Re: Trying to model loop antenna (why does capacitance go down when adding parallel cap?)

"Stephen" <stephendeese@xxxxxxxxxxx> wrote in message
news:1114459743.952089.255040@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> My goal is to be able to model the antenna accurately.

Are you interested only in modeling it at frequencies
near 60 KHz, within maybe +/- 10%? An LCR
model is probably reasonable in that case.

> I would like to be able to reasonably estimate the amount of
> capacitance needed to get the thing to resonate at 60 khz. It very
> easy for me to just start adding capacitors by trial and error until I
> get there, but that doesn't really teach me much.

If you measure the first resonant frequency and Q,
then measure inductance at a much lower frequency,
you (or many folks here) should be able to calculate
the capacitance needed to tune to 60 KHz. That
will be a simple reactance or admittance summing
and differencing calculation. (I assume here that
you have a simple loop antenna, not the thing you
described in your initial post.)

If you want to learn antenna theory, this is not the
best place to go, except for some pointers to good
sources for that knowledge.

> The measured values (no capacitor) work pretty well. From what I've
> studied, resonance should occur when the two impedances equal one
> another, so F =1 / sqrt(LC) = 438 khz.

Strictly speaking, the impedances are complex
conjugates at resonance. This will matter when
you do the tuning calculations.

> I measure a resonant frequency
> of 442 khz. Not bad! This leads me to believe that the L/C meter can
> indeed give the right answer for L and C in cases where they both
> exist. Is this a bad conclusion?

It is a dangerous generalization. It may not apply
to devices operating at other frequencies. I think
Mr. Larkin's note on that subject is pertinent.

> So, I'd really like to learn here. In adding the eternal 10,000 PF
> capacitor.. why did the total measured capacitance decrease? Is it an
> error in the instrument? or something else?

As others have stated, there is some reactance
cancellation going on. Without knowing more
about how your LC meter works, it is difficult
(and impossible for me) to explain in detail why
that result is obtained.

> Should my model consist of:
>
> A) (A capacitor in series with an inductor in series) in parallel with
> a cap

I doubt your antenna looks like a trap. That
circuit would.

> or
>
> B) (A capacitor in series with an inductor in series with a cap) in
> parallel with a cap

No, for the same reason. (And the two caps within
the parentheses are redundant for a model.)

> ?

Given that you intend to operate the loop quite a
ways off of its resonance, I think the loop model
that does you the most good will most resemble
an inductor with series and parallel resistance to
reflect radiation and heating losses. Those will
probably not be the same values that model the
antenna at it natural resonance.

> I realize that the six loops of coaxial will add some capacitance due
> to thier geometry (being next to each other). Should that be
> considered a series or a parallel capacitance? Is a good model
> possible, or am I searching for something that will not be found?

I have to skip that question. Your arrangment
of coax is confusing to me. Your shield hookup
is unclear, and its purpose a mystery. What are
you trying to do that simple wire could not do?

> Thanks to anyone willing to help here.

You're welcome, FWIW.

--
--Larry Brasfield
email: donotspam_larry_brasfield@xxxxxxxxxxx
Above views may belong only to me.


.

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