Just to expand my previous posting:
With a 20ohm ground connection at the bottom of the braid, if the
reactance seen at the dipole feedpoint looking into the braid path were
cancelled by the CM choke, the resulting CM path resistance would be
less than 100 ohms for any braid length between 0-12ft and 22ft-35ft
(and "repeats" thereafter). In other words, this effect is not
constrained to braid lengths close to an electrical half-wavelength -
over 70% of all possible braid lengths could exhibit significantly
increased braid current from the insertion of an Reactive choke.
Steve G3TXQ
Steve Hunt wrote:
> It's actually short of an electrical half-wavelength, which gives it
> about 300 ohms of series capacitive reactance. That reactance is then
> cancelled by the inductive CM choke, leading to a lower CM path
> impedance and increased braid current. The effect is most noticeable,
> not when the braid is an electrical half-wavelength and the CM path
> impedance is lowest, but when it has a significant reactive component
> which may be offset by the choke impedance.
>
> Even with 16ft of coax a Reactive choke will increase the braid current;
> albeit the current is low to begin with.
>
> The point is that most operators will never know what their braid
> impedance will be; and it will likely be very different on different
> bands. So the safe option is a CM choke with a high RESISTIVE impedance.
>
> 73,
> Steve G3TXQ
>
>
> Dr. Gerald N. Johnson wrote:
>
>> Of course there's lots of current in the braid, you made it nearly a
>> half wave long. Try the same experiment with that braid 16 feet long and
>> you will find much less current because then its a quarter wave long
>> terminated in a low impedance.
>>
>> 73, Jerry, K0CQ
>>
>>
>
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