[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [TCML] OT: Help with special LF antenna design
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@xxxxxxxxxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
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
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla