Hi Bill,
I strongly disagree that the early tests and articles meant
very much. First, they deluded Hams into thinking 120
radials are needed to achieve maximum efficiency in an
array. The truth is the efficiency curve of eff vs. number
of radials flattens out in most cases with as few as 40-50
radials.
Second, it wasn't a good test for measuring a variation of a
few dB.
>I am ONE of the people who claim that four elevated radials
>can have
> approximately the same efficiency as 120 buried quarter
> wavelength radials.
The only possible way to make that test with any reasonable
accuracy would be to install four elevated radials over
virgin soil and then remove them and replace them with 60 or
120 radials on the ground while measuring FS at the very
same points. I'll explain why below.
> I have installed such systems at three Standard Broadcast
> stations in the
> United States, and made field strength measurements that,
> when analyzed in
> accordance with FCC procedure, showed that the
> unattenuated field strength
> at one kilometer was essentially the same as the FCC
> criteria for broadcast
> antennas with 120 buried 90 degree radials (Figure 8 of
> Part 73 of the FCC
> Rules).
The FCC test works like this:
1.) A person measures the slope of the FS changealong
straight lines radiating out from the antenna for a
considerable distance.
2.) From that slope a person finds a "best match" to a
graph. This graph then estimates the average conductivity
along that path.
3.) From that estimated conductivity a person then estimates
the expected FS and expresses it normalized to some
standard, like a kilowatt at a kilometer.
The problem is the variables and the human element. There
are many wobbles and variations in the slope of FS curves.
Sometimes even moving just a few feet, a person can make an
array have any efficiency he wants within reason! It's easy
to have 10dB of variation with radial distance with just a
few feet of movement!! So what does the person use?
As an example if we look at the FS measurements Belrose used
in an elevated radial article, the slope of the FS could be
matched to two or three different curves. Not only that, he
picked the "best" radial direction for estimating a high
efficiency. The "wobble" or deviation in slope was 2-3dB or
more over very small distances in points along a radial
direction. It just happened that picking the worse sloped
curve made the earth look lossy, and that made the array
look better because it makes the EXPECTED FS look much
lower. This means the array efficiency looked better. If the
mean value of slope was used the earth loss would have
looked better, and the array efficiency looked worse. Even
by picking another direction the array would have looked
worse.
The FCC estimate is also an average of the entire path, not
the localized conductivity. It treats the earth as a
homogeneous media with constant conductivity. That isn't the
real earth.
Making the tests even less reliable, one of the early
elevated radials tests was made over soil that contained
unknown remaining radials from a earlier ground. Who can say
how much effect that added to the mix??
So how would we do a good test with minimal chance for
error????
We could do a good test by changing nothing except the
radials, and measuring at exactly the same points. Then we
don't need to match dozens and dozens of readings into a
sloped curve, and then transform that result into another
estimate of what we should have at a certain distance, and
then find a spot at that distance that proves we get what we
guessed we should get.
We simply go back to the very same point and record the
change with only one thing changed, the radials. The result
is the exact deviation caused by the ground system, within
limits of human error and instrument calibration. The result
is not a long process of matching curves to estimate the
soil conductivity, and then extrapolating that conductivity
data into an estimate of efficiency based on the estimated
soil conditions along the path and expected FS at a certain
distance.
We actually did several tests where FS was measured with a
small elevated system and with no changes at all other than
the radials FS was remeasured. In those tests four elevated
radials that were ground isolated were compared to a
conventional ground of 60 radials with no change except the
radials themselves. The results showed the small elevated
system was a few dB to 6 dB down from 60 radials. This test
repeated with similar results at three locations, one of
which was WVNJ radio. WVNJ was a good test because they
never had a conventional ground, they started with four
elevated radials. It was easy to watch the FS at measurement
points increase 2-5dB as the ground system was changed by
adding more and more radials.
Roy Lewallen was out here last year, and we found four
elevated radials about the same as 10-15 radials in the
dirt. That was on 40 meters. Why would I put up with all the
headaches of suspending an antenna and radials up several
feet and NOT having good lightning protection when I could
just tack 10-15 radials down on the ground and have a normal
system that works exactly the same?
73 Tom
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