[Antennas] Current balun vs. voltage balun

Sun, 22 Dec 2002 01:08:44 -0600

With all due respect and not unappreciative of a lot of RTFM on Jim's
part to gather this information, the main point of the question and
the answer appears to have gotten a little distributed, Let me try to
summarize the voltage/current balun picture.

A voltage balun is a device which when driven by an unbalanced (to
ground) power source produces at its two balanced output terminals
equal and opposite voltages, relative to the system reference point.
The currents which flow in the loads connected to the balanced output,
usually a balanced transmission line, will depend upon the impedance
seen by each line to a reference point in the system.

A current balun is a device which when driven by an unbalanced (to
ground) power source produces at its two balanced output terminals
equal and opposite currents, whose magnitudes are theoretically
independent of the impedance of each terminal to the reference point
or "ground."

The essential difference is that when we attach a ladderline feeder to
a voltage balun on the output of a tuner/transmitter, the currents
which will flow in each side of the feeder may or may not be equal and
opposite and thus prevent line radiation. In the real world, there are
many situations in which unbalanced currents are almost guaranteed.
For this and other reasons, the voltage balun has fallen from favor
for transitioning balanced lines to unbalanced sources.

On the other hand, a current balun will force equal and opposite
currents in the lines of the ladderline and virtually guarantee little
or no line radiation. The current balun is now the preferred balun for
use with transmission lines.

The voltage balun has another problem in that all the load current
must flow through one or more windings that  create magnetic flux
within the ferrite cores. This can readily lead to core heating if not
saturation and it a leading cause for baluns overheating and sometimes
literally catching on fire when operated at adverse impedance levels
at high power. The voltage balun is generally much more dependent upon
source and load impedance levels in its operation than is the current
balun.

The current balun does not require any of the load current to pass
through any of the core windings,  thus only the common-mode current
is available to set up flux in the core. Since this current is usually
relatively small even at QRO, current balun core materials are much
less likely to heat or saturate and can be relatively much smaller
than those used with voltage baluns.

One parting thought: a balun is a *transmission line transformer* not
a transformer. The concept of turns ratio of windings on a core has no
meaning when discussing a balun. A balun can be constructed such that
some "impedance transformation" is provided, but this is not the
function of a balun. A balun is constructed of transmission line
sections which generally are much shorter than 1/4-wavelength and that
have specific required Zo values. I

Inherently a current balun is a 1:1 device since it generally consists
merely of a short section of coax or balanced line wrapped around a
ferrite rod or toroid.  A 4:1 balun is constructed by providing two
1:1 lines which are fed in parallel at the input and whose outputs are
placed in series. Three lines gives 9:1, etc. But these ratios apply
only to the specific Zo values of the lines and are not general
impedance matching methods.

For example, a 4:1 balun designed for 50 ohm and 200 ohm impedance
levels can be made to work very well. That same balun can present all
manner of problems if used to connect a 75-ohm source to a 300-ohm
load, even the impedance ratio is still 4:1. It is composed 50 ohm
line sections which when driven by 75 ohm sources and 300 ohm loads
can be expected to set up internal SWR conditions that can lead to
surprising performance outcomes.

Again, these are not impedance transformers. Use the balun to make the
balanced/unbalanced transition and to reduce or block common-mode
currents that result in line radiation, etc. Use an r-f transformer or
an "LC" tuner to accomplish whatever impedance transformation is
required.

Any balun has a frequency range and source and load impedance ranges
over which it performs the best. Current baluns tend to be more
tolerant of impedance levels than are voltage baluns. A balun that
works very well at 10 meters may be virtually useless at 160 meters,
unless it is designed specifically to cover that frequency range. To
do so may require restricting impedance levels, power levels, etc.

Finally, the old myth that baluns are lossy originated with and
applies largely to the voltage balun. Current baluns can be designed
and readily constructed that have almost immeasurable loss.

I realize that this brief summary hardly progresses beyond the tip of
the balun iceberg, but perhaps it will be a start. I strongly suggest
reading Reflections II by Walt Maxwell, and Transmission Line
Transformers by Jerry Sevick if you want to start learning about
baluns and related topics.

73/72, George
Amateur Radio W5YR -  the Yellow Rose of Texas
In the 57th year and it just keeps getting better!
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
K2 #489  IC-765 #2349 IC-756 PRO  #2121 IC-756 PRO2 #03235

----- Original Message -----
From: "Jim Shaw" <[email protected]>
To: "Charles Greene" <[email protected]>; <[email protected]>
Sent: Saturday, December 21, 2002 11:28 PM
Subject: [Antennas] Current balun vs. voltage balun

> 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?
> <snip>
>
> 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  2002.12.21
>
> 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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