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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