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

TRABEM wrote:
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

Some possibilities might be polystyrene and metalized polypropylene.
They will be large, but not so expensive in that size.

Thanks John,

I did find metalized polypropylene caps in the Mouser catalog. Some
are quite expensive for no apparent reason and a few are inexpensive.
I still don't know why the big cost difference.

There were many pages of metalized polyester film caps, not sure if
they are ok or not. But, they are way cheaper, about half the price of
the lowest priced metalized polystyrene caps.

Are metalized polyester caps just as good as metalized polypropylene
caps?

Thanks,

T






Some possibilities might be polystyrene and metalized polypropylene.
They will be large, but not so expensive in that size.

TRABEM wrote:
Thanks John,

I did find metalized polypropylene caps in the Mouser catalog. Some
are quite expensive for no apparent reason and a few are inexpensive.
I still don't know why the big cost difference.

There were many pages of metalized polyester film caps, not sure if
they are ok or not. But, they are way cheaper, about half the price of
the lowest priced metalized polystyrene caps.

Are metalized polyester caps just as good as metalized polypropylene
caps?

Mylar caps are pretty good, but not as stable or as high @ as polystyrene.

You might read through this dielectric tutorial:
http://my.execpc.com/~endlr/film.html

TRABEM wrote:
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.

Is there any possibility that you could add turns to your loop, to
lower the size of the capacitor needed?

John Popelish wrote:
TRABEM wrote:

Thanks John,

I did find metalized polypropylene caps in the Mouser catalog. Some
are quite expensive for no apparent reason and a few are inexpensive.
I still don't know why the big cost difference.

There were many pages of metalized polyester film caps, not sure if
they are ok or not. But, they are way cheaper, about half the price of
the lowest priced metalized polystyrene caps.

Are metalized polyester caps just as good as metalized polypropylene
caps?


Mylar caps are pretty good, but not as stable or as high @ as polystyrene.

Well, it sort of looks like a Q. That should have read:

Mylar caps are pretty good, but not as stable or as high Q as polystyrene.

John Popelish writes:

TRABEM wrote:
Thanks John,

I did find metalized polypropylene caps in the Mouser catalog. Some
are quite expensive for no apparent reason and a few are inexpensive.
I still don't know why the big cost difference.

There were many pages of metalized polyester film caps, not sure if
they are ok or not. But, they are way cheaper, about half the price of
the lowest priced metalized polystyrene caps.

Are metalized polyester caps just as good as metalized polypropylene
caps?

Mylar caps are pretty good, but not as stable or as high @ as polystyrene.

You might read through this dielectric tutorial:
http://my.execpc.com/~endlr/film.html

You can easily get Q1000 with polypropylene capactor. Wima made some
really nice ones.
--
Steven D. Swift, , http://www.novatech-instr.com
NOVATECH INSTRUMENTS, INC. P.O. Box 55997
206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA

"TRABEM" bravely wrote to "All" (22 Oct 05 23:58:10)
--- on the heady topic of "High Q caps for 60 Khz loop antenna?"

Hi,

60KHz really isn't all that demanding. How about polycarbonate,
polystyrene, or mylar? The cheaper the better for receiving.
There are 0,22uF and lower standard value caps in all the above types.
Polystyrene are possibly the more stable but more easily damaged by
the heat of soldering. Just heatsink the lead with needlenose pliers
as close to the body as possible while soldering.

A*s*i*m*o*v


TR From: TRABEM
TR Xref: core-easynews rec.radio.amateur.homebrew:88419

TR What caps should I use for a resonant loop antenna for VLF? I have
TR some antenna plans and need .2 uf total capacitance to resonate a loop
TR antenna at 60 Khz.
[,,,]
TR Any suggestions for 50,000 pF caps that don't cost a fortune?

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

TR Thanks.

.... A mind is a terrible thing to ...oooohhhhh, pastel Wonderbras.

Hi John,

The loop is already bigger then my house and buying the large diameter
wire to make it was not cheap. Buying an additional length just to add
another turn would not be easy on me physically. Further, the R(ac),
and therefore the Q would suffer since the loops resistance is now
doubled by the addition of a second turn.

I am going to feed this into a quadrature front end, which has a
problem with harmonics getting into the receiver. My hope is to use
the loop as the sole tuning for the front end of the receiver, so
maintaining the highest possible Q is imperative.

In terms of labor and cost, it is much more practical to use a larger
value capacitor to get higher Q than it would be to use larger cable
and more turns of it for the loop.

So, I will try to stick with the larger diameter single turn loop,
which I why I asked for suggestions on which type of capacatior to buy
to resonate the loop with.

Thanks to all who replied. I appreciate all the comments.

T


Is there any possibility that you could add turns to your loop, to
lower the size of the capacitor needed?

Are metalized polyester caps just as good as metalized polypropylene
caps?

Mylar caps are pretty good, but not as stable or as high @ as polystyrene.


OK John,

I didn't realize that a polyester cap was the same as a 'mylar' cap.
But, it makes sense now that I know.

Some polyester film caps are pretty inexpensive. I might have gone for
them if I didn't need all the Q I can get for this particular antenna.

Thanks for the tip and to all who commented.

T