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Unun or common mode choke

Mar 19th 2015, 18:55

kd8azo

Joined: Feb 22nd 2005, 15:07
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I put up a half sloper antenna 2-3 years ago with a homebrew common mode choke (12 wraps of RG8X around a plastic coffee can) at the feed point as a preventative measure. I'm thinking about replacing the choke with a commercially made 1:1 unun. Will the unun perform the same function? Thanks in advance for the help.

Dave KD8AZO
Mar 19th 2015, 19:56

W1VT

Super Moderator

Joined: Apr 4th 1998, 00:00
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The terminology folks use is quite confusing. A "unun" is normally an impedance transformer, with unbalanced input and unbalanced output. Typically the idea is to go from one impedance to another. But, some folks do use the term 1:1 unun to mean an isolation transformer, like 600 to 600 ohm transformers used in audio circuits. While you could have magnetic coupling and some degree of choking action, you could also have a 1:1 transformer with the shields tied together, resulting in no choking action whatsoever. But, such a device would provide DC isolation, while shunting any static charge to ground.

Zack Lau W1VT
ARRL Senior Lab Engineer
Mar 19th 2015, 20:44

kd8azo

Joined: Feb 22nd 2005, 15:07
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Total Posts: 0
Thanks for the quick reply Zack. Well, it was marketed as and I bought it as a 1:1 unun, made for amateur antennas. It is pvc pipe with a cap on each end, an SO-239 at the bottom and a connection point on each side. My understanding of 1:1 is that the impedance is not changed. So what would be its purpose? I thought about using it because the half sloper antenna is unbalanced, as is the coax, therefore the use of the unun would apply. So, in your opinion, would this also serve as a common mode choke?

Dave KD8AZO
Mar 20th 2015, 12:32

W1VT

Super Moderator

Joined: Apr 4th 1998, 00:00
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If you have an antenna analyzer, you could measure the common mode impedance to see if it is high enough to be useful at the operating frequencies.

For single band applications, your original coax choke is usually the best option--you can get very high choking impedances. Ferrite loaded chokes tend to be a compromise, with moderate impedances of several hundered to a thousand ohms over a much wider bandwidth.

There are some good ferrite designs--they usually involve large toroids and winding turns to get a lot of choking impedance--you can't get that by slipping a few beads over some coax.

http://forums.qrz.com/archive/index.php/t-259423.html
Here is a discussion "what is an unun?" that may help.

The good common mode rejection of a balun allows it to be used as common mode choke. But, an unun may or may not have useful common mode rejection, so the only way to know is to measure it or trust the recommendation of the manufacturer.

Zack Lau W1VT
ARRL Senior Lab Engineer
Mar 20th 2015, 18:40

kd8azo

Joined: Feb 22nd 2005, 15:07
Total Topics: 0
Total Posts: 0
OK Zack, I appreciate the help and info. I have passed it along to a few friends who had similar questions.

73 and may God bless.
Dave KD8AZO

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