[TenTec] The OCFD Mystique

[Rick - DJ0IP / NJ0IP]

Man, I hope I don't come across as evil after this response.
This is to Wade and Barry.

Guys there are a lot of myths and fallacies in the paragraphs below.

First Wade, there is no reason on earth not to use Openwire with OCFD
antennas.
If you back in the QST archives, back in the 30s, it was very popular.
Many people do it today.

Second, ignoring matching losses, the gain and radiation pattern of 130 ft.
of wire is the same regardless of where you feed it.  You can feed it in the
center, at either end or anywhere in between without changing gain or
pattern.

Matching Losses is the "gotcha" here, and here's where the openwire fed
antenna sometimes comes out ahead.  Sometimes.  It depends on the quality of
the matchbox and the band you are working on.  Most matchboxes are
inefficient on 10m when trying to match high impedances and that is what you
have with the antenna you suggested.

With the OCFD antenna it is possible to get the mismatch to under 3:1 on
many bands and with short runs (less than 100 ft.) of good coax, the loss is
negligible.  What remains is balun and choke loss.
THEY ARE THERE, and as I stated earlier, that is the price one pays for the
convenience of not running openwire through the air and not having to have a
matchbox that can cope with high impedances efficiently.

I ran openwire all my life (not to my beams of course) until I moved here.
Can't run it here.  Landlord won't allow the ugly stuff.  This is what
forced me to switch to OCFD.

Now to Barry, whose post included good recommendations and some.... well,
read below.

The OCFD is not really a variant of the Windom.  W8JI does a fine job of
explaining why.
The Windom was not a harmonic usable antenna, but rather was designed to be
used only on the fundamental.  Of course people soon began using it on the
harmonic frequencies.  As it turns out, the Windom is only a Windom for a
very narrow frequency (couple of KCs).  Above and below that it converts to
a form of an Inverted-L.  How that works is described in detail in an
article that appeared in the May 1978 issue of Ham Radio magazine, written
by John Nagle, K4KJ (SK).

The feedpoint of the Windom was off center but not the same place people
later began feeding the coax-fed OCFD.  It was originally believed by Byrne,
the primary developer of the antenna, that the optimum feedpoint was based
upon a coefficient and this coefficient was related to the wire size. It
worked for their tests but they never varied the height, nor tried the
antenna over other types of earth.  In other words, their work was flawed.
Forget the Windom.

The original OCFD concept did indeed define a point (in theory) in which the
impedance of the fundamental was the same as that of its even harmonic bands
and varied between 235 and 285 Ohms, height dependant.  Lower heights means
lower impedance, higher heights means higher impedances.
THAT TOO WAS FLAWED.  It works in theory in outer space but down here in the
mud on mother earth, we have a thing called "End Effect".  Due to the
different behavior of end effect on the harmonic bands, the impedance is not
the same, nor is the antenna actually harmonic.  The higher up you go, the
more it wanders away from the harmonic frequency.  This causes the impedance
to be different on different bands... albeit, close enough to be fed with
the same balun.

Saying that SWR is a red herring is kinda true but also very dangerous.
Indeed with the classical OCFD and its 1/3 - 2/3 split, the even harmonic
bands have a fairly good SWR.  However the odd ball her, 15m is a show
stopper.  At the 1/3 point, the RF current on 15m crosses the zero axis and
the impedance goes to infinity (in space), or about 2k Ohms here in the mud.
Unfortunately the random length of coax often transforms its impedance such
that it "looks like" 5:1.  Yes, you can certainly match this, but it doesn't
change the fact that it's still 10:1 at the feedpoint.  If you run high
power into it here, in Jim's words YOU WILL FRY THE BALUN.

OCFD WAS YESTERDAY.

For the past 5 years I have been building antennas called "Stromsummen
Antennas".
I have never seen, heard or read about this in the English language.
It is well documented in the German Rothammel Antenna Book and there is an
entire book written on just this antenna.  In German.  Sorry.

This concept is a twist of the OCFD design.  It dictates moving the
feedpoint just far enough that the RF current on 15m shoots back up to a
level that produces an impedance in the ball park with the other bands.  In
doing so, the impedances of the other bands also go up or down a little but
not as fast as 15m is changing.  If you prune carefully, you can find
several feed points which have a usable SWR on 6 or even 7 bands, AT THE
FEEDPOINT.

THERE IS A COST.  It is higher SWR in general but still quite usable.  As
Jim points out, you also lose power in the balun but the picture is not a
black as he paints it.  Might be if you insist on running 1500w but at
friendlier power levels, say up to about 800w, and with proper balun design,
these antennas are very reliable.

There's a lot to be said about "Never touch a running system."
If you are using anything and are happy with it, by all means keep using it,
regardless of what anybody else says.
However should it break, it is worthwhile to do your homework.

The sound ideas put forth in the ARRL handbook, our Rothammel, Low Band
DXing, etc. are still outstanding and probably the best thing you can did...
if you can do it.

