Jim Brown wrote:
> There is a MAJOR advantage to using a ferrite core -- it makes the
> choke RESISTIVE rather than inductive. A feedline shorter than a
> quarter wave looks capacitive in the common mode circuit. Adding an
> inductor in series (the coil of coax without the ferrite) tunes out
> that capacitance, which reduces the impedance and INCREASE THE
> CURRENT.
>
>
K9YC is correct in pointing out that a coax choke's impedance is due to
its reactance and the impedance obtained when using a ferrite is
predominately resistive. In some situations this can cause problems if
a reactance is added in series with the coax choke which causes the
feedline plus the choke to be resonant close to operating frequency.
Here are a few cases to consider:
At less than 1/4 wavelength a line will look capacitive only if one end
is open, but that never happens with a feedline. If it did, the
feedline would be open at the station end and there would be no current
going into the station equipment anyway. Since both ends of the
feedline are connected to low impedance loads, the line looks inductive
and adding any inductance will lower the current. (Adding a series
capacitor will increase the current. It's like tuning a small loop
antenna. You do that with a cap.)
You could possible create a resonance at 1/2 wavelength. If the
feedline plus the added inductance causes a resonance close to the
operating frequency this would be a case for maximum feedline radiation,
but it's like feeding a halfwave vertical, the current is very low at
the feedpoint. So the addition of the added coax choke inductance
results in lower common mode current into the station equipment,
although the radiated signal from the feedline could possibly cause a
problem.
The real problem happens when the feedline length plus the choke
reactance causes a resonance close to one wavelength. (Think full wave
loop.) That may produce significant current because the feedpoint
impedance is low, and if the choke moves the resonance frequency closer
to the operating frequency then it increases the common mode current,
not decreases it. Of course this also happens at integer multiples of
one wavelength. This might happen at feedline lengths of 400 to 600 feet.
I'm not saying a coax choke is better than a ferrite choke because it's
not. Ferrites have several advantages. I'm just trying to be accurate
in describing what happens when you use a coax choke.
Jerry, K4SAV
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160 meters is a serious band, it should be treated with respect. - TF4M
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