Jim, your comment is both accurate and inaccurate.
In general it is accurate but does not have to be like that.
Things have changed.
The amount of common mode current on the feedline of an OCFD is determined
by many factors. One of them is "how far off center" the feedpoint is. The
closer to the center, the less CMC gets onto the feedpoint. OLD OCFD's have
used 33% (from one end) as their feedpoint.
Recently other splits have shown up, some of which are closer to the center.
These newer ones have inherently less common mode current.
ALL commercial OCFD antennas of the past have used a single-core 4:1
Guanella balun (no exceptions that I know of). These not only do a lousy
job of impeding common mode current, they actually generate common mode
current, as Steve Hunt (G3TXQ) has mathematically shown us and then shown us
the lab measurements to prove it.
Bottom line there: everything we know of to date was of poor design.
Finally, I have spent several hundred hours in the field measuring common
mode current on all sorts of OCFD antennas, strung in various configurations
and using both types of baluns. I've bent the coax all over the place and
re-measured, each time comparing the amount measured common mode current to
its impact on the SWR curve across each ham band.
I have over 500 sets of measured data.
About half of these are posted on my web site.
If you use a single core balun like all the US commercial vendors are
selling, then you are correct Jim. You do get CMC onto the feedline.
But if you use a well designed dual-core balun, then you are incorrect. You
must do some really dumb things to it (like running the coax just a few feet
away from one leg) before you get significant amounts of common mode
current.
BTW, I tried that on a dipole too and it also got common mode current on the
coax, albeit, not as much as the OCFD did. However surely nobody would ever
erect an antenna like that!
So in practice you are 100% correct if a person goes out and buys a
Buckmaster or Carolina Windom. However it is possible to build one
yourself, or buy one from Scott W4PA, which does not have this problem.
DISCLOSURE: I have a commercial interest in the antenna products Scott is
selling.
73 - Rick, DJ0IP
(Nr. Frankfurt am Main)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Jim Brown
Sent: Tuesday, April 14, 2015 10:30 PM
To: tentec@contesting.com
Subject: Re: [TenTec] Field day antennas
On Tue,4/14/2015 12:36 PM, John Bescher via TenTec wrote:
> And for your comment on the OCFD,
Off-center fed antennas put lots of common mode RF on the feedline, which is
bad news for interaction between stations.
Yes, stubs and bandpass filters can help reduce interference between
stations. So can a better rig -- cleaner TX and more bulletproof RX.
That's why K3s are the current weapon of choice, with rigs like the KX3 and
TS590 a decent second place.
Don't even think about bringing an el-cheapo do-everything wonder like an
IC706, FT100D, or newer equivalent Yaesus to FD -- you'll be spewing trash
into all the other stations!
73, Jim K9YC
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