[TenTec] The OCFD Mystique
k3ndm at comcast.net
Tue Sep 22 18:09:32 EDT 2015
The Windom was invented when tube transmitters were around. The
output stage of most, not all, could handle a fair difference in
impedance, but most could not handle the very high impedances reflected
by feeding in a voltage loop. 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.
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
center. Most Windoms, OCFDs are designed for a 300 Ohm feed point making
it necessary to use a 6:1 balun transformer. 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 angles.
I do not intend to flame you so take off the fire retardant clothes.
However, I will say you have fallen victim to a lack of understanding of
antennas. I really don't mean to criticize, but more than half of the
hams today really don't understand the very basics of antenna design.
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. This antenna will work all
of the bands 80-10 with a tuner. The losses will be very low, even on 10
meter. You are correct about losses, keep'em down.
So, why do I use a Carolina Windom type antenna. Because I bought
into having the ability to do some fill in at the lower angles on the
lower bands. The center fed dipole would have been a bit simpler, but I
felt at the time this would be better; mine is a modified Carolina
Windon that I built using EZNEC modeling. I have a really good choke 15
feet below the feed, so I have no problems with RF in the shack. My
tuners seem to have no problem with tuning on 40-10; I do have a little
trouble on 80 as my antenna is only 66' vice 132' long.
My advice is to just pick a design and put it up as high as you can.
Let your tuner do all of the worrying for you. Feed the antenna with the
least loss transmission line you can find or afford that fits your
chosen design, and then use the heck out of it. Most of this makes
little difference from a practical perspective, yet it's more than a
semester for the engineering student. Just remember the ionosphere will
mess with your signal in the 10s of dbs while antennas may only amount
to a few.
------ Original Message ------
From: "Wade Staggs" <tvman1954 at gmail.com>
To: "Barry LaZar" <k3ndm at comcast.net>; "Discussion of Ten-Tec Equipment"
<tentec at contesting.com>
Sent: 9/22/2015 4:49:14 PM
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 ..
> 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>
>> 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.
>>TenTec mailing list
>>TenTec at contesting.com
>Living one day at a time with Jesus as my Savior. But, still having
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