First, absolutely the least reliable model is one used when
a wire is parallel with and close to earth. That's pretty
well established.
Second, most of the work available to us simply involves
models. Almost no one actually built an antenna and measured
FS with only a radial change. People use a model that is
known to not be very reliable (with 5dB or more), and
publish results to the tenth of a dB.
Third, models like Eznec and so on don't even calculate
groundwave. The programs assume the model is over infinite
flat earth and FS is taken a few hundred WL (or whatever)
from the antenna. The signal really isn't zero at zero
degrees at reasonable distances.
> 1) In your opinion where is the cutoff between "elevated
> radials" and a "ground plane" system. How high off the
> ground does the system need to be before it becomes a GP
> rather than an "elevated radial" system?
It won't be a cutoff, it'll always be a very gradual
transition. The wires wouldn't really be divorced from earth
below the antenna until the earth is much further away than
the area of the groundplane. It's a geometry thing that
depends on wavelength and occupied area.
The only actual measurements I've seen were at a BC station
in Brazil. They showed 8 elevated radials 1/4 wl above
ground were just ever so slightly weaker than a conventional
system on the ground. GAP actually was handing out a copy of
that paper.
I measured two systems here where I compared low radials to
ground mounted radials on 3.5MHz. The resonant radials were
perfectly horizontal. Looking at old data the results were:
elevated radials (6-8 feet high)
4 wires -4.3 dB 20.1 mV
8 wires -2.4 dB 25 mV
16 wires -1.2 dB 28.7 mV
60 wires -.1 dB 32.5 mV
ground mounted radials
4 wires -5.5 dB 17.5 mV
8 wires -2.8 dB 24 mV
16 wires -1.3 dB 28.4 mV
60 wires (used as 0 dB reference) 33 mV.
I repeated the basic test using a Rohn 45 insulated
tower ground mounted and sloped the radials up to 10 feet.
Results
were almost the same, but I saw a bigger difference in the
advantage
of elevated radials when only 4 wires were used. However, at
16
radials the ground mounted radials were a tiny bit better.
WVNJ showed just under 5dB average change when we went from
six elevated (~25 ft high) radials to a conventional system.
In some directions and distances the change was as little as
2dB or so.
> 2) How does the efficiency of 50-60 on-the-ground radials
> compare to a reasonable number of radials (insert your
> number of radials required here) that are installed under
> the feedpoint of a vertical when the feedpoint is let's
say
> a 1/4 wave off the ground.
Never measured that. 60 radials is obviously getting very
close to 100% efficiency so efficiency can't be a
consideration. I know my 5BTV mounted right above a 20 meter
5 element yagi at 150 feet AGL was almost always less than
10dB weaker than the Yagi. Unfortunately I didn't have one
at ground level at the same time to compare.
I'm sure results would vary a great deal depending on
clutter around the ground mounted antenna and soil
conductivity. Living on the ocean or in a salt marsh or
open pastures would be different than in a city.
Further, if it
> was practical to install a vertical either on the ground
> with an excellent radial system as you've described or
> elevated a 1/4 wave with the appropriate number of redials
> which would you choose?
I'd use a dipole with the tip of the elevated vertical
supporting it! Seriously, the only measurements I've seen
show a 1/4 wl high GP with eight radials just ever so
slightly weaker than a full system on the ground, and that
was on the AM BC band. I'd expect to not see much difference
between the two unless some absorber or reflector was
cluttering the area around the antenna. I do have an
aversion to needless things hanging in the air. How could I
cleanly install a dipole with all those GP radials
interacting?
This whole thing is really mostly just a matter of the
specific situation.
I won't use an elevated wire system because they are a PITA
for multiple reasons.
73 Tom
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