[TCML] OT: Help with special LF antenna design

[Gary Peterson]

> To obtain maximum sensitivity, is it better to have a bigger diameter loop 
> with fewer turns, or a smaller diameter loop with more turns?

The physical size of the loop affects the capture ability of the loop.  The 
larger the winding size the greater the pickup.  In other words, the larger 
the loop diameter, the greater the efficiency.

 "A small loop antenna appears as a very large resonant circuit.  The loop 
itself can be viewed as a large single turn inductor of this circuit.  Due 
to the large (relatively speaking) size of this inductor, radiation very 
easily takes place.  Experimenters have noticed that the more turns there 
are to this inductor, the less efficient it is.  For some low frequency loop 
antennas several turns may be required to obtain resonance but radiation 
efficiency usually suffers.  Basically, the radiation resistance increases 
as the loop size increases.  The higher the radiation resistance, the higher 
the efficiency assuming a constant loss resistance.  Increasing the number 
of turns in a loop increases the inductance but also increased the loss 
resistance.  Taken to the extreme, ordinary tank circuits don't radiate much 
at all due to the small, multi-turn inductors. . . .

 "[Interestingly] There have been claims that a vertical small loop does not 
suffer from the pseudo-Brewster angle notch-out effect that other vertical 
antennas will have when installed over real ground.  I have not uncovered 
any theory to back this up and my first thought would be that a vertical 
small loop should act like any other vertical antenna in this regard. 
However, the real world experience of users would indicate that this 
phenomenon may need further investigation. . . ." [ 
http://www.aa5tb.com/loop.html ]


----- Original Message ----- 
From: "David Speck" <Dave at davidspeckmd.org>
To: "Tesla Coil Mailing List" <tesla at pupman.com>
Sent: Thursday, April 24, 2008 6:23 PM
Subject: [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