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Author Topic: OCF antenna question  (Read 1738 times)
W6AOA
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Posts: 35




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« on: January 03, 2012, 11:45:44 AM »

Just recently I put up a 7-Band Off Center Feed (OCF) antenna and it works quite well. I should have tried this years ago.  Anyway I have heard of a vertical coaxial add-on with an isolator or another balun (unsure). This can be added to the existing OCF and it gives you a vertical (polorization) element to your radiation pattern and lowers the angle of radiation for DX'ing.  What am I looking for Huh

Ed Schaff W6AOA
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WB6BYU
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Posts: 13029




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« Reply #1 on: January 03, 2012, 12:58:30 PM »

I have seen advertising claims to that effect, but never any supporting proof that it
really works as claimed.  In the common implementation the "vertical radiator" is too short
to contribute significantly on the lower bands where it might be useful because the
antenna is low in terms of wavelengths, while on the higher bands where there could
be significant radiation from such a device the antenna is probably high enough in the
air that horizontally polarized radiation from the main part of the antenna actually does
a better job for DX.
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W1JKA
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Posts: 1619




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« Reply #2 on: January 03, 2012, 02:21:36 PM »

     Ed,if this is a windom type antenna your speaking of,Len Carlson K4IWL web site has a great explaination about these vertical coax Q sections with balun and isolator. HNY  Jim
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W5TTW
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Posts: 42




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« Reply #3 on: January 03, 2012, 02:35:59 PM »

I'm running an OCF at 33' (inverted V) with a 'vertical radiator' attached to the balun. It's made from 10' of coax with a line isolator at the bottom.  I can't say that it's responsible for my DX contacts, but they are plentiful.  Moreover, it eliminates RF in the shack.  In my case, $20 well spent.
« Last Edit: January 03, 2012, 02:44:59 PM by W5TTW » Logged
K4SAV
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Posts: 1825




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« Reply #4 on: January 03, 2012, 07:38:17 PM »

It's a Caroline Windom (bad name but that's what they call it).   They claim the vertical section provides some vertical polarization on 15 meters and lowers the SWR on that frequency.  A model for the antenna shows a significantly different pattern on 15, but I can't say it looks any better than an OCF of the same dimensions.  The increase in vertical radiation is very small.  I would say it's insignificant since it's down about 10 dB from the horizontal component.  On 15 meters the peak gain in the largest lobe is about 1 dB more than the OCF.  The effect on 10 meters is about the same, different pattern, not overall better, and about 0.5 dB more gain in the strongest lobe.  The vertical section seems to have no effect to bands below 15 meters.

I don't think it is designed to be an add-on.  The current mode balun on an OCF has to be changed to a voltage mode balun and a choke has to be added to the feedline a few feet down.  That makes the center pretty heavy which can be a problem because it's difficult to use a metal mast to support it in the center because of the radiating feedline.  There are several versions of OCFs with different dimensions so what you have may not match what is required for the Carolina Windom.  Sounds like you would be better off just starting from scratch.

Jerry, K4SAV


edit:

Wow.  I just looked up some Carolina dimensions and found a whole bunch.  I'm not sure the info I gave is correct because I'm not now sure what the dimensions should be.  For an 80 meter version I found:

side 1 _ side 2 _ vert
51 ___  82 ____ 22
51 ___  85 ____ 7
51 ___ 82  ____ 20
50 ___ 83  ____ 22
51  ___ 85 ____ 10

Then there are a bunch of 160 meter versions, and a bunch of shortened 40 meter versions, and some 20 meter versions.
« Last Edit: January 03, 2012, 09:22:57 PM by K4SAV » Logged
K3TN
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Posts: 282


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« Reply #5 on: January 04, 2012, 02:42:54 AM »

I've been using a 135' Carolina Windom for 3 years now (at about 50') and can't say I see any difference over previous use of a standard OCF dipole - other than a bigger problem with RF getting into the shack. I had to add a choke balun at the feedthrough panel into the house.

From a performance point of view, I've had great success with the OCF antennas. They were both superior to a G5RV on all bands except 10m - the Windom seems to have a porcupine pattern on 10 and performs poorly. On 15m, it plays pretty well - but the addition of the vertical coax/isolator on the Carolina design didn't seem to change anything.

From a tuning perspective, pretty much the same report: easy to tune on all bands except 10m, no real difference with the vertical coax/isolator added.

John K3TN
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John K3TN
KK4GER
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Posts: 54




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« Reply #6 on: January 04, 2012, 04:43:08 AM »

I am in the process of finishing a New Carolina Windom.  This is the 40M version described by Len Carson in his article.  I mounted it to the chimney at the end of my house, which puts the wires about 40 feet off the ground.  Here are a few pictures (I posted this link on a thread I have going about Baluns):

               http://photobucket.com/Antenna-Pictures

I mounted the ferrite choke to the bottom section of mast which is bolted to the chimney mount, and the 4:1 is at the top in the box and is able to pivot on the top section of the mast so it can allign when pulling the wires to the trees.

I have some issues (I think) but will post them in a new thread, so as not to disrupt this thread too much.  I thought the pictures might trigger some construction ideas.  (It appears to recieve well and tunes up on all the bands (160 - 10) with the internal tuner
but I haven't tried transmitting (except for about 10W when tuning), as I still have too many questions about SWR and impedance.)
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