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 <[email protected]> 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 <[email protected]> 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 <[email protected]> wrote: > > On Thu, Dec 31, 2020, 9:10 PM Artek Manuals <[email protected]> > 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 > > _________________ Searchable Archives: http://www.contesting.com/_topband - Topband Reflector
