Re: [TCML] OT: Help with special LF antenna design

["DC Cox" <resonance@xxxxxxxxxxxx>]

Contact me off-list if you need the circuit.  I have one using an N-channel
FET that works great.  I use approx 10 feet of wire on a circuit board.  I
also have a "directional" antenna I devised that gives directionality in a
plane.

Regards,

Dr. Resonance




On Thu, Apr 24, 2008 at 5:23 PM, David Speck <Dave@xxxxxxxxxxxxxxxx> wrote:

> 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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