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Author Topic: Carolina Windom and swr  (Read 1746 times)
ZS1VK
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Posts: 2




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« on: March 07, 2012, 04:27:05 AM »

Hi All.

I need help. I am busy constructing a Carolina Windom and have cut it for 40m. ie 41ft and 25ft and have 10ft of RG58 which I have wound 10 times at the 10ft mark. This then continues into my shack. I am using a 4:1 voltage balun. I have to use an inverted V as I dont have supports for the end. I have pulled it up the tower and it is sitting at about 30ft. The angle between the two legs is about 110 degrees. When I attach the analyser it indicates a resonating frequency 11,600mhz with swr at 1.2
At 7Mhz the swr is about 4. The only other place where it resonates is on 10m where it is also 1.2.
Is this the way it should be or should it resonate on say 7.100 seeing that that was the frequency I cut it for. If it should resonate on 40m any idea what the problem might be. I have tested the balun, antenna wire, accuracy of the tape measure and the coax.

Thanks.
Russell ZS1VK
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W5DXP
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Posts: 3546


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« Reply #1 on: March 07, 2012, 06:10:45 AM »

... and have 10ft of RG58 which I have wound 10 times at the 10ft mark.

That is not nearly enough decoupling for 40m so your coax is acting like a detuning stub. How about a nice 17T choke wound on an FT240-43 toroid?

http://www.karinya.net/g3txq/chokes/
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
K4SAV
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Posts: 1831




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« Reply #2 on: March 07, 2012, 07:44:36 AM »

There are a couple of possibilities.  Cecil already mentioned the choke.  An air wound choke will look like an inductor.  That inductor could be looking like a loading coil on the feedline causing the whole feedline to radiate.  Reducing feedline common mode currents with this antenna is very difficult.  You need a lot of impedance.

The other possibility is interaction with the tower.  Since with this antenna there is a vertical section of feedline that radiates there should be significant coupling to the tower.  How far away from the tower is the feedline?  An inverted vee configuration makes that worse because the fields from the antenna do not cancel at the point where the feedline attaches. 

You didn't mention any characteristics of the tower which might give a clue as to the resonant frequency of the tower (such as height, antennas on top, guy wires, and other wire antennas on the tower).  Interaction with other wires can have some weird effects.

Jerry, K4SAV
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ZS1VK
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Posts: 2




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« Reply #3 on: March 07, 2012, 07:52:13 AM »

mmm, thanks for the responses so far. Yes it is right next to the tower by about a foot and I do have 2 steel guys below it. The interesting thing is how well it "hears" on 40m. Will work on the choke situation as well as that was a suggestion from elsewhere earlier. Will pursue this to the end.

Russell ZS1VK
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KG6YV
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Posts: 506




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« Reply #4 on: March 07, 2012, 01:19:18 PM »

From your dimensions it looks like the feedpoint impedance at the antenna is somewhere around 200 ohms on 40M.  You will need a 4:1 balun right at the antenna to feed it with 52 ohm line.  Your description indicated you have the 4:1 balun further down the feedline.  That won't work. 

DX Zone has some good articles on OCF diploles.  Google it and find the one written by an ON4 station.

Greg 
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W1JKA
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Posts: 1619




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« Reply #5 on: March 07, 2012, 05:16:22 PM »

    I built the same type antenna from (Len Carlson-K4IWL) internet design new Carolina widom, same dimensions. However mine is horizontal and up 25 ft .As mentioned above,the 4:1 balun must  be at the antenna connection.I cheated with the 1:1 choke (line isolator) at the bottom of the 10 ft. Q section,I bought one from MFJ. .I do not use a tuner on 40/20/15/10 meters,all 1.2:1 swr .I originally wanted to put this up as an inverted vee but some local hams all told me I would have to shorten the ends in proportion to the offset lenghts of each leg because the swr will change,same as a horizontal dipole swr changes when changing it to an inverted V . I did not want to get involved with the trimming ratios, so I put it horizontal. If other posted suggestions are checked out and swr is still to high, experiment with folding back the ends an inch at a time in proportion from the design lengths. This is a great antenna and I hope you get the swr issue resolved.
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