On Sun, 23 Oct 2005 21:08:29 -0400, TRABEM wrote:

Hi Reg,

Didn't know you monitored here!

Try googling the archives to see he does more than monitor.

The normal 5 or 10 percent tolerance means that you will have to
measure and select values from a larger batch of inexpensive
capacitors.

Don't forget you will have to select from a small number of standard
values such as 0.22, 0.1, 0.047, 0.033, etc.


I've ordered a good supply of polystyrene caps in the standard values.
I also have an RCL meter, which is cheap and not accurate, but I can
measure the actual value of the caps I get from Mouser and assemble
them to hit the target frequency pretty easily.

Most capacitors are rated at the same frequency that most RCL meters
(for good reason) run at: 1KHz. This is not to say capacitance is the
same at 60KHz. It would be simpler to build your antenna with a
tickler coil in an oscillator circuit and measure the resonance
frequency.

So, I'm actually hoping the values they send are somewhat
dispersed from the marked values.

They usually are, but are also within the stated error band. That is
to say if they are spec'd at ±5% you might them spread from +1% to
+5% of nominal rather than across the full range of -5% to +5%. The
upside is tighter matching, the downside is no offsetting average
centered around nominal.

To reduce size of the capacitor just increase the number of coil
turns. You will notice little or no difference in operation.

The most efficient loop has a single turn of very thick wire.

Which is exactly what I intend to do! I had a choice between large
copper welding cable and 3 inch copper pipe. I chose the welding cable
because it was actually cheaper although I'm not sure which would have
the best Q.

And this is where most of the advice so far (and the perception of the
problem) comes into a clash with reality. I will quote from your
earlier post to make the point:

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

Current carrying capacity is an issue, that is why it is called Q!

Simply having smaller currents does not boost the Q of these pitifully
meager leads on the capacitors going to the comparatively garganthuan
copper pipe. Even more, and especially at this frequency, contact
resistance of the plates of the capacitor to the leads is an issue
that has been studied and resolved for designers of switching power
supplies (which typically run in this frequency neighborhood).

What you need to research for are caps with low ESR (Equivalent Series
Resistance), or test your selections along the lines offered at:
http://octopus.freeyellow.com/99.html

This returns us to the discussion of Q (we never really left), but for
capacitors is measured as DF (dissipation factor, something that is
generally ignored for transmission lines because the loaded Q is so
very low - as Dorothy would offer "Toto, we're not in Kansas
anymore!"). You could be killed by ESR and not know what hit you.

The suggestion for Polystyrene may be good, but only for a particular
manufacturer or for a particular run. It requires individual
examination and specification, especially when you've been warned away
from ceramics which can exhibit ±15ppm/°C (how good do you want it?).
The real trick is to simply accept you are going to get some that
drift and plan to offset that drift with another parallel formula that
drifts in the opposite direction such as: polyester and polypropylene,
or polystyrene and polycarbonate.

73's
Richard Clark, KB7QHC