"Reg Edwards" wrote in message
...

"Richard Clark" wrote -

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

Richard,


Snippppppp
Assume the tuning capacitor is comprised of ten 0.02 uF capacitors in
parallel. The resistance of 10 capacitor leads in parallel is
negligible in comparison with the loop's single-turn conductor RF
resistance of 0.23 ohms.

He, Trabem, will be obliged to use a bundle of capacitors to make an
EXACT value for the tuning capacitor because of the impossibiltity of
obtaining a single capacitor of exactly the correct value, at a
particular temerature, and of sufficient long-term stability. He
can't afford it!

Immediately, the ESR of a 0.02 uF capacitor, whatever it is, is
divided by 10. Yes, I know that the ESR of a capacitor at 60 KHz
involves a little more than lead resistance. But it's too small for an
American General Radio bridge to accurately measure it.

But to return to Earth, the working Q of the 5-metre-per-side loop is
a function of the sum of the conductor resistance, plus the small ESR,
plus the radiation resistance (which is also negligible), PLUS the
loss resistance due interaction of the loop with its environment.

Unless the loop is removed from ground and other foreign structures by
at least 1/3 of its diameter the losses due to its environment will
greatly exeed all other losses.

If the environmental loss is equal only to conductor resistance loss,
the working Q of the loop will be reduced to 25.

With a Q of 25 the bandwidth will be of the order of 60/25 = 2.4 KHz,
or enough to accommodate an audio SSB transmission.

And getting down to practicalities, this means that the 0.2 uF tuning
capacitor has to be adjusted to an accuracy of about 0.3 percent, or
within a few hundred pF.

That is why I suggested a 2000 pF variable capacitor be included in
the bunch. A 2000 pF variable capacitor consists of an old fashioned
4-gang, 500 pF, receiving-type capacitor with all sections connected
in parallel.

As the loop is to be installed outdoors (with 5 metre sides it HAS to
be) the variable 2000 pF component might be useful to re-tuning it
between summer and winter temperature variations.

It's surprising what can be gleaned from a knowledge only of the value
of the proposed tuning capacitor.

Its all guesswork of course.

Incidentally, if Trabem obtains batchea of nominally identical value
capacitors, he will probably find they are all on the same side of the
tolerance. They probably all came from the same production line and
machine settings. Production values are not distributed at random.
This can seriously handicap his choice of particalar values to make up
the total of 0.2 uF.
----
Regards, Reg, G4FGQ.



The idea of having to use identical values of caps is a bit off track. You
build the capacitor bank up with obtainable value units, but add smaller
values (it's called trimming) to get to the exact capacitance you need.
I saw a VLF loop antenna that used such an arrangement, and as the fine
tuning element, there was a couple varicaps in parallel. There was also a
thermistor that fed temperature data back to an opamp circuit that
controlled the bias on the varicaps. The end result was an antenna that was
very well compensated for temperature. The loop stayed tuned within a few
hertz of it's tuned frequency over temperatures ranging from around 0F to
over 100F. The antenna was part of a VLF frequency calibration system.
Can't remember the make/model... it was a number of years ago. Tracor
seems to ring a bell, but not sure.

Cheers!!!
--
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!!