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Re: Topband: Elevated radial number vs efficiency

To: topband@contesting.com
Subject: Re: Topband: Elevated radial number vs efficiency
From: Artek Manuals <Manuals@ArtekManuals.com>
Date: Fri, 1 Jan 2021 16:37:10 -0500
List-post: <mailto:topband@contesting.com>
Correction that should be N6LF (not N6FL)

NR1DX

On 1/1/2021 4:26 PM, Artek Manuals wrote:
Mike & Bill

Thank you!!! I sometimes think there is too much emphasis on NEC modelling and never enough real world verification with actual field measurements

{I have hijacked the original thread and changed the subject. to be more on point }

N6FL� was quoted earlier in the previous thread, for his work on studying the effects of elevated radials and he in fact states on his web page which IS by all means� worthy of reading and close study.� I am reposting the link here https://www.antennasbyn6lf.com/design_of_radial_ground_systems/

However N6FL� states "The article is primarily intended to show why I (he, N6FL)� suggest that 10-12 elevated radials should be used if possible. " . His own data however is a bit contradictory and his comment lacks� the context of radial length. Radials of .25 wavelengths (Page 37 figure 12, QEX, March 2012) produces a gain identical to 16 radials of the same length. His data suggests that if you lengthen the elevated radials to .6 wavelengths then 16 elevated radials do indeed produce ~.6db improvement over four radials of that� same length. Most of us are unlikely to want to invest in the almost a mile of additional wire on 160M to get that .6db improvement, let alone the labor involved in stringing it up and keeping it up.

What I also stumbled on in reading that same article is that only two elevated radials is only down by .4db compared to four radials, which would suggest that even only two (elevated) radials would perform as good or better than ground mounted radial fields of a couple of dozen radial range. A quick google search did not produce a similar study to N6LF's work for ground mounted radials though I am sure it is out there and the readers of this thread will find it for everyone's reading enjoyment !

Cheers and HNY
Dave
NR1DX


On 1/1/2021 3:09 PM, Mike Waters wrote:
This link at to top of that page is a must-read, too.
https://web.archive.org/web/20180923221943/http://lists.contesting.com/_topband/2007-11/msg00248.html.
Guess I might as well include the text...

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


73 Mike
W0BTU

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Dave Manuals@ArtekManuals.com www.ArtekManuals.com

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