The OCFD is a compromise antenna for someone wanting a simple, single wire
antenna that runs all bands and does not need an expensive matchbox. It is
not the antenna of choice for those trying to win the contest.
To all of you guys commenting on the common mode current of an OCFD, I have
one simple question:
HAVE YOU EVER PERSONALLY INSTALLED AND OCFD AND MEASURED IT?
If the answer is no, then it is speculation.
Common mode current is not hocus pocus, it is simply RF flowing on the
outside of the coax and it can be easily measured with a clamp-on RF
Ammeter.
I have spent about 500 hours investigating this antenna and doing so with
all sorts of configurations. I began by testing a dipole with various balun
types, then repeated the same tests for different OCFD antennas, different
feedpoint splits and different baluns.
The results speak for themself:
The single core 4:1 Guanella balun does indeed have common mode current
problems... and this is what all of the vendors are selling. THIS is the
reason for the bad reputation of the OCFD.
However the OCFD using a dual-core 4:1 Guanella balun behaved much like a
dipole in common mode current tests, until I did some drastically stupid
things with them; specifically, running the coax parallel to one leg of the
antenna with just a few ft. of separation from the leg of the antenna. Then
the common mode current on the OCFD was greater than that on the dipole.
But nobody installs antennas like this! (I hope)
The OCFD is not the greatest thing since sliced bread but it is quite
capable of being a wonderfully simple multi-band antenna, "IF" you use the
right kind of balun in it.
I'm very happy to review any measured data suggesting this wrong, and we can
compare it to my measured data (posted on my web), but speculation doesn't
really prove anything.
The rubber hits the road in the field!
Certainly I would prefer Loops over an OCFD. No question!
But the OCFD is the simpler antenna.
Certainly I would prefer monoband Yagis over everything else.
But in most of the clubs I have been in, the best we could do is simple wire
antennas, unless I brought along one of my Spiderbeams.
I have two friends who did a mini expedition to an island, using 2 OCFD
antennas in close proximity of each other and only running 100w. They had
no problems. But they operated on different bands.
Obviously someone going to all the effort to set up a multi-multi station is
going to want (need) better antennas than a simple OCFD.
It's horses for courses.
73 - Rick, DJ0IP
(Nr. Frankfurt am Main)
-----Original Message-----
From: TenTec [mailto:tentec-bounces@contesting.com] On Behalf Of Don Jones
Sent: Thursday, April 16, 2015 8:32 AM
To: k9yc@arrl.net; Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] Field day antennas
BINGO!!! YOU GET THE PRIZE!!!
Common mode RF is a huge intra-station killer at a Multi-Multi field site.
Last year at W7PIG 6A WWA we used resonant antenna's fed with RG-213. We
used 2 tri-band yagi's that were 300 feet apart. Each yagi was fed with a
tri-plexer that had a ground rod to it. On 80 & 75 meters we used inverted
vees. On 40M we used a 2 element driven delta loop arrays. The loops were
optimized delta loops from the ON4UN Low Band DX'ing book.
They werevertically polarized and identical in resonance. The loops were fed
90 degrees out of phase. The apexes were 45 ft high and the performance was
outstanding.
With everything being 50 ohms the performance of the bandpass filters and
coaxial stub filters were flawless.
We had CW & SSB run stations on 15, 20, & 40 meters. We had little or no
intra-band/station interference using a mix of older rigs.
I believe a pair of OCF Dipoles would be OK in a multi-two deployment. I
have my doubts that two stations could operate at the same band at the same
time. The antenna's would have to have adequate separation, the ladder line
would have to come straight down to a remotely located antenna tuner, I
would not try this without a decent ground at the tuner to terminate to
common mode RF prior to bringing a coaxial feed line back to a common
operating location.
Just my $.02.
Don KO7i
On 4/14/2015 1:30 PM, Jim Brown wrote:
> 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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