----- Original Message -----
From: "Tom Rauch" <w8ji@contesting.com>
To: "Jim Lux" <jimlux@earthlink.net>; <towertalk@contesting.com>
Sent: Thursday, June 03, 2004 2:20 AM
Subject: Re: [TowerTalk] matching to screwdriver antennas
> >> One approach that has been used is to use a 4:1 (or other
> ratio, as
> > suitable) transformer, with capacitive coupling between
> primary and
> > secondary (either parasitic, or explicit). The idea is
> that as the frequency
> > goes up, the capacitance tends to dominate over the
> transformer. At low
> > frequencies, the impedance of the capacitor is high enough
> that it is
> > irrelevant.
>
> I'm missing something Jim.
>
> How does the transformer decrease magnetic mutual coupling
> effects from primary to secondary as it increases "bypass"
> capacitance to effectively reduce turns ratio with
> increasing frequency?
>
> All I see happening is it making the system reactive while
> the effective ratio barely changes if flux coupling is high.
Assume an ideal transformer, in series with an inductor (the leakage
inductance)
The capacitor is across the whole thing (i.e. in parallel with the series
inductor and the transformer)
I think that the scheme (which, as I say, I haven't analyzed in any detail)
depends on there being leakage L in the transformer (which is the same as
saying the coupling isn't perfect). Then, you have the transformer in
series with a low pass and a short in series with a high pass. (and all
sorts of weird reactances, but those can be cancelled by tuning the antenna
itself away from resonance)
But, even with the transformer being perfect, thinking early in the morning,
it might still work. Take a limiting case, where the capacitor is very
large (i.e. the reactance is very low). The transformer is essentially
shorted, so the impedance seen at one port of the network will be the same
as the impedance seen at the other. (probably, plus some shunt L, due to the
transformer.. It's too early for me to work out what the two terminal
characteristics are of a 4:1 transformer with windings shorted to each
other.) Now take the other limiting case, where the capacitive reactance is
very high (low frequency). The impedance at one port will be transformed to
the other port by the transformer. Somewhere in between, the effect should
be somewhere in between. The real question is whether the transition
between "transformer" and "capacitor" is "well behaved". It could
(especially if there's some other reactances in there) have ugly resonances.
As far as the reactance introduced by the network, it is essentially
cancelled by adjusting the L in the antenna to higher or lower, as required.
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