[Elecraft] OCF antennas

[Jim Brown]

On 3/9/2012 5:48 PM, Guy Olinger K2AV wrote:
> OCF antennas, under miscellaneous names, have been working fine, just fine,
> just about as long as radio.

"Working fine" depends on your definition of "the big picture," and your 
ability to diagnose problems. The problem with OCF antennas is COMMON 
MODE FEEDLINE CURRENT, which causes the feedline to receive noise, make 
the shack hot with RF, and radiate RF to the TV and stereoand computer 
in our living room (and our neighbor's living room).   Fifty years ago, 
there was relatively little man made noise to be picked up on the 
feedline of our antennas, the equipment in our living room did not have 
Pin One Problems that turned all of the wiring into receiving and 
transmitting antennas, and the equipment in our neighbors' living room 
was not full of noise generators (other than their TV set's horizontal 
flyback system).

Equally important, we had not LEARNED about common mode current on 
feedlines, and its contribution to these problems.  We lived in blissful 
ignorance.  We called CQ, we got responses, we had fun, but we also had 
TVI!  And when the electronics world changed, introducing Pin One 
Problems, digital equipment, and switching power supplies to create 
noise, COMMON MODE CURRENT on feedlines started biting us in the behind, 
WHETHER WE KNEW IT OR NOT.

  As Guy and I have both noted, we can get away with unbalanced antennas 
if we choke them to death, but we're going to fry chokes if we run power 
unless we use MULTIPLE chokes.  And "choking them to death" means 
multiple turns around ferrite cores (multiple cores and multiple chokes 
for high power).

A FAR better solution, if we can do it, is resonant antennas for each 
band, well choked.  If you're limited on space that can be done with fan 
dipoles or traps or loading coils.  And there is NO MAGIC to parallel 
wire feedline -- there can be just as much common mode current on 
parallel wire line as on coax if the antenna itself is unbalanced.  
Balance is determined by the entire circuit -- the antenna, the 
feedline, and the transmitter (including the tuner), not ONLY the 
feedline. That's why I object to  the words "balanced feedline" -- they 
are are pure fiction.

73, Jim K9YC