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Walter Maxwell wrote:
"To anyone who believes the W5QJR EH conceptis valid." OK. I read the first page: "Welcome to the Wonderful World of EH Antennas". It said nothing of why I should be interested. Why convert an existing broadcast antenna to EH? FCC has a publication, "Rules of Good Engineering Practice for Standard Broadcast Stations" which includes Mv/m at 1 mile on a radial over perfect earth from a vertical antenna of various heights. It shows about 195 mV/m for a 1/4-wave grounded vertical. This can be adjusted for any power or diistance from the sender: E = Eo sq rt P/d Eo = 195 mV/m for the 1/4-wave antenna at 1 mile P = the actual radiated power d = distance from antenna in miles The mV/m at a mile assumes a perfect ground and a perfect antenna ground system. FCC says 120 radials equally spaced and 1/2-wavelength long are its standard. Efficiency typically exceeds 95%. If the vertical radiator is higher (longer) than !/8-wavelength, the 150 mV/m at 1 mile, required minimum efficiency, can still be met with 120 radials on the earth that are only 1/4-wavelength long. The reduction in efficiency is small. Nobody has perfect ground unless he is at sea. For imperfect ground, the FCC publishes "Ground Wave Propagation Curves" for various soil conductivities. In the FCC millivolt per meter numbers for vertical antennas of various heights, the field strength only increases 5% in going from very short to a full 1/4-wave height. This requires the near perfect ground. A 3/8-wave radiator only has a 15% advantage over a very short radiator. If the radiator is a thin wire, bandwidth may be only + or - 1% of the wire`s resonant frequency. Broadcast stations use towers of substantial cross section as antennas. These provide several percent of bandwidth and allow full audio range in the medium wave band. A short antenna has low radiation resistance and high capacitive reactance. This requires tuning out the large capacitive reactance (small capacitance) with an equally large inductive reactance (large reactor), and matching the very low drivepoint resistance of an end-driven vertical to the higher impedance sending circuit. Resistance involved in neutralizing reactance and matching the antenna to the source is likely to be lossy for the too-short antenna. Walter has already pointed this out. Why would the EH antenna have interest? Best regards, Richard Harrison, KB5WZI |
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