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Author Topic: Balun for a Cushcraft 402CD  (Read 522 times)
K4JRB
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Posts: 56




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« on: December 06, 2017, 04:49:52 PM »

I have a 402CD 40 meter beam that had  a coil balun of 12 turns of 6 inches in diameter and the coil got messed up taking it down.  I know later versions of this beam (not the XM240) switched to a balun.

Can someone point me to the correct balun.

72 Dave K4JRB
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KU3X
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Posts: 434




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« Reply #1 on: December 07, 2017, 12:40:16 PM »

Make things easy. Get hold of an FT 240-43 core. Wrap 8 turns of your coax through the core and place it next to the feed point of the antenna. I learned a long time ago, every electrical connection (PL-259) is a possible point of failure. By doing it this way, the only connection you have is where the coax connects to the driven element of the shorty forty. I wish I would have done this 32 years ago when I put my shorty forty up. Too late now, it's on top of the mast and a major  project to get to the balun.

Barry, KU3X

http://www.ku3x.net/primary-station-antennas
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K6UJ
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Posts: 1138




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« Reply #2 on: December 07, 2017, 02:01:31 PM »

I agree with Barry.  Coax coiled thru the Amidon ferrite core is the way to go.  Much effective than simply coiling the coax.  I use a stack of 4 of the ferrite core donuts and coil 5 turns of RG213 thru it.  I can't get much more than five coils with RG-213.  Makes an excellent choke. 

Bob
K6UJ
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N2SR
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Posts: 655




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« Reply #3 on: December 08, 2017, 08:06:25 AM »

K9YC has done a lot of research about CM chokes.   Take a look at his website and you can easily roll your own. 

Or, you can purchase one from Balun Designs.  Great company.   I have two of their low frequency baluns, and I had one on my TH7 (sold it with the antenna). 

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If no one is doing it that way, there is a probably a very good reason.
K4JRB
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Posts: 56




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« Reply #4 on: December 08, 2017, 11:13:33 AM »

I looked at Balun Designs and need to know  which model and what value.  As I recall the 402CD is a low value at the feed point of 10 or 15 ohms so I need to bring that up to 50 ohms.  I agree that trying to wind 12  6 " dia loops of RG213 is difficult especially since the old windings fell apart .

73 es tnx  Dave K4JRB
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KU3X
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Posts: 434




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« Reply #5 on: December 08, 2017, 02:17:50 PM »

  As I recall the 402CD is a low value at the feed point of 10 or 15 ohms so I need to bring that up to 50 ohms. 
73 es tnx  Dave K4JRB


10 to 15 ohms? Where did you ever get that idea from? Not even close to that low of a feed point impedance.

You can use a 1 to 1 current balun on that antenna. I know, I have one. I guess you did not like the idea of using an FT 240-43 core?
Since you want to add connectors and possible points of failure, search this on google. It's the perfect balun for the beam. "gu6-160-5kw"

Or use: http://www.hypowerantenna.com/products/balun/yagi-beam-balun

For this antenna, they all use pretty much the same ferrite material......43. Some use 31 material but on 40 meters they both pretty much give you the same results.
Barry
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K4JRB
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Posts: 56




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« Reply #6 on: December 10, 2017, 09:01:43 AM »

I was told by Mr. Cushcraft (now SK) that the coax balun (Collins balun) was required because the 402CD was about 10 to 12 ohms to bring the impedance up to 50 ohms.  I have the problem fixed with a new piece of coax and 12 windings as shown in the manual.  SWR is now 1.2 to 1 as it was before. Thanks for all the help.

73 Dave K4JRB
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K6UJ
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Posts: 1138




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« Reply #7 on: December 10, 2017, 09:35:08 AM »

Dave,
Glad you got it resolved.  I learned something here about the 402CD.   I thought for sure all you needed was a 1:1 choke.   You are ready for the Bouvet Dxpedition in January now.   40M is a key band for working them according to the prop charts.  I am working on my 40M beam, replacing the old coax, etc.  this month so I will be ready too.     

73,
Bob
K6UJ
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KU3X
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Posts: 434




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« Reply #8 on: December 10, 2017, 02:24:05 PM »

Unless they redesigned that beam?Huh??
I have a 1 to 1 current balun on mine, the beam is set for 7.080 mhz and I see 1.2:1, in the shack and at the base of the tower at 7.080 mhz.

My manual even says to wrape a few turns of coax to make an ugly balun and have it at the feed point. Ugly baluns DO NOT change the  impedance ther are just
suppose to choke off the RF.....which they don't do a good job. A few months later both my friend an I changed our ugly baluns to a 1 to 1 current balun.

If my antenna was 10 to 12 ohms, there is no way I would see a 1.2 to 1 SWR at its resonant frequency. I actually had the antenna anaylzer at the top of the tower
to trouble shoot a connector issue. So at 12 feet away from the feed point, the SWR was the same, 1.2 to 1.

Good luck,
Barry

Update:
I just downloaded the MFJ / Crushcraft manual for that antenna. It shows an ugly balun at the feed point.
12 turns of coax, 8 inch diameter. That does not have a step down or step up value to it. What you see at one end of the coax you will see at the other end.

http://www.k0to.us/HAM/Articles/Cushcraft%2040-2CD%20Manual.pdf

Page 4

If that antenna was 10 to 12 ohms, you would see a 4 or 5 to 1 SWR by following these instructions.

What am I missing?
« Last Edit: December 10, 2017, 02:29:54 PM by KU3X » Logged
KM1H
Member

Posts: 2614




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« Reply #9 on: December 10, 2017, 05:44:40 PM »

Quote
I was told by Mr. Cushcraft (now SK) that the coax balun (Collins balun) was required because the 402CD was about 10 to 12 ohms to bring the impedance up to 50 ohms.

Les Cushman was long gone by the time that beam came out. Ive installed dozens using a pair of 43 Mix Fair-Rite cores and CC has since gone far away from the potentially squirrely all coax version.

Quote
If that antenna was 10 to 12 ohms, you would see a 4 or 5 to 1 SWR by following these instructions.

What am I missing?

You are missing nothing Barry but others have a poor understanding of the subject but are obsessed by posting......

Carl
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