[TowerTalk] Common Mode RF choke on a 50 MHz yagi

[Richard (Rick) Karlquist]

On 6/28/2020 3:25 PM, David Gilbert wrote:
> 
> 
> That's a standard quarter wave coaxial choke that has been in antenna 
> books for several decades in one form or other.  I used that technique 
> on a triband quad (with separate feedlines) more than 50 years ago.
> 
> Off the top of my head, though, I think your version requires that the 
> driven element not be isolated from the boom.  Not sure it would work 
> with the driver isolated from the boom.  I'll have to draw it out later 
> to double check.
> 
> 73,
> Dave   AB7E
> 
> 

If you want to be a perfectionist:

Cut a second identical 56 inch cable and connect its
shield to the feedpoint terminal that is connected
to the center conductor of the balun cable.  Bond
the shield at the other end to the boom in the
same location as the balun cable.  Do not connect
the center conductor of this second cable to
anything at either end.

You could get away with just taping these two
56 inch cables together, but it might be
desirable to space them apart somewhat.

The second (dummy) cable results in both driven
element halves having the same (very high) parasitic
impedance applied to them.  IE, perfect balance.

I believe I saw this in Marchand's 1939 balun paper.

Rick N6RK