,
to simulate Dave's system. What I found was the current near the base
of the antenna rose, while the current further up in the radiator
dropped, when the radials were shortened and resonated with inductors
(like Dave did).
You can have Kenneth Starr call Dave and "check this out".
The reason for this is the electric field between the radial and the
lower area of the antenna increases, because the voltage on the
radial near the antenna increases. This "short circuits" the path to
the upper area of the radiator and robs the upper part of the antenna
of current.
Yet things at the base of the antenna "looked like" they got better.
The impedance was lower, the current in the base of the radiator was
higher. Only the FS went the wrong way!
Second, when measuring my mobile antenna I noticed the base impedance
of the antenna was about 26 ohms with a five foot hat twelve feet
above the vehicle. The coil loss resistance was about 8 ohms, so I
assumed the ground loss resistance was about 18 ohms. That was when I
believed base resistance meant something useful.
Fooling around, I discovered by lowering the hat I could get the
base resistance down to 12 ohms! Base current went up. Yet the field
strength dropped almost two dB. I stopped believing.
If you look in the ARRL Antenna Handbook, they claim the resistance
of a car at HF is about two ohms! I suspect that is because someone
measured the base resistance like I did, and used that value to
"guess" the real ground loss resistance. Let's face it... if a car's
ground loss resistance was really two ohms, I'd buy a junk car and
mount my tower on the roof. If a loaded radial worked magic I'd be
using Texas Bugcatchers for all my grounds. The heck with radials.
> I went to the elevated radials at this QTH since I did not want to go
> thru the hassle of 25,000 feet of radials that I had at the old QTH. The
> system here works very well and the system is now 2 verticals. With only
> 1200W...QRP as someone mentioned....I rarely have a long wait or miss a
> new one.
Nor do I. Even when I run 100 watts. During the DX contest, I ran 100
watts and got answers mostly on the very first call. As a matter of
fact the only time I miss not having a big PA was with the 9M0, or
during afternoons when I hear the Europeans ragchewing and they can't
hear me (or when calling EA3JE, hi).
In spite of this, I have no idea if I am one dB down from ideal
or ten dB. I certainly have no idea how my antenna efficiency
compares to my antenna system in Conyers three months ago,
or how it compares to my signal when I lived in Ohio ten years ago.
Any claim I would make would be a gut feeling, not a fact.
On the other hand I measured a system (four elevated were down about
5 dB from ideal on 80 meters), I know as a fact what was measured at
WVNJ AM (six elevated up ~30 ft on each tower were about 7 dB from
ideal on a four tower array), and I suspect what N7CL says the
military measured (about six dB) is true, and that other people who
have tried both systems on ONE antenna say they notice an
improvement.
Myself, I'll go with a few actual direct comparisons over computer
model "measurements" or loosely applied "FCC proof estimates"
any day.
I'm hoping to do some tests on 160 this spring, as I add a second
tower in an empty pasture far from the present antennas. I'll let you
know when and if I do that test.
73, Tom W8JI
w8ji.tom@MCIONE.com
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