Unfortunately HOA restrictions and diminishing size of city lots have caused
most of us to seek compromise antennas.
Believe me, there has been a lot of research and advancement in these areas.
Most of all we now have ultra lightweight and strong building materials that
were not even dreamed about in the 50s and 60s.  There is no reason to build
antennas as heavy as battle ships.

With HF antennas, HEIGHT IS MIGHT.
Today we have very tall, lightweight fiberglass poles that go up and down in
a minute.
You can get them all the way up to 85 ft. if so desire.
Of course they come in several smaller sizes.

You can push a wire high into the air at night when nobody can see it, and
work the world.
But you can't hang a Carolina Windom on it.  You'll break the pole.  You
must build ultra-light antennas and fly them high.

I do not recommend feeding an OCFD at 1/3 as Barry suggests.  I prefer to
move the feedpoint such that the antenna also works 15m and even 30m if I so
desire.

However he is right about the feedline.  Don't skimp on coax.  Don't buy
RatShack.  Same goes for the coax connectors.  By name brand.  If you buy
cheap, you buy twice.
 
73 - Rick, DJ0IP
(Nr. Frankfurt am Main)


-----Original Message-----
From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Wade Staggs
Sent: Tuesday, September 22, 2015 10:49 PM
To: Barry LaZar; Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] The OCFD Mystique

*Once again I walk forward with my Asbestos Suit on hi .. hi .. In the QST
Article linked below, modeled on a 66 foot dipole. The SWR and Loss Figures
comparing Ladder Line ( Windowed Line ) and Coax is Drastically Different.
I know that the Antennas are different Beasts in many ways. And also that
the SWR Figures should be lower on an OCF Antenna. We are also very aware
that Ladder Line can't be used on an OCF Antenna because it is an Unbalanced
Antenna. But .... if you are going to need a Tuner anyway to cover the Full
Bands? Why do folks even bother with OCF Antennas? Just take a quick look at
the Gain an 130 foot dipole has on 20 Meters using Ladder Line. Sure there
are lots of Major and Minor Lobes, but the gain in the Major Lobes is
Outstanding. Plus you can run QRO without having enough RF in the Shack to
cause major problems. Hey guys, just asking here. There are many people here
that are Much smarter than me. You never learn unless you ask the
questions.... Someday when we can afford a Faraday Cage.... I would like to
play with OCF Antennas again .. hi ... hi ..*

http://www.arrl.org/files/file/Technology/tis/info/pdf/QST_Dec_1993_p70-71.p
df
           *So ... we sit here with the 1% Silvadene Cream and Asbestos
Underwear on. Please, be kind fellas This is an honest question because I
want to learn something.*
*                               73 from Wade/KJ4WS*

On Tue, Sep 22, 2015 at 12:35 PM, Barry LaZar <k3ndm at comcast.net> wrote:

> Fellas,
>     There is nothing magic about an OCFD. It just a variant of the 
> Windom that was invented, I believe, in the 1920's. It's nothing more 
> than a dipole fed off center so that when you use it on even multiples 
> of a 1/2 wave the impedance is either 50 Ohms or in the region of what 
> your tuner can handle. If you can get it up 35 or more feet, it does 
> make for a pretty good all around antenna. I have a variant called a 
> Carolina Windom. It's up about 40 feet and I've worked DXCC, mixed CW, 
> and phone. I've also gotten my WAS using it and 100 Watts.
>
>     The issue of SWR is a red herring. So long as you can tune the 
> "system" such that your rig can accept what the tuner gives it, that 
> is really all that matters. Yes, there are losses associated with high 
> SWR, but if you use low loss transmission line, the additional loss 
> due to high SWR can be pretty trivial, a 1db or so, depending on SWR 
> and band being worked. On 10 meters my system losses total around 2 db 
> which includes everything behind the tuner.
>
>     One thing that must be remembered is that the transmission line 
> from the antenna to the station can be an impedance transformer if the 
> SWR is greater than 1:1. What this means is that what you measure at 
> the station may not be reflective of what the antenna is reflecting. 
> So, some of the comments about this antenna works well over all but 
> this band or another may not be accurate unless the effects of the 
> length of the transmission line is included.
>
>
>     Bottom line from here is put up 66' or 132' of wire. Feed it ~ 1/3 
> of the way from an end. Use a 4:1 current balun if you don't want the 
> feed line to radiate or voltage balun if you do. And, if you force the 
> feed line to radiate, put a good RF choke about 15 or so feet down 
> from the antenna feed point point, load it up, and start working the 
> world. Just remember to use a good low loss coax like Belden 9913 or Times
Wire LMR-400.
>
>
> 73,
> Barry
> K3NDM
> _______________________________________________
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> TenTec at contesting.com
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>



--
*Living one day at a time with Jesus as my Savior. But, still having Fun.*
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