Caps to consider: as you've already figured, polypropylenes can do
very well. Mylars/polyesters are not very stable, and don't have as
high a Q as polyprops. They also show greater distortion, which may or
may not be an issue to you. C0G ceramics should work fine. Be aware
that the loop inductance will change with temperature, too, as the loop
size changes. Presumably your loaded Q won't be so high that it's a
problem: 50ppm/C over 20C is 0.1%, which wouldn't be noticable, most
likely, with a loaded Q of up to 500 or so. At 60kHz, Q=600 is only
about a 100Hz bandwidth, so I suppose you won't want a higher Q than
that anyway (assuming you could get it). I'm curious: what loaded Q
do YOU expect to get? How big is your loop going to be? What
impedance do you expect with the loop resonated?

Cheers,
Tom

Thanks Tom,

I hope for a Q of 600.

At 60kHz, Q=600 is only
about a 100Hz bandwidth, so I suppose you won't want a higher Q than
that anyway (assuming you could get it). I'm curious: what loaded Q
do YOU expect to get?

How big is your loop going to be?

Around 5.2 Meters per side.

What
impedance do you expect with the loop resonated?

It should be under 1 ohm. I don't exactly know the ac resistance or
how the Q of the C and the Q of the inductor combine.

My loop material is 2/0 copper welding cable, many fine starnds. I
considered 3 inch copper pipe, but couldn't get an estimate of the ac
resistance for either, so I chose the copper cable.


Regards,

T

I'm puzzled. My copy of rjeloop3 suggests the Q will be about 200 at
60kHz with a 9mm wire diameter, and you'll see about 2kohms when it's
resonated. Are you not taking the output across the ends of the loop
(across the capacitor)? And with a skin depth of about 0.01" at 60kHz
in copper, certainly 3" diameter soft copper pipe would have the lower
resistance. You might have some trouble finding soft copper pipe,
though. But even hard copper pipe should have a low RF resistance.
"Reference Data for Radio Engineers" (or "Reference Data for Engineers"
in newer incarnations) has lots of good info for figuring out things
like RF resistance of copper wire. I assume your welding cable doesn't
have strands that are insulated from each other like Litz wire.

Consider that Q is energy stored divided by energy dissipated per
radian (1/2pi of a cycle). Then the net Q will be 1/(1/Q(inductor) +
1/Q(capacitor)). So if the cap and inductor have the same Q, the net Q
will be half that. And if you put a resistive load across the
coil+cap, that will dissipate power and lower the Q further.

Cheers,
Tom

Another set of questions: Given the high atmospheric noise level at
LF/VLF, is there really a need for such a large loop as you propose,
for receiving? How quiet is your receiver front end? In other words,
will such a large loop significantly improve your SNR on weak signals?
Do you have a reason other than signal level for using such a large
loop? What about the response to nearby strong electric-field noise
generators of a large loop versus a smaller one?

Cheers,
Tom

TRABEM wrote in message ...
What caps should I use for a resonant loop antenna for VLF? I have
some antenna plans and need .2 uf total capacitance to resonate a loop
antenna at 60 Khz.

I know I should avoid electrolytic and tantalum due to their poor
temperature stability.

Disc ceramics are so poor thermally that they are out of the question,
especially since the caps are going to be outdoors.

Silver Mica's are horribly expensive these days, and aren't available
much past .01uf anyway.

I have a Mouser catalog and plan to order from them soon, but it's not
clear which type of cap I should order.

Any suggestions for 50,000 pF caps that don't cost a fortune?

Note that this is a receive only antenna so voltage rating and current
carrying capacity are not an issue.

Thanks.

T

Polystyrene caps are very temperature stable, typically changing only 0.5%
over their full temp range. And they're available in high capacitance
values. Check Mouser and Digikey for good prices.
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!