Total posts : 45366
Ermi wrote: I don’t know if Brown’s 22 degree antenna was top loaded or not. I presume that it was top loaded, since, with a radiation resistance of 1.474 ohms, the ground resistance would be .182 ohms, which is too low to be practical. With a radiation resistance of 5.9 ohms, the ground resistance would be .729 ohms, which is also low.
None of the monopoles in the BL&E experiments was top loaded. As for the r-f ground resistance, the analysis in their IRE paper states, “We see that the power lost in a ground system consisting of 113 wires, each 0.4 wave length long is insignificant.”
This result also is shown by their Figure 18 plot of power lost in 113 each, 0.4-lambda radials at all monopole heights measured ranging from 22 to 88 degrees. Of interest is that Figure 18 shows 745 watts as lost in the ground system of a 22-degree monopole using only 15 radials each 0.4-lambda long, for 1 kW of applied power. All of this is shown for a ground conductivity of 2 mS/m.
Some additional metallic ground plane is necessary to ensure that the needed efficiency is obtained.
None is shown needed or used in the BL&E experiments to produce the results they measured for the 22-degree monopole with 113 each radials, each 0.4 lamda long.
At this point I’ll include two links.
This one http://i62.photobucket.com/albums/h85/rfry-100/BLEMonopoleComparison.gif shows NEC-2 calculations and a comparison of the elevation patterns and gains for a lossless 22-degree monopole without top loading, and a 1/4-wave monopole — both over perfect ground.
This one http://i62.photobucket.com/albums/h85/rfry-100/BLESimulation.gif uses the 0.38 dB difference in h-plane gain for the 22-degree, perfect monopole from the NEC comparison, along with values taken or implied by the BL&E paper for this configuration. The frequency, monopole height and monopole OD are those used by BL&E.
This simulation shows that the BL&E-measured field equal to 283 mV/m at 1 km for 1 kW of applied power is achieved by this configuration as long as the matching and r-f ground loss total is not more than 0.149 ohms. BL&E state that the reactance of the monopole was determined by varying the value of a condenser (capacitor).
As for Brown’s results, the ground resistance clearly has to be too low for them to be completely correct.
The r-f ground loss for the BL&E system of 113-radials each 0.412-lambda long certainly is much lower than most licensed AM stations. But then very few of them use radials longer than 1/4-lambda. Dr Trainotti suggests that about 120 buried radials each 1/2-lambda long used with a monopole virtually is a perfect r-f ground. BL&E’s measurements show that they got very close to that.
He [Brown] probably reported exactly what he measured. As has been already pointed out, it is not easy to measure field strength with high accuracy.
My money is on Dr Brown and his team, who still are very highly regarded as pioneers in broadcast engineering. Their field measurements were within 1.5% of the theoretical value for that radiated power for a perfect 1/4-wave monopole over a perfect ground. Accurately measuring the change in that field from a 22-degree monopole, other things equal, is not technically difficult. The BL&E findings in these experiments became the basis for the minimum performance required by the FCC for licensed AM stations, to this very day.
For those who may be interested, paper 4 at this link http://rfry.org/Software%20&%20Misc%20Papers.htm is a PDF of the IRE publication of BL&E’s experiments, and their results. It makes for very good reading and study. I hope anybody finding anything in conflict with what I have posted based on the BL&E paper will let me know.