"i4jmy@iol.it" wrote:
> Nobody told that beyond the resonance the choke impedance goes further
> increasing,
> of course it doesn't because of inductor self capacitance.
The actual mechanism is that of a transmission line,
not of a simple capacitance.
> Disregarding self capacitance, which is a parasitic effect anyway, and very
> little felt on
topband, the inductor reactance is multiplied by four when doubling the number
of turns.
My own experience from winding and measuring tens if not
hundreds of chokes and baluns has been that it is usually
impossible to get a sufficiently high impedance without
entering the "transmission line" domain.
> The only way to have more reactance in real life chokes is to work in the
> around of an
> inductor self reasonance, whose thing is obtained acting with the number of
> turns,
> if and when inter-wire capacitance is a fixed value.
There are two ways, and both are known from transmission lines:
* use as high a characteristic impedance as possible;
* operate as near to a parallel resonance as possible.
> Self capacitance anyway, is a matter of capacitance between adiacent turns,
> and in a single layer coil it doesn't add for increasing the number of turns.
This is a common myth, not supported by actual measurements.
Even good old Terman warns that current distribution near
resonance is quite different from the one at low frequencies,
and there is only one possible reason: standing waves on a
transmission line.
The capacitance that "counts" is NOT the one between adjacent turns.
A properly applied choke (i.e. one whose only interaction with environment
is via terminals) develops a wave like this. Imagine a very short pulse
applied to terminals. After a bit of time the pulse has progressed
toward center. At any one instant the electric field starts from
a point near the left terminal and extends to the corresponding
point near the right terminal. At that instant the only capacitance that
"counts" is between those two points, etc. Therefore it is the
"differential" capacitance that counts, not the one between
adjacent turns. This is exactly the same as in any other transmission
line, which is to be expected knowing that lumped elements (R, C, L)
are just abstractions. Transmission lines are also abstractions,
but on a "higher level".
> Talking about 1.8 MHz it's uncommon to go beyond resonance with common coil
> sizes and
> standard RG213 lines. The warning to have an excessive number of turns is a
> purely academic
> speech with a little practical application.
No, it's not academic, it's just for fun like the rest we do :-)
> Chokes, moreover, out of the TEM mode which is confined inside,
> are not lines and do not behave like lines. In the "outside" of the choke,
> there is one, and only one, self resonance.
I'd take this as said in good faith, but even good faith errs occasionally.
Everything that happens inside the choke is visible
in it's outside appearance - impedance.
Please go to my site to see multiple alternating parallel and
series resonances of an ordinary HF amplifier choke:
http://www.geocities.com/va3ttn
Thank you for the opportunity to discuss the matter.
73,
Sinisa YT1NT, VA3TTN
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