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[TCML] OT: Help with special LF antenna design



List,

With your indulgence, I'd like to ask a question about optimum design of a low frequency antenna, not exactly a TC question, but sorta, kinda close.

With the thunderstorm season upon us again, I'd like to build a sferic detector like the one I made back in high school some time in the Middle Ages. For those not familiar with sferics (a contraction of "atmospheric electricity"), these are audio frequency EM waves radiated by lightning and other natural phenomena. You can make a pretty neat lightning detector which gives an indication of bearing and distance to a lightning strike by using two circular loop antennae mounted at right angles to each other, which are connected to the inputs of a simple X-Y oscilloscope display. If you use an omnidirectional vertical wire antenna connected to a monopolar amplifier driving the Z-axis of the scope, you will get an unambiguous bearing indication to the strike. My question relates to the optimum design of the antenna. The original Scientific American "Amateur Scientist" article used a loop of 100 turns of wire wound around a split hula hoop as a coilform. The whole antenna was wrapped in aluminum foil as an electrostatic shield, with a small gap in the shield to avoid acting as a shorted turn. I understand that the more turns there are in the antenna, the more sensitive it is, but the more inductance it would have, which I think would limit the high frequency response. Making the loop larger in diameter with the same number of turns would also increase its sensitivity because it would enclose more lines of magnetic flux, but again, also its inductance. Of course, there are limits to a convenient size and unobtrusive location for erecting a pair of big silver loops. My question to those more facile with these concepts is: To obtain maximum sensitivity, is it better to have a bigger diameter loop with fewer turns, or a smaller diameter loop with more turns? After extensive web searching, I've seen it done both ways.

Thanks in advance,

Dave


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