[Elecraft] OCF antennas

[Guy Olinger K2AV]

OCF antennas, under miscellaneous names, have been working fine, just fine,
just about as long as radio.

We just have 50 ohm coax and SWR meters on the brain, and have a terrible
time seeing outside the rather narrow 50 ohm coax/low SWR box.  And we're
boxing ourselves in further with single-Z transistor amps that fall off the
table and go blurg off 50 ohm Z.  (Whatever happened to
I-don't-give-a-d*mn-about-SWR tetrode amps, like 807's, 6146's and
4CX1000A?  I worked DXCC and had a BPL medallion before I ever had an SWR
meter.  Just can't understand how I managed.  :>)    Oops, forgot, Alpha's
8410 monster uses a pair of those 4CX1000A's, and it really doesn't give a
d*mn about swr, either.)

One famous (back in the day) OCF oldie is the ORIGINAL Windom, the single
wire feed version Windom, decidedly an OCF.  It's only a single band
antenna, but you feed the single up wire with a 9:1 auto transformer at the
ground against what can be minimal  to almost non-existent radials since
the Z is 400-450 ohms at the bottom of the feed wire, and it takes an
absolute totally ugly resistive stinking ground connection to mess it up.
 Very useful on field days.  It's supreme mechanical advantage is that the
single wire feedline can be a single stranded #18 wire, even for QRO, far
and away lighter than any other feedline. This means you can get what
amounts to weird-fed 80 and 40 dipoles WAY up there, that are only
supported at the ends, without all the weight issues.

How does it work?  It USES the common mode feedline induction from the long
side of the OCF dipole, to work against the opposite phase coming from the
feed (it's the same piece of wire).  If you monkey with it a bit in a
model, you can pick a connection point for the single wire feed where the
current is flat all the way up, no standing waves, which means it is
radiating very little from the feed. Or you can move it around a bit and
PICK the amount of vertical radiation you would like to mix in.   But that
changes the feed Z.

 Disadvantage?, you can't buy the 9:1 autotransformer off the shelf any old
where and you may have to roll your own.

OCF antennas in general CAN be tamed at QRO, but one needs to know to do
it.  Just running a feedline to it and putting power on it WILL get you RF
in the shack as others have opined.

Using an isolation device at the antenna end only gets PART of the problem.
The other part is that the now isolated feed line is STILL off center
approaching the OCF antenna.  This in and of itself will cause the
"isolated" antenna to induce current on the feedline shield, because the
long part of the antenna at right angles to the feed line induces more
current to the feedline than the opposite phase current from the short
part.  This is not a problem in a dipole where ordinary (and fairly weak)
devices pull off any needed isolation.  "Weak" isolating devices will not
cut it for OCF.

A SECOND isolation device down the coax a bit, that breaks up the coax
shield into a non-resonant length next to the antenna, will prevent the RF
in the shack AND it will clean up the pattern to what the models say it
should be, usually a performance plus.  Care needs to be taken to the
suitability of the isolation devices for the two uses.

Some antennas use the second device as the ONLY device and deliberately use
a length of the shield as a common mode radiator.  No advertising needed.

If you suspect that cleaning up all this tends to work against multiband
use and makes doing OCF well a chess game, you suspect correctly.

Portable use at 100 watts without all the isolators will work just fine.
 OCF's have been a mainstay of field day use as long as that has been going
on.  My K2/10, with the built-in ATU and the itty bitty balun love those
antennas and never had the first problem.

73, Guy.

On Fri, Mar 9, 2012 at 7:09 PM, Don Wilhelm <w3fpr at embarqmail.com> wrote:

> Well said Jim,
>
> There is no way to obtain equal and opposite currents on the feedline
> with an offset fed antenna - they are a sure recipe for
> RF-in-the-shack.  As much acclaim as the Carolina Windom gets,  that
> fact is still true, one just cannot run high power with such an antenna
> - at low power the RF levels may be tolerable, but at high power, they
> can wreak havoc in the shack with RF all over everything.
>
> 73,
> Don W3FPR
>
> On 3/9/2012 5:58 PM, Jim Brown wrote:
> > On 3/9/2012 1:50 PM, Bill wrote:
> >> I have found the easiest single antenna to utilize is the OCF
> > Off center fed antennas are a recipe for noise pickup on the feedline.
> > While a good common mode choke can help, off-center feed can also burn
> > up a common mode choke if you're running much power.
> >
> > Bottom line -- off-center-fed antennas are a bad idea.
> >
> > 73, Jim Brown K9YC
> >
> >
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