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Re: Topband: Elevated Radial Efficiency - an inordinately long post

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Subject: Re: Topband: Elevated Radial Efficiency - an inordinately long post
From: "William Culpepper" <>
Reply-to: William Culpepper <>
Date: Wed, 28 Nov 2007 23:53:39 -0500
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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 

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 

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 

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.



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