[Antennas] Current balun vs. voltage balun

2002年12月21日 21:28:52 -0800

Topic was "From transmission lines to tuners"
<snip>
Would someone tell me in an understandable way what is the difference(s)
between a current balun and a voltage balun?
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Please don't consider this the final word on the topic as I am no expert.
But I did a little research as this question since interests me as well.
Here is what I have found so far in the ARRL Antenna Book, 19th Edition
Voltage balun ==> transform impedance up or down (e.g. 200 ohms down to 50
ohms)
Current balun ==> choke off (stop) current from flowing on the outside of
coax shield.
pg 26-20 "The simplest method to create a common-mode choke balun with
coaxial cable is to wind up some of it into a coil at the feed point of the
antenna. The normal transmission- line currents inside the coax are
unaffected by the coiled configuration, but common-mode currents trying to
flow on the outside of the coax braid are 'choked off' by the reactance of
the coil."
pg 26-22 "Ferrite-core baluns can provide a high common-mode impedance over
the entire HF range. They may be wound either with two conductors in
bifilar fashion, or with a single coaxial cable. Rod or toroidal cores may
be used, although the latter is generally preferred because greater common
common-mode inductance can be achieved with fewer turns."
pg 26-22 "Another type of choke balun that is very effective was originated
by M. Walter Maxwell, W2DU. A number of small ferrite cores may be placed
directly over the coax where it is connected to the antenna. . . . The beads
fit nicely over the insulating jacket of the coax and occupy a total length
of (about) 9 inches. . . . Type 73 material is recommended for 1.8 to 30 MHz
use, but type 77 material may be substituted; use type 43 material for 30 to
250 MHz. The cores present a high impedance to any RF current that would
otherwise flow on the outside of the shield. The total impedance is in
approximate proportion to the stacked length of the cores."
pg 26-25 - (Summary). Voltage baluns are one way to step up or step down
impedance in a balanced feed line (e.g. a 4:1 voltage balun in a balanced
200 ohm system can transform the impedance to 50 ohms). Generally, this
page takes a cautious position regarding the use of voltage baluns because
if the system is not balanced (which is often the case), common-mode
currents still occur. My reading of the book's pages on voltage baluns is
that matching stubs are preferred over voltage baluns and that current
baluns are still probably required regardless of how the impedance
transformation is made.
Hope that is helpful.
73 de Jim WA6PX
[email protected]
-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Cletus W Whitaker
Sent: Saturday, December 21, 2002 8:44 AM
To: Charles Greene
Cc: Sandy and Kees Talen; [email protected]
Subject: Re: [Antennas] From transmission lines to tuners
de WB2CPN South Central Pennsylvania 2002年12月21日
Would someone tell me in an understandable way what is the
difference(s) between a current balun and a voltage balun?
If I wind an rf auto-transformer on a a ferite stick, or
on a ferite ring, what is it? 73 Clete
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