I am ONE of the people who claim that four elevated radials can have
approximately the same efficiency as 120 buried quarter wavelength radials.
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 first station was in 1990 and it was WPCI, 1490 kilohertz, Greenville,
SC where the height of the tower steel was 93 degrees above the base
insulator and 87.2 degrees above the point of attachment of the four
elevated radials. The radials were horizontal all the way to the tower
where they were attached with an insulator and connected to the outer
conductor of a coax cable. The coax center conductor was connected to the
tower at that point. The license application containing the field strength
measurements, measurement analysis and explanations can be found in the FCC
Public Reference Room under file number 900615AE.
Measurements were made on eight equally spaced azimuths out to three
kilometers using a Nems Clark model 120E field strength meter. 146
measurements were made for an average of over 18 per azimuth. Power was set
at one kilowatt using a General Radio model 916A RF impedance bridge for the
radiation resistance and a Delta Electronics precision RF ammeter for the
antenna current. The measurement data was analyzed with EDX Engineering
program AMDAT which is described in IEEE Transactions on Broadcasting, Vol.
BC-32, No. 2, June 1986.
The result was an RMS value of the eight radials of 302.7 mV/m/kW at one
kilometer. This compares with the FCC Figure 8 value of 307.8 mV/m/kW for a
93 degree tower with 120 ninety degree buried radials, however, a tower 87.2
degrees (the height of the WPCI tower above the four horizontal radials) has
an FCC rated efficiency of 303.7 mV/m/kW, one mV/m more than our measured
value.
The WPCI radials were number 10 copper wire 90 degrees long and 8.7 degrees
(16 feet) above ground. A coax cable was fed through the inside of the
tower from the T network at the tower base to the point of radial
attachment. The top of the base insulator was approximately five feet above
ground. The impedance was measured at the input to the coax which was the
point of current measurement for determination of power. The bridge
measurement was R 78 +j56.4.
The FCC personnel in the Broadcast Bureau were initially reluctant to
entertain the notion of an AM broadcast ground system which was so radically
different from what had been used from the beginning of vertical broadcast
antennas in the 1920s, and as refined by the classic article on broadcast
ground systems in the thirties (Ground Systems as a Factor in Antenna
Efficiency by Brown, Epstein and Lewis, Proceedings of the IRE, June 1937).
They finally agreed to permit the elevated system on the condition that
field strength measurements would be submitted prior to a license being
issued, and that if it did not perform as represented that the elevated
system would be abandoned and a conventional 120 buried wire system would be
installed. Fortunately, they approved the measurements and granted a
license. I believe that WPCI was the first broadcast station in the United
States to ever be licensed to use a small number of elevated radials as its
ground system. It is operating with the four elevated radials to this day.
You can tune it in as you drive in the vicinity of Greenville on Interstate
85, and you can find it with your GPS at 34-51-38 north and 82-24-31 west.
The other two broadcast stations where I was instrumental in installing a
four wire elevated radial system were KVML, 1450 kilohertz, Sonora,
California and WGCM, 1240 kilohertz, Gulfport, Mississippi. The FCC
required measurements on both, and the results were similar to WPCI. A
license was granted to both stations.
I did extensive experiments at other sites in the 1990s which I will not
bore you with except to say that for amateur applications, the four radial
wires can be brought down to the base of the tower at a 45 degree angle for
a more convenient feed arrangement than the method at WPCI. (The wires can
be hazardous to humans and other animals.) The efficiency is about the same
as the non-sloping radials as described for WPCI. Also, as long as the
radials are near 90 degrees, it seems to work very well with towers much
less than 90 degrees in height as indicated by the measured antenna
resistance becoming very low with short towers. This would suggest that the
loss resistance is very low. With a short tower and a low driving point
resistance the normal reactance will cause the bandwidth to be very narrow.
As an aside, with a 120 foot tower (27.4 degrees) and four elevated radials
of number 2 copper wire 20 feet high and 267 feet long (61 degrees) at 625
kilohertz, I measured R 1.45 -j380 (that is R 1pt45). This was with the
battery powered signal generator/detector and bridge isolated from the earth
to prevent ANY current from flowing through the earth back into the system.
This indicates that the loss resistance was incredibly low. I had a single
wire lying on the ground 250 feet long which I connected in parallel with
the elevated radials thinking that it would further lower the radiation
resistance. Wrong - the resistance shot up to about eight ohms indicating
that the antenna was then collecting return current that was flowing through
the dirt and substantially increasing the R loss. With more normal
impedance values this is not such an extreme problem as the WPCI system was
not isolated from earth. However, as just shown, isolation from earth is an
interesting subject.
As demonstrated above - do not connect a mediocre buried radial system in
parallel with your elevated radials as it will increase the loss resistance
and impair the efficiency. In fact, why connect any buried system in
parallel with elevated radials.
Do my measurements in the broadcast band mean that four elevated radials
will work on 160 meters as well as 120 buried wires? I have not proved it,
but my opinion is that they will work very well. But that is just my
opinion.
At the invitation of Tim Duffy (K3LR), I covered all of the above and much
more in my talk at the Antenna Forum at the Dayton Hamfest in 1996.
73,
William
W4BZ
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