I'm not advocating bead chokes; I fully appreciate the benefits of "N^2"
on a toroid core and the ability to put the SRF somewhere useful because
of the increased stray capacitance. Rather, I am trying to understand
why you would label a choke "And it's _Inductive!_ (Bad)", whose
Resistive component is greater than its Reactive component over the
large majority of the HF spectrum.
On a separate topic - one where I think we could agree - I recently made
measurements on my ladderline-fed doublet to determine its 3-terminal
equivalent circuit. This is a doublet which is supposedly balanced:
equal length legs; ladderline coming away at right angles etc. On 80m
the 3-terminal Star network equivalent turned out to be:
Z1 = 15 - j70
Z2 = 1.6 - j109
Z3 = 31 +j110
Compare those figures with a completely balanced and floating load,
where Z1=Z2 and Z3=Infinity, and it's clear why we need to consider a
choke even on a nominally "balanced" antenna.
On 17/04/2012 20:27, Jim Brown wrote:
> Now, here's where we diverge a bit on our analysis of the usefulness of
> strings of beads. Yes, a single turn through #31 (that is, simply
> clamped onto the cable) shows resistance the same general order of
> magnitude as reactance between 4 MHz and 20 MHz, you've got to have a
> LOT of beads in that string to hit 5K ohms resistive, which is known to
> be a good design value to make a dent in RX noise. By contrast, it's
> easy to hit 5K with the multi-turn chokes on #31 or #43. The only times
> I use a "string of beads" in single turn fashion are either at VHF, or
> where it simply isn't possible to wind turns.
TowerTalk mailing list