Ian White GM3SEK wrote:
But maybe Cecil is setting the bar too high by imagining the worst
possible case.
I immediately followed that posting up with another in
which I said 500 ohms of choking impedance is sufficient
for a resonant 1/2WL coax-fed dipole. But assuming the
1/2WL dipole is resonant on 3.75 MHz, 500 ohms of choking
impedance is really not enough at the band edges.
There's another consideration. The location of the common-
mode nodes and antinodes are not necessarily related to the
differential signals because the velocity factor is different
for common-mode signals Vs differential-mode signals. One
could encounter the "worst possible case" and not even be
aware of it.
The moral is: Follow Ian's advice and actually make common-mode
current measurements before and after installing the choke.
I have seen the installation of a choke actually increase the
common-mode current at a point 1/4WL away from the choke.
--
73, Cecil
http://www.w5dxp.com