[TenTec] The OCFD Mystique
k3ndm at comcast.net
Wed Sep 23 10:30:57 EDT 2015
You may be correct about the very early work on the Windom, but
later on it was definitely accepted as a multiband antenna. However, in
those days we only had 15 meters that was not an even multiple of 80
meters; I'm not including 11 meters as I'm not sure how many hams really
used that band seriously.
When the crowd set out to discover where and how to feed an antenna,
there was little or no understanding of SWR. It was demonstrable by
using leacher lines, that instrument before the slotted line, but few
cared from my reading. The concerns were RF in the shack and successful
loading of the final amplifier meaning impedance matching the plate
circuit to the antenna system. Nothing else seems to have been
addressed, not even feed line loss in those days.
I still stand by my comments about the Windom, OCFD, and dipole
being the same antenna with different feeds. The are 1/2 wave, Hertzian
antennas. Ergo, there patterns must be the same except for the
perturbation cased by the feed.
Any antenna has a standing wave on it. If it is a 1/2 wave, there is
a current loop at its center, meaning that is where the impedance is
lowest. And, there is a voltage loop at its end meaning this is high
impedance. I can feed an antenna any where along its length and hit any
impedance I want from very low to high. That means I can hit the 300 or
200 Ohm spot on the antenna. Whether I use a 4:1 or a 6:1 balun in my
design is a function of what I infer the feed impedance to be
considering where I feed the antenna and its height above ground.
You are correct about concern for common mode currents. I do use a
4:1 current baluns which we bought for our FD operation. We got them
from Balun Designs. The other alternative is to use ferrite or powdered
iron RF chokes to keep common mode currents in their place. I've also
used coax windings on PVC pipe as a choke. that works pretty well also.
In fact, I make this recommendation to those of you who are reading this
and have a shack RF problem use a choke outboard of your shack to help.
Just make sure it can handle the power level you are running and at high
SWRs; a 1KW rated choke when you are running 100 watts should suffice.
------ Original Message ------
From: "Rick - DJ0IP / NJ0IP" <Rick at dj0ip.de>
To: "'Barry LaZar'" <k3ndm at comcast.net>; "'Discussion of Ten-Tec
Equipment'" <tentec at contesting.com>
Sent: 9/23/2015 5:03:29 AM
Subject: RE: [TenTec] The OCFD Mystique
>Boy there are a lot of wrong statements in that one, Barry.
>There was good sound advice, mixed in with some totally wrong
> "So Windom said I have the answer. I'll feed the antenna at a point
>that would reflect a reasonable impedance regardless of band. THERE
>NEVER was a consideration for SWR. The SWR issue ONLY came up after
>coax cable began to be used widely."
>THIS IS TOTALLY WRONG! Windom never said anything of the kind!
>Windom NEVER set out to create a harmonic antenna with equal impedance
>on all bands.
>His only goal was to find a position to feed the Herz, such that there
>were NO STANDING WAVES ON THE FEEDLINE! THAT WAS HIS ONLY GOAL!
>Read Kruse's 1926 QST article and read the original Windom Paper (1929
>I gave exact details of where these are in my previous email.
> "Next, a Windom type antenna, dipole, and OCFD are the exact same
>thing. The only difference is the feed. A 132' wire will have generally
>the same gain regardless of how you feed it. However, the pattern will
>be skewed a bit showing a bit more gain in one direction feeding it off
>THE ONLY THING THESE ANTENNAS SHARE is approximately the same radiation
>The Windom (the real one) as well as the false Windom which is what
>most people today believe to be the real Windom, both needed a good
>ground network to be efficient. The dipole and OCFD do not.
>The dipole and OCFD are similar but the OCFD has significant common
>mode current characteristics that must be addressed.
>"Most Windoms, OCFDs are designed for a 300 Ohm feed point making it
>necessary to use a 6:1 balun transformer."
>WRONG: Most OCFD antennas today use 4:1 baluns. You can't get the
>OCFD to have 300 or 200 Ohms. You might get a single band to match
>that, but that's it. The impedance is on the order of 230 to 240 Ohms
>when it is elevated around 30 to 50 ft - and it is about 260 to 280
>Ohms when it is above 70 ft. Obvious ground type and other
>environmental thing influence this.
>This point was researched by Buck Rogers (K4ABT) long ago and has been
>on his web site for as long as he has had one. At one point in time
>Buck sold 5:1 baluns with his OCFD's but these were complicated beasts
>and he soon went back to 4:1.
>"And if you have RF in the shack, you are not using a good balun or
>have coupled energy from the antenna to the transmission line, not an
>uncommon issue if you do not come away from the antenna at a right
>BARRY IS ABSOLUTELY RIGHT ON THIS POINT! THIS IS VERY VERY IMPORTANT.
>Unfortunately I know of no commercially available balun at this time
>that will sufficiently deal with the common mode current problems of
>the OCFD on 80m. As a result you should always use an additional RF
>choke at the feedpoint. A Maxwell might suffice. A Guanella will.
>If anyone thinks (s)he know of one, don't just call it by the
>manufacturer's name, tell us the make and model number and be prepared
>to tell us what's inside the box. If you don't know, then how can you
>know it is good enough?
> "Now having gone through all of that, let me say, A very good
>antenna is a center fed 1/2 wave on 80 meters fed with open wire, if
>you have a fair run. Where you enter the house/shack, you transition to
>something like LMR-400 using a good 4:1 current balun."
>SORRY, NOT CIGAR. DO NOT (only) USE A 4:1 BALUN in front of your
>The problem here again is the common mode current and it must be
>removed. A 4:1 balun, regardless of how good it is, is rarely good
>enough. YOU MUST USE A 1:1 GUANELLA BALUN HERE!
>If your matchbox is incapable of matching the 2K+ Ohms on some bands,
>then it is fine to insert a 4:1 between the 1:1 and the openwire
>feedline. But it is used "in addition to", NOT "instead of" the 1:1
>balun. BUT REMOVE THE 4:1 BEFORE OPERATING ON THE FUNDAMENTAL
>FREQUENCY OF THE ANTENNA.
>This is explained (proven) by W8JI and G3TXQ.
>I have proven it too (in practical testing) but I'm not an engineer so
>my word does not count. ;-)
>PEOPLE, FORGET ALL YOU THINK YOU KNOW ABOUT BALUNS & TUNERS, MASTER
>CLEAR AND DO YOUR OWN DUE DILIGENCE.
>(use the links below to the real gurus of the industry)
>Balanced Tuners and common mode current, by VK2OD
>Tuner Balun Ratios: 4:1 or 1:1? , by G3TXQ
>Tuner Baluns, by W8JI
>Choosing the Correct Balun, by W8JI
>RFI-Ham, by K9YC
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