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Author Topic: dipoles OCF7  (Read 387 times)
K9FLO
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Posts: 13




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« on: November 08, 2008, 10:38:25 AM »

I have an OCF7 dipole about 30' horz end to end above the ground, feeding it with 65' of 9913F7.  All but 80m the swr is inspec. On 80m (3880)tsting w/mfj269 I have 1.4 when I grab the coax the swr goes up and when I ground (grd window) the coax at the mfj269 it jumps to 3.1. It looks like common-mode current issues, can it be corrected for 80m without effecting 40-10?
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N1LO
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« Reply #1 on: November 08, 2008, 12:30:36 PM »

Hi Dean,

What's an OCF7? 30' sound waaay to short in general. I assume it's got loading coils for 40 & 80.

All that aside, sure sounds like the outside of the shield is hot with RF, which goes with OCF's. The vertical section of the coax shield radiates and is part of the antenna system.

See the recent thread about OCF consensus.

I believe you need a choke at the bottom of the vertical section of the coax to stop the RF on the outside of the shield.

I think this can be accomplished using either a line isolator or a W2DU bead balun. I would also put another choke on the jumper between your entrance panel and your radio.

GL,

Mark
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K9FLO
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« Reply #2 on: November 08, 2008, 01:06:51 PM »

OCF7 is a 7 bander made by Buckmaster.  I think your are right and that's what I'll check on Monday.
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K9FLO
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« Reply #3 on: November 08, 2008, 01:13:31 PM »

More info, Mark, it is a non loaded 135' OCF dipole with a custom voltage balum by Buckmaster
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W8ATA
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Posts: 327




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« Reply #4 on: November 08, 2008, 07:42:43 PM »

Hi Dean,

I am the one who started the thread that Mark refered to by trying to find if there was a consensus on OCF dipoles as I plan to build one. I was looking for some consensus on over-all length and the location of the feed point. I got a lot of good info on my task at hand. Further I have extensively studied OCF's from all aspects. And I have two local friends who have the Buckmaster that you have. They went with it because it was what would fit their lot and both are generally pleased with it. Both have installed their's per the instructions, that is the center around 35-40' and the ends sloping to 8' and 10'. That then provides for the correct apex angle. Just for starters I would suggest that you try it that way if your property and possible support will accomodate that. Not having mine built yet, I can't report on actual experience. However a review of the literature suggests a strong correlation between the center elevation and the impedance at the feed point. Also it appears that these OCF's can be a bit tempermental and you get it working right by trying different heights and end elevations. I will know about that once I get mine up and play with different elevations both at the feed point and the ends. And one caution, during testing limit your key down time as you can smoke one of these baluns without expecting that to happen. One of the local friends I mentioned smoked his 300 watt version with too much key down time and a messed up end due to a limb falling over it which apparently also caused the SWR to climb.

73 and best to you with your new antenna,
Russ, W8ATA
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AC5UP
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« Reply #5 on: November 09, 2008, 04:53:19 AM »

Since the OCFD appears to have eclipsed the Fan Dipole in terms of general interest on eHam... Even though you thought that could never happen... It might be worthwhile to bookmark this link in advance of the next OCFD topic:

http://www.qsl.net/on4baa/mirror/antennas/windom.html

Looks like almost everything anyone might need to know.

Wink
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N1LO
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« Reply #6 on: November 09, 2008, 07:38:52 AM »

OK on the length and configuration, Dean.

Russ is right about the height and shape affecting the feedpoint impedance. The feedline impedance and length also have a major effect.

Thanks Nelson for that link. I've saved it to disk in my OCF folder.

Mark
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W8ATA
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Posts: 327




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« Reply #7 on: November 11, 2008, 07:22:46 AM »

Dean,
I just checked my ever expanding folder on the OCF and found a note to this effect:

20 to 40 feet above ground Z=200 ohms, use 4:1 balun
40 to 55 feet above ground Z=250 ohms, use 5:1 balun
55 to 70 feet above ground Z=300 ohms, use 6:1 balun

Now those Z numbers are close entimates and will vary. Also you mention your Antenna came with a voltage balun. My understanding is to use a current balun and Bob Rumsey at Balun Designs and others confirmed that and suggested a 4:1 which I purchased as the heart of my soon to be built OCF. This isn't a put down of the antenna you purchased. As I previously mentioned two buddies have it and like it.

73,
Russ
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