[Elecraft] OCF antennas

[Guy Olinger K2AV]

Well, I don't think you can blame OCF for TVI/RFI.  You can blame radiating
RF for TVI/RFI.  Some of the best improvements in RF antennary were
accompanied by increases in TVI/RFI, and that would include cleaning all
that common mode current off the feedline so it only radiated off the
driven element down toward the horizon like it should be.  BTDT.  Back in
the day of 21 mc TV IF's, just sneezing on fifteen meters would cause TVI.

If one only uses antennas that you don't have to think about, what fun
would that be?  Part of the draw of ham radio is making metal stuff work
like an antenna, in spite of itself.

73, Guy

PS, who doesn't know what "mc" is?

On Sat, Mar 10, 2012 at 3:27 AM, Jim Brown <jim at audiosystemsgroup.com>wrote:

> 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
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