[TowerTalk] Choking on chokes

[Earl Morse]

Don't feel bad David.  There are lots of engineers that can't grasp the difference between common and differential mode currents.

W8JI and K9YC both have pretty good explanations of it on their sites.

Unless you have a really bad case of common mode (CM) currents running back into the shack and you are getting RF bites from the key you won't be able to tell if you have RF running on the outside of your coax without equipment to measure it.  If you have an antenna that is well matched to the feed line then the CM currents are going to be minimal.  However, that match is frequency dependent so while you may have minimal CM currents on the feedline at the resonant point of the antenna they will change as you move away from resonance and the antenna impedance is no longer matched perfectly to the feedline.

The simplest choke is winding the coax to make the outer braid an inductor coil that has enough impedance to choke the CM currents at the frequency band of interest.  Next would be ferrites placed over the cable to provide the same impedance without having the loop.  Finally there are baluns/ununs that will provide this isolation while making an impedance match or balanced to unbalanced conversion.

The simplest chokes have little effect on anything other than the CM current they are choking and don't even add another connector to the system that might increase your chances of system failure.  The baluns/ununs do a job but have a little more complexity.  They might be required if your antenna needs the impedance transformation anyway.

Putting the CM choke in when you don't need it won't hurt anything, just extra cost and in some cases another point of failure.  Adding it later because you feel you need it is a whole different story after the antenna is in place at 100 feet.  Grasp these basic concepts before moving onto the design of the CM chokes/baluns/ununs and having to worry about which permeability of ferrite to use.  That's a whole other issue.

Earl
N8SS  



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Message: 3
Date: Wed, 3 Dec 2014 15:16:38 +0000 (UTC)
From: David Gallatin via TowerTalk <towertalk at contesting.com>
To: "towertalk at contesting.com" <towertalk at contesting.com>,
	"towertalk at contesting.com" <towertalk at contesting.com>
Subject: [TowerTalk] Choking on chokes
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	<114057225.4439249.1417619798633.JavaMail.yahoo at jws10639.mail.bf1.yahoo.com>
	
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Hello fellow listers,I have tried to educate myself as to whether my antenna's need common mode chokes and what to use if so. ?Lessons on permeability are not really permeating my brain tissue and I'm rather dazed and confused. I want very badly to get this right so I'm asking for help. So I am going to list each one in hopes of someone telling me whether A) it needs a choke and B) what that choke should consist of. The coax used is DXE 400, except DXE 213 on the Mosley per their spec.My antenna's are as follows:Diamond X200 vertical 2M?
M2 6M3 6 meter yagi
Mosley TA33 JRN 20,15,10 tri band yagi
EARCHI design homebrew 9:1 unun wire antenna of 32' which I wish to use on 40, 30, 17 and 12 and (maybe) 80. Will run horizontally
Thank you for your time,?73,
David, AA9G

ex W5DCG and KC9EEV



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