Topband: Elevated radial number vs efficiency

Fri Jan 1 16:26:33 EST 2021

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
-- 
Dave Manuals at ArtekManuals.com www.ArtekManuals.com

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