Topband: Elevated radial number vs efficiency

[Artek Manuals]

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 at ArtekManuals.com www.ArtekManuals.com

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