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

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.

All this makes sense.

Further, the R(ac),
and therefore the Q would suffer since the loops resistance is now
doubled by the addition of a second turn.

This, however, doesn't, necessarily. The resistance would double, but
the inductance would quadruple. The impedance would also quagruple
from about 119 ohms to 476 ohms, so the Q would roughly double when
tuned with a .055 uF capacitor. This, of course, neglects the loading
effect of the receiver input resistance and the radiation resistance
of the loop, and the eddy current loading of nearby conducting objects.

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.

Understood.

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.

I can certainly see the cost advantage. Even an expensive capacitor
is probably cheaper than more wire.

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.
(snip)

Makes lots more sense, now that I understand you are not using a
ferrite rod antenna.

Makes lots more sense, now that I understand you are not using a
ferrite rod antenna.


Ouch! Dern it. I left out a really important detail. Sorry for the
oversight.

Thanks so much for the comments.

T

On Sun, 23 Oct 2005 20:34:17 -0400, TRABEM wrote:

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.

Before going to such extreme measures, are you absolutely sure that
the "harmonics" are entering the receiver from the actual loop due to
insufficient Q and _not_ sneaking in through the cable shield, power
supply lines etc. or other antenna effects ?

Even if you have a single strong unwanted signal at some harmonic
frequency, taking it out with a high-Q notch filter prior to the
mixer(s) would be easier. An isolation stage (emitter follower etc.)
might be needed between the receiver loop and the notch filter to
prevent too much interaction.

Paul OH3LWR

Yes, I'm sure.

It's a 60 foot run to the house, which is relatively short in terms of
wavelength.

I am feeding it with a balanced line (actually both conductors
floating). So, the pickup by the line to the house should be quite
minimal. I've tried 300 ohm twinlead and cat5 twisted wire cable. Both
work well.

Would never use anything as crude as coax for something like this, it
is just asking for trouble.



On Mon, 24 Oct 2005 08:32:58 +0300, Paul Keinanen
wrote:

On Sun, 23 Oct 2005 20:34:17 -0400, TRABEM wrote:

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.

Before going to such extreme measures, are you absolutely sure that
the "harmonics" are entering the receiver from the actual loop due to
insufficient Q and _not_ sneaking in through the cable shield, power
supply lines etc. or other antenna effects ?

Even if you have a single strong unwanted signal at some harmonic
frequency, taking it out with a high-Q notch filter prior to the
mixer(s) would be easier. An isolation stage (emitter follower etc.)
might be needed between the receiver loop and the notch filter to
prevent too much interaction.


Paul, there are no mixers. It's not a superhet. No mixers, no diode
detector, no tuned circuit between the antenna and the detector. An
active component anywhere between the antenna and the detector defeats
the purpose of a quadrature detector...and, since we don't need an rf
stage at all, the receiver is much cleaner than conventional superhets
ever could hope to be.

Strong signals on adjacent frequencies that force their way into the
front end just end up being cancelled out by the quadrature detector
(witht he exception of harmonically related signals)...which is
another advantage.

The audio is remarkably clean and crisp.

One would NEVER use any sort of a preamp in a quadrature based
receiver.
Regards,

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

Someone tell Trabem about polystyrene and/or NPO disc please!!!!!!!!!! N5TDE

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

wrote:

Someone tell Trabem about polystyrene and/or NPO disc please!!!!!!!!!! N5TDE

Where can you buy .2uf NP0 discs?

-Bill

On Tue, 25 Oct 2005 21:08:34 -0400, Bill wrote:

wrote:

Someone tell Trabem about polystyrene and/or NPO disc please!!!!!!!!!! N5TDE

Where can you buy .2uf NP0 discs?


And, even if we could buy them, why would we use a disc ceramic cap in
an environment that was not controlled due to stability issues?

I'm totally lost by the suggestion of a disc ceramic.

T

TRABEM wrote:
On Tue, 25 Oct 2005 21:08:34 -0400, Bill wrote:


wrote:


Someone tell Trabem about polystyrene and/or NPO disc please!!!!!!!!!! N5TDE

Where can you buy .2uf NP0 discs?



And, even if we could buy them, why would we use a disc ceramic cap in
an environment that was not controlled due to stability issues?

I'm totally lost by the suggestion of a disc ceramic.

You shouldn't lump "ceramic cap" into a single group and assume it has
one set of characteristics. There are many types of ceramic capacitors,
with widely varying characteristics. The four most common groups a

1. Hi-K. These are made with a class of ceramic having a very high
dielectric constant. This results in physically small capacitors which
are excellent for many RF applications such as bypassing. Their small
physical size results in a high self resonant frequency for a given
capacitance. The tradeoff is instability in nearly every other respect.
They have a very high temperature coefficient, are sensitive to humidity
and physical stress, and the capacitance varies with frequency and
applied voltage. A common type is Z5U, which has a specified maximum
capacitance change of +22 - 56% over the temperature range of +10 to +85 C.

2. "Temperature stable". These are made with ceramic which has
considerably lower dielectric constant than used for the hi-k types.
Consequently, they're larger for the same capacitance. Their temperature
coefficient is much lower, and they're much less sensitive to other
factors. They're good for many general purpose uses such as low- to
moderate-Q filters and resonant circuits, coupling, and so forth. Q can
be quite good. A common type is X7R, which has a specified maximum
capacitance change of +/- 15% over the temperature range of -55 to +125 C.

3. Temperature compensating. These are much larger yet, being made from
ceramics with a low and controlled temperature coefficient. They're
typically very stable with respect to voltage, frequency, mechanical
stress, and humidity. A common type is P2G (N150), which has a
temperature coefficient of -150 +/-30 ppm/C.

4. NPO. (Common type is C0G.) These are made from a sandwich of two
temperature compensating types with equal and opposite coefficients.
They have excellent Q and are very stable. They're good for use in
resonant circuits including oscillator tanks, high-Q filters, and other
demanding applications. Q can be very good. A common type is C0G, which
has a temperature coefficient of 0 +/-30 ppm/C.

NPO capacitors are substantially larger than "temperture stable" types,
and much larger than hi-k types. You won't typically find them in large
values.

Roy Lewallen, W7EL

"Bill" bravely wrote to "All" (25 Oct 05 21:08:34)
--- on the heady topic of " High Q caps for 60 Khz loop antenna?"

Bi From: Bill
Bi Xref: core-easynews rec.radio.amateur.homebrew:88513

Bi wrote:

Someone tell Trabem about polystyrene and/or NPO disc please!!!!!!!!!! N5TDE

Bi Where can you buy .2uf NP0 discs?

Bi -Bill

How about 10 of 0.02uF?

A*s*i*m*o*v

.... "If it ain't broke, you're not tryin!" - Red Green