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Author Topic: A question about Balun 'Isolation'  (Read 493 times)
G7MRV
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Posts: 472


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« on: April 27, 2008, 02:54:12 AM »

Perhaps 'isolation' is the wrong word here...

This months issue of RadCom (the RSGB's inhouse mag)has an article on a doublet antenna, and includes a design for a toroid wound Balun. This balun design has, obviously, a balanced port, but it also has BOTH a 1:1 and a 4:1 unbalanced port. Seemingly this would be a very usefull feature, allowing a rapid change in ratio and hence allowing more variety in matching options.

However, i have a reservation. What effect would the unused port have on performance? for instance, if i had my 450R ladder line from my doublet connected via this designs 1:1 option, would the fact that the 4:1 port is not connected to anything cause losses? should this port be left open, or terminated, or what? How much a loss could i experience, bearing in mind i use a multiband doublet?

If the losses/isolation are minor, then the use of this balun, with both ports connected by short (say 1-2ft) RG-213 patch leads, to a coax switch, and then hence to the tuner, would give great possibilities.

So far i have tried a 10t 6" coax 1:1 balun (coax choke), and a 4:1 RG58 built collins balun on this antenna with varying results, this design being on a toroid would be easier to handle and customise for me


Martin G7MRV
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G8JNJ
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« Reply #1 on: April 28, 2008, 05:35:12 AM »

Hi Martin,

I saw the article, the balun is a useful item to have in the shack, and the design seems OK. However they have got the 4:1 and 1:1 unbalanced ports swapped around in the drawing.

Leave the unused ports un-terminated. Don’t leave even short lengths of coax connected to the unused ports as this will add varying impedance loads which will adversely affect performance.

10 turn 1:1 coax baluns does not provide sufficient choking impedance on 80 or 160m, although they are generally adequate on 40m and above. If you intend to use your doublet on 160 or 80m use several turns of coax through something like a 240-61 ferrite core (or similar).

My home brew coax 4:1 balun is not good when driving into low impedance capacitive loads (event though it has a very low through loss when terminated correctly), so I’d suggest the T200-2 transmission line balun as per the Radcom article would be better. Please make sure you wind the transformer neatly with suitable diameter wire, as this has a large effect on performance, particuarly at HF frequencies. There are some really bad examples shown in construction articles on the internet. Winding a transformer badly can easily result in 3 or more dB loss, so I'd recommend making loss measurments, when you have built yours. You can find some easy methods on my website.

Also for 160m you can strap the doublet as a T and feed it against ground. If your tuner can’t handle the low impedance load reverse the 4:1 balun and use the unbalanced ports as a 1:4 UnUn.
 
Regards,

Martin – G8JNJ

www.g8jnj.webs.com
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KC7NOA
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Posts: 64




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« Reply #2 on: February 21, 2009, 01:31:39 AM »

Is there a reprint of this T200-2 transmission line balun as per the Radcom article?? or a link ...

Please dont say to use a search engine ....

thats like saying to the question of "how do you shoot a gun" with "with a bullet" --- duhhhh
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W3LK
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Posts: 5644




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« Reply #3 on: February 21, 2009, 09:11:28 AM »

<< Please dont say to use a search engine .... >>

Is this because you're too lazy to search the web for yourself. Smiley

It took me all of 15 seconds to find the RadCom web site.

73,

Lon - W3LK
Naugatuck, Connecticut

PS, your analogy is incorrect.
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A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.
PHILIP_EX_KC7FWB
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« Reply #4 on: February 21, 2009, 06:19:27 PM »

"It took me all of 15 seconds to find the RadCom web site. "

There is a website www.radcom.com, which has absolutely nothing to do with the RSGB Radio Communications magasine.

This isn't what you found is it?
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K7KBN
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Posts: 2764




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« Reply #5 on: February 21, 2009, 08:33:09 PM »

I Googled "radcom magazine".  Took me less than 15 seconds.

73
Pat K7KBN
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73
Pat K7KBN
CWO4 USNR Ret.
KB1NXE
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Posts: 301




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« Reply #6 on: February 24, 2009, 12:35:59 PM »

I shoot a gun by pulling it's trigger.  It fires a bullet as a result.
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