[Elecraft] open wire feeders

[Jim Brown]

On 12/31/2011 9:42 AM, Don Wilhelm wrote:
> My first rule is to use balanced antennas - off center fed antennas are
> famous for feedline radiation and RF in the shack.

RIGHT!  Some important fundamental principles here.

1) A two wire line will be balanced ONLY if what is connected at each 
end is balanced.

2) Any imbalance in the antenna or the turner will cause un-equal 
currents in the two conductors. The DIFFERENCE between those two 
currents is called common mode current, and it will radiate, just like 
the current on the outside of coax.  Feeding an antenna off-center 
creates a LOT of imbalance, which, as Don has said, puts a lot of common 
mode current on the line, and thus a lot of RF in the shack.

3) Most practical ham antennas, even those that we try to build as 
balanced antennas, like center-fed dipoles, are unbalanced a bit by 
their surroundings -- trees, buildings, unequal height, sloping ground 
under the antenna, etc.

4) This unbalance may not hurt the antenna's performance too much, but 
because antennas work essentially the same on both TX and RX, that 
feedline picks up noise and couples it to the receiver.  THAT'S why we 
need a really good common mode choke at the feedpoint of ANY antenna.

There's another HUGE problem with off-center-fed antennas -- the 
unbalanced current on the feedline -- ANY feedline, including any form 
of parallel wire line -- can be quite large, and can easily cause even a 
very good common mode choke to fail with high power.

73, Jim Brown K9YC