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***EDIT*** The 5 dB advantage I mention in the next paragraph is for the modeled helical 3 meter antenna compared to a modeled 3 meter whip and not compared to an isotropic radiator or a 1/4 wave ground plane antenna. I should have made that clearer. Both of these 3 meter antennas have very low efficiency at AM frequencies. ***
I too am surprised by the apparent 5 dB advantage (from the plot) of the helical wound antenna over the wire antenna. Since the patterns are about the same it looks as if the higher field strength results from the higher radiation resistance.
Earlier in the thread I asked ” So the question is does a helically wound antenna provide gain above a straight conductor? “. It looks as if the answer is yes. Therefore my argument about equivalent field strength from these antennae satisfying the legal intent is not valid. As Rich says “you are on your own” with the legal issues.
I spent over an hour searching for actual gain numbers for a short, small diameter helical antenna and the closest I came was with a VHF “rubber duck” which has a reported gain around -6 to -10 dBi. I did not extrapolate this to the 3 m antenna because of the vast difference in dimensions in terms of wavelength. Rich’s work fills a gap in knowledge regarding extremely short antennae. Thanks Rich.