Topband: Vertical choke needed?

Fri Jan 1 15:58:27 EST 2021

Check out those links on my old ground radials page, especially the ones by
N6LF. See what you think. :-)

73, Mike
W0BTU

On Fri, Jan 1, 2021, 2:51 PM CUTTER DAVID <d.cutter at ntlworld.com> wrote:

> Groundbreaking work for the BC industry.
>
> Is the short distance coverage required of BC stations always relevant to
> amateur DX? Or is it a happy coincidence?
>
> David G3UNA/G6CP
>
> On 01 January 2021 at 20:09 Mike Waters <mikewate at gmail.com> 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
>
> On Fri, Jan 1, 2021, 2:01 PM Mike Waters <mikewate at gmail.com> wrote:
>
> On Thu, Dec 31, 2020, 9:10 PM Artek Manuals <Manuals at artekmanuals.com>
> wrote:
>
> >> Elevated radial systems NEED a choke ..PERIOD. Been there , done that,
> >> got the RF burns to prove it...8^(
> >>
> >
>
> You bet they do! Otherwise, the coax feedline on the ground seriously
> degrades the antenna's performance! I don't recall anyone mentioning this
> important fact in these recent threads here.
>
> See
> https://web.archive.org/web/20180815141931/http://w0btu.com/160_meters.html
> ,
> partway down the page; and the links about elevated radials below the
> graphs at
>
>
> https://web.archive.org/web/20180815154501/http://w0btu.com/Optimum_number_of_ground_radials_vs_radial_length.html
>
> These are must-reads for anyone with elevated radials, IMHO.
>
> 73 Mike
> W0BTU
>
> _________________
> Searchable Archives: http://www.contesting.com/_topband - Topband
> Reflector
>
>