[TowerTalk] Best Balun

[Jim Brown]

On Wed,11/26/2014 1:50 PM, Wilson wrote:
>   As for balance, there’s a good bit of impedance in a wavelength or two of coax shield, so I doubt if the shield end at the top is actually very near ground.
>          This is mentioned in the list of factors someone gave.  No doubt, but how much does it affect the pattern?

WL,

A good common mode choke prevents the feedline from being part of the 
antenna. Does that matter much on transmit?  NO, EXCEPT if it puts RF in 
your shack. Does it matter on receive? YES! because the signal picked on 
the feedline couples to the antenna and fills in the nulls in the 
pattern.  Goodbye front-to-back and front-to-side rejection. Hello NOISE.


> I’m not competent to make analytical arguments about which choke is best, but rather I maintain that using the ARRL choke and getting the Z of the choke close to ten times the feedline Z is likely to be all anyone needs.

In the link I posted, I showed that RESISTIVE choking Z on the order 
5,000 ohms (10X your "rule of thumb") is a far better design goal, both 
for the decoupling concerns (TX and RX) and for power handling.  I also 
showed why chokes must be resistive, not inductive, which means that 
they must be multi-turn chokes on #43 or #31 cores.

73, Jim K9YC