One has to take care that the added reactive impedance doesn't become
series resonant with the available wire length. You wish to shift to a
wire plus reactance that makes an odd multiple of a quarter wave to
minimize the current in that wire. If you slip up and the combination
makes any multiple of a half wave, you have maximized the current in
that wire.
73, Jerry, K0CQ
On 8/7/2010 4:26 AM, Steve Hunt wrote:
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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