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

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

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

TRABEM writes:


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.


Wrap-and-fill polypropylene are a lot better than metalized polyester (mylar
is a trade name for polyester). Mylar also absorbs water and shifts its
Q with time rather badly.

If you care about Q for large values, polypropylene is going to be your
best shot. There are other caps (teflon, and other exotic materials) that
are better, but you also seemed concerned with cost. Higher voltage for the
same value with give a better Q.

Good luck.

Steve.
--
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