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Author Topic: 1:1 choke balun question, please ..  (Read 554 times)
KC9KEP
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« on: October 06, 2009, 12:06:25 PM »

I'm in the planning stage of fabricating a Moxon 20m antenna as featured in the August 2009 issue of QST magazine.

I'm considering whether I need a balun but I know little about them.

The author of the article says:

"I often include a 1:1 choke balun, to prevent common mode current on the coax shield, by strapping it to the mast."

He includes several photographs in which he's got the feed coax loosely wound around the antenna-mast a few times.

Is this practice considered a 1:1 choke balun?

Thanks!

--Tom Nickel
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W5DXP
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« Reply #1 on: October 06, 2009, 12:28:42 PM »

> KC9KEP wrote: He includes several photographs in which he's got the feed coax loosely wound around the antenna-mast a few times. Is this practice considered a 1:1 choke balun? <

No, there is no choke shown in the article. He is apparently talking about maybe 6 turns x 6" diameter coils of coax taped to the support, i.e. an ugly balun designed for 10m.

If the center support were made of 1.25" fiberglas pipe, for instance, the coax could be coiled around it to create a choke.
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73, Cecil, w5dxp.com
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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.
WA3SKN
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« Reply #2 on: October 06, 2009, 12:39:23 PM »

That is not a balun (at least a good one), but remember that the coax needs enough slack to be able to rotate 360 degrees. I mis-judged once and when the antenna got to about 270 degrees the feed line came off.  I never made that mistake again!
With a directional antenna like the Moxon, you will probably want a balun though.  And ferrites make the best, although adding a bit of weight.  The coil of coax balun is a poor substitute, but is lighter.
73s.

-Mike.
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KC9KEP
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« Reply #3 on: October 07, 2009, 06:14:57 AM »

Thank you all for the prompt and helpful information!
73
KC9KEP
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W4HV
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« Reply #4 on: October 08, 2009, 09:08:00 AM »

The choke balun works very well..Using several turns for one has been in Hygain's design for their TH6 and TH7 beam for decades..It is a cheap and effective method for the homebrewer. Just remember it's not truly a balun...Its a big choke, but it works!..I use them daily!
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W5WSS
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« Reply #5 on: October 09, 2009, 03:08:59 PM »

I used a 1:1 choke balun on my 6 element mono bander. I needed it because 4 elements were driven and balanced. I noticed common mode problems prior to the choke install which afterwards remedied it. The antenna however presented 52 ohms of Rr at the feed point so I used rg-214 and air wound the choke 4 ft back from the end of the line to allow rotation The construction technique I used care when wrapping the turns....I did Not use a form so in essence the RF choke was air wound and not as heavy and a continuous run of feed line no breaks no corrosion problems etc. I used 6 turns and 6" diameter a 10M version and many black nylon tie wraps to keep the coils solidly in place for this 10 meter log-yagi. The coils or turns if you will should be void of over lapping resting side by side uniform shape etc or the choke will simply not work as intended...mine was installed for 10 years and was working fine until the time when I moved I have no reason to think the choke would need service before the feedline or antenna etc. 73
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