> To quote Tom N4KG: <Time for a repeat of this 'oldie
but goodie' from
> the archives by Ed Gilbert, WA2SRQ. See below.>
>
http://lists.contesting.com/archives//html/Towertalk/2001-03/msg00125.html .
>
> What I got out of the posting is that a random-wound
choke is not as
> effective at high HF as is a solenoid (i.e., carefully)
wound coil of coax.
The difference with scramble wound is it moves the resonant
frequency lower, making the choke more effective with fewer
turns but over a smaller usable bandwidth.
As a matter of fact, the following statement was extracted
directly from that link:
"All the baluns start out looking inductive at low
frequencies, as
indicated
by the positive phase angles. As the frequency is
increased, a point is
reached where the capacitance between the windings forms a
parallel
resonance with the coil's inductance. Above this frequency,
the winding
reactance is reduced by this capacitance. "
That might lead you to think impedance falls off right above
resonance.
Parallel capacitance does NOT reduce impedance of a winding
until Xc is less than 1/2 of XL.
This means the reactance INCREASES at frequencies from
nearly dc up to 1.414 times the self-resonant frequency by
the addition of shunt C. Say you add enough stray C to
equal XL at 21 MHz. Now the choke has VERY high impedance at
21 MHz, limited only by Q. At 10 meters, the impedance is
the same as it was on 29MHz before you added the stray C!!!
It never got worse. The 14MHz impedance gets better also, as
does the 7MHz and 3.5MHz impedance.
So if you don't have enough turns to be at self-resonance,
adding shunt C always helps increase impedance.
I think W7EL has some articles in the Antenna Compendium on
scramble wound choke baluns.
The only reason I don't suggest them is because it is easier
for me to describe the size of a solenoid for optimum
impedance. But don't fear scramble wound! It can work OK.
73 Tom
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