Thanks for the response, Steve.
Yes, I know what the function of a balun is, and have read (and re-read
several times over the years) the books you probably are referring to,
namely those by Jerry Sevick, W2FMI, and Walt Maxwell, W2DU. But my
question is about the (non) difference between a balun and a choke.
For example, in "Building and Using Baluns and Ununs: Practical Designs for
the Experimenter," Jerry describes Joe Reisert's, W1JR (a frequent
contributor to TT), toroidal balun and Walt Maxwell's "bead balun." Joe's
design is the proper number of turns of coax through a proper ferrite
toroid. Walt's design is the proper number of ferrite beads (of the proper
type) over the coax. What both of these designs do is inhibit current flow
on the outside of the coax shield. Walt even called his a "choke balun."
So it seems to me that the only difference between a "choke" (as I
understand it from the original post (feedline wound into a coil)) and the
above-mentioned baluns is that the "baluns" have ferrite cores, and the
"choke" has an air core. Otherwise, they are the same. As I mentioned
earlier, the air-core choke probably will have a narrower rejection
bandwidth, but otherwise don't they both perform exactly the same function?
As you say, "[The RF choke] 'chokes' RF from going down the coax shield;" I
agree with that. However, when you say, "[The RF choke] does nothing for
the balanced-to-unbalanced feedline/antenna situation," I have to disagree:
isn't that exactly what the "choke" does? Doesn't the choke have an
unbalanced end (going to the shack) and a balanced end (the coax conductors
on the antenna side?
There probably are mechanical reasons for preferring a specific design. but
it certainly seems that either a "balun" or an air-core choke (both with the
proper inductance and self resonance) would do the job.
I may be off base here, and I'm always willing to learn. Additional
comments?
Thanks much.
73
Jim, W3WV
________________________________
Jim Wilcox
----- Original Message -----
From: <K7LXC@aol.com>
To: <jim.wil@cox.net>; <towertalk@contesting.com>
Sent: Monday, 02 June, 2003 12:27
Subject: Re: [TowerTalk] rf choke or balun?
In a message dated 6/2/03 8:59:01 AM Pacific Daylight Time, jim.wil@cox.net
writes:
> What am I missing here--I don't understand the difference between a
"balun"
> (at least a 1:1 balun) and a "choke." Possibly K9MI is referring to a
4:1
> (or something else other than 1:1) balun?
>
> Aren't they in fact the very same thing? If they each were in a "black
> box," would they not both appear the same?
A balun is a balanced (antenna) -to- unbalanced (coax) transformer
typically wound on some sort of ferrite core. The newer versions are the
"current
type". There are a couple of good books on the subject.
An RF choke is a air wound coil at the antenna end of a coax run. It
'chokes' RF from going down the coax shield but does nothing for the
balanced-to-unbalanced feedline/antenna situation.
An important purpose of these devices is to keep the outside of the coax
shield from radiating - an important consideration for RFI problems.
Cheers,
Steve K7LXC
TOWER TECH
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|