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80m Carolina Windom mod




 
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Author Topic: 80m Carolina Windom mod  (Read 1407 times)
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KA2PTE
KA2PTE
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« on: February 27, 2021, 03:41:16 PM »

This is my first HF antenna, was donated to me by a local ham back in 2018 who said it was resonant on the CW portion of 80m, and I confirmed early on its best SWR is about 3610kc on the swr portion of the band.

Been very satisfied with it, even used it on 160m in a pinch, though it was a poor radiator at 1/4 wave
and had about a 10 swr unless you used a tuner.

Im told my type is slightly different than the normal one which uses a voltage balun at the feed point and I think
a 1:1 from the coax. This one has _no_ balun at the feedpoint, just straight 300 ohm ladder line that goes about 25 feet from there, to a 300W 4:1 balun that converts to 50 ohm coax the rest of the run to the building. That coax run is about 50-55 feet.

Im ready to upgrade to a linear and am in the final stages of hooking it in, but will need a legal limit balun, which I do have.

Since the 300W balun is small and light, theres no problem with it supported via the ladder line and coax, but this new balun for sure wont be able to just swap in with that setup. As theres not much room logisticly to build a support
or mount the balun on the outside of the building, I am considering mounting the balun inside the room near the window where the coax is coming in now, and splicing in another 50-55 feet of 300 ohm ladder line to make it to where the old balun was located.

Im curious how this will change performance, and after consulting some other hams, 1 is of the mind its not a good idea and resonance will shift to below the 80m band.

I figured since theres no voltage balun at the feedpoint, and a true Windom needs it, mines more an OCF dipole fed directly with 300 ohm line. The line will just be longer now, with a bit more loss, but hows that changing the resonance of the fixed length antenna?

This is a picture of the balun I will use , it can be dismounted from the support and already I have removed it and started scoping out how to mount it near the window:

http://www.mediafire.com/view/au7a0y7ahku2laf/HPIM0700.JPG/file

Other concern would be RF radiating inside the room, but as there would only be maybe 6 inches of line from the window to the balun, I am thinking it would be more or less negligable. Is there a small current loop on the 300 ohm side of a baluns terminals, even if theres ladder line transfering the energy to the real antenna?


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W1NB
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« Reply #1 on: February 27, 2021, 09:27:44 PM »

Steve,

The antenna you describe sounds more like a traditional OCF dipole. The Carolina Windom is a variation of the Windom (which used a single wire feed) designed by Edgar Lambert, Joe Wright and, I think, Jim Wilkey in the mid 80s. Joe was my neighbor and one of the VEs when I took and passed my General. Edgar was a friend, as well. Anyway, they took their design to Jim Thompson who made a few refinements and began selling it through his business, The Radio Works. The Carolina Windom uses the 4:1 current BalUn at the feed point to a 26 foot vertical coaxial radiator. At the end of the vertical radiator a line isolator, essentially a foot long ferrite sleeve, is placed to choke off the common mode current that form on the outer braid of the coaxial feed line. I donít recall all the theory but the vertical radiator increases the low angle vertical radiation of the antenna, improving its DX performance.

The ladder line OCF dipole uses a BalUn at the end of the ladder line to help step down the impedance to 50 ohms. The antenna line impedance at that point is about 600 ohms so a 4:1 doesnít quite make is, but it gets you close. The length of the ladder line, from what Iíve read, makes little difference.

 Both versions, as well as the single wire feed Windom have issues with common mode currents so itís recommended to route the coaxial feed line at right angles to the dipole, avoid sharp turns, and preferably, bury the coax for as much distance as possible. You can also minimize the RF from entering the shack by choking the common mode currents just before shack entry by coiling 8 to 10 turns of the coax into ~6Ē tight coil.

I hope this helps.

Scott, W1NB
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KA2PTE
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« Reply #2 on: February 28, 2021, 02:14:20 PM »

Hi Scott

Wow you do have a connection back to the original people who envisioned the Windom, thats interesting.

I thought a dipole's impedance is 300 ohms, regardless if its OCF but I could be wrong. The configuration
of this antenna if I remember right, just has a standard 300 ohm feed type mount where there are bolts
connecting the radials to it, and clearly the 300 ohm ladder line is attached to the bottom of it, so that was why
I viewed this really as an OCF dipole early on, but the person who donated it, said its a Windham.


After surfing the web some more found this statement :
"For an OCF Dipole, the height determines the type of balun required.  At 33í, the feed point impedance would be 200 ohms, requiring a 4:1 balun.  The higher it goes, the more the impedance rises, so the balun must change with it. There are a few web sites with the exact impedance at each height. At the 20 feet you have, the impedance might be low enough to use a simple 1:1 to get RF off of the coax."

If true, you could apparently feed a dipole very low off the ground with 50 ohm coax, and not need a matching balun. I believe the lower it is to ground, the more ground absorption, so this makes sense in an "ohms law" kind of way.

I remember the goal was to have this one up 40 feet for the "sweet spot" and I think we hit that or are close. If true then I guess we are at about 300 ohms - hence the 300 ohm ladder line coming down etc. Starting to think this is merely a OCF, not a Windom.


Here is a plot of it from back in 2020 when it was resonating on the cw portion of the 80m band, just like the previous owner said it was made to do.

Question would be is that typical for an OCF or for a Windham ? Or would they be mostly identical...?



* windham.jpg (264.12 KB, 1824x864 - viewed 150 times.)
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W1NB
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« Reply #3 on: February 28, 2021, 09:02:27 PM »

I got lucky. I was in upstate NY working for a division of Regency. We were up for sale and a deal with Westinghouse/Sanyo was pending. At the 11th hour the deal fell through and Regency shut us down and closed the doors. I was cleaning out my office when I got a call from a colleague who had recently left working for Jones Intercable to work for a startup cable company. After he found out we were shut down he asked me if I was interested in going to NC. A few months later our house in NY was sold and we were living in Edenton, a small town just south of Elizabeth City. I started to frequent the Elizabeth City repeater where I met both Joe and Edgar. I suspect I also spoke to and met Jim but Edgar and I spoke daily as I made my rounds out to the outer banks and south across the Albemarle Sound. They were great guys. Very supportive and active club. I got both my General and Advanced while I was there.

Anyway,

If youíre referring to the plot, I believe that is the predominant characteristic of the OCF so youíll see those recurring resonance points in all three; the OFC, the Windom and the Carolina Windom.

The change in impedance from the ladder line length in the OCF is interesting. My OFC reference is a ARRL handbook from the mid 40s. It doesnít mention that but itís not like they had antenna analyzers and software modeling back then.
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KB2WIG
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« Reply #4 on: March 01, 2021, 10:48:18 AM »

Cebik, w4rnl, has analized the OCF and it's brethren. Its worth a look.

IIRC, the Windom was a single wire feed, with the connection point 1/3  distance to the end of the ant.

For what its worth, the classical Z for a center fed dipole,  in free space is ~73 ohms. Free space Z is 377 ish ohms.

As as F. Gump would say, thats all I'll say about that.

KLC
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KA2PTE
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« Reply #5 on: September 11, 2021, 01:52:36 AM »

So I have obtained some more window line, thought it was the same gauge as the length reaching the balun,
but its 20ga 300 ohm window line. The 25 feet or so coming from the feedpoint is 16 or 18ga.

I plan on splicing this in to the existing line, and wonder if it will effect the power capacity very much
as its a thinner gauge. If it can handle legal limit, or a kw that would be about what I am gonna try putting on it
with some aircraft grade splice crimps joining them together.
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