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
On 8/6/2010 3:43 PM, Steve Hunt wrote:
> It might be worth expanding on Jim's valuable point about the
> undesirability of chokes which have Reactive CM impedance, rather than
> Resistive.
>
> Here's an experiment those of you with antenna modelling software can try:
> Take a 20m half-wave dipole 30ft above average ground. Connect coax
> braid between one side of the feedpoint and ground. Insert a 20 ohm
> resistor at the bottom of the braid to represent a moderately effective
> ground connection. Now look at the current flows at the feedpoint.
>
> With no choke:
> Dipole Leg1 = 1A; Dipole Leg2 = 0.96A; Braid = 0.15A
>
> With 270 ohm INDUCTIVE choke at the feedpoint:
> Dipole Leg1 = 1A; Dipole Leg2 = 0.31A; Braid = 0.7A
>
> With 270 ohm RESISTIVE choke at the feedpoint:
> Dipole Leg1 = 1A; Dipole Leg2 = 0.92A; Braid = 0.1A
>
> See how adding an Inductive choke has INCREASED the braid current
> dramatically!
>
> Also note how the 270 ohm Resistive choke has improved things, but not
> by much; you need a Resistive choke of at least 4000 ohms to get the
> braid current below 0.01A in this instance. That should dispel the old
> Rule-of-Thumb about needing 5 times the antenna feedpoint impedance for
> the choke impedance.
>
> Steve G3TXQ
>
>
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