[Antennas] Re: Current balun vs. voltage balun

2002年12月22日 06:10:00 -0500

Jim,
You have defined very well a 1:1 "current" balun, but a 4:1 current is not 
just a choke. The 4:1 current balun is basically a broad band transmission 
line transformer. There are two coiled transmission lines in a 4:1 current 
balun, either on two cores or on a single core. Usually the cores are 
toroidal, but a rod core(s) will also work. The inputs of the two 
transmission lines are connected in parallel and the outputs in 
series. The series portion adds the voltages in wires 1 through 4, thus 
you get a transformation ratio of 1:4, step up ratio. You can also have 
three transmission lines, with all inputs in parallel and all outputs 
connected in series adding for a transformation ratio of 1:9. The 
transformation ratio is N^2 where N is the number of transmission lines 
connected inputs in parallel and outputs in series. The balun will also 
operate step down; just reverse input and output. It does work but at 
reduced efficiency, as a balun is optimized for either step up or step down.
The voltage balun however, operates in an entirely different manner. It 
uses a transmission line on a core and is broadband, but that is where the 
similarity ends. There is only one transmission line, and it is connected 
as a phase inverter. The input voltages are added to the inverted output 
voltages in such a manner that the sum of the all the voltages in the 
network are 4 times the input voltage, thus a 1:4 step up voltage ratio, as 
long as the voltage from the phase inverter is in the proper phase to add 
to the other voltages. The voltage balun does not work in reverse, it is 
just step up and not step down. I really can't get into more theory than 
this without quoting Sevick, and he takes seven pages in his book, 
"Transmission Line Transformers" to do it.
What you described below is a 1:1 current balun. There is also a 1:1 
voltage balun, but it is not as common these days. Sometimes it uses a 
third wire on the core particularly if the core is a rod instead of a 
toroidal core. Generally speaking, the 1:1 current balun is an unbalanced 
to balanced device like to a dipole antenna, where the 1:1 voltage balun 
can be used to a 1:1 unbalanced to unbalanced load like you would see on a 
Yagi beam that uses a matching network on the beam to raise the input 
impedance on to 50 ohms. Its purpose is to minimize RF currents on the 
coax which distorts the beam pattern and/or FB ratio.
This is probably more than you ever wanted to know about baluns. Check the 
above reference or Sevick's other book "Building and Using Baluns and 
Ununs" if you want to go into it in detail. Both books are available from 
the ARRL library.
Merry Christmas and Happy new year to everyone.
Chas, W1CG
At 09:28 PM 12/21/2002 -0800, Jim Shaw wrote:
>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