[Elecraft] Slightly Unbalanced dipole?

[Don Wilhelm]

Al,

If not placed at the antenna feedpoint (up at the antenna), then the 
feedline may radiate due to common mode current.
For feedlines using parallel conductors, the problem is not as bad if 
the feedline exits at right angles from the radiator for at least a half 
wavelength.  In that case, the common mode choke can be placed at the 
junction of the parallel line and coax.

In all cases of a radiator fed with coax, a good common mode choke at 
the radiator feedpoint is necessary.  Even though the RF on the inside 
of the coax is balanced, when the coax reaches the radiator, the side of 
the antenna connected to the coax shield sees two paths.  One is the 
side of the radiator, and the other is the outside of the coax braid - 
it will dutifully split to follow both paths depending on the impedance 
of each one.  A good common mode choke will prevent the RF current from 
seeing the coax shield as another conductor.

A similar situation exists at the transition between parallel feedline 
and coax.

73,
Don W3FPR


On 1/13/2019 6:42 PM, Al Lorona wrote:
> By the way, whenever Jim says, "...a very good common mode choke at the feedpoint of an antenna...," he means an antenna fed with coax. For the rest of us, of course, that choke would (should) go at the output of the antenna tuner, whether in the shack or close by.
>