A balun tutorial discussing a number of aspects of the design and construction of baluns for the amateur radio design or your next electronic project. A balun is a transmission line transformer for converting balanced input to unbalanced output or vice versa. It may or may not provide wide frequency range impedance transformation depending upon the configuration used.

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Sunday, 23-Jun-2013 12:55:30 PDT

BALUNS


 

Authored by Ian C. Purdie VK2TIP

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What are baluns?

"A transmission line transformer for converting balanced input to unbalanced output or vice versa. It may or may not provide wide frequency range impedance transformation depending upon the configuration used."

An old typical usage of the balun was (and still is) with TV antennas. The folded dipole is part of a yagi antenna which looks something like a pole with rods set across it at right angles. The second last one is folded into an oblong or rectangular shape. A folded dipole exhibits two important characteristics (a) its bandwidth is good for over an octave (e.g. 50 Mhz to 100 Mhz or say 120 Mhz to 240 Mhz) AND its characteristic impedance is a more or less a constant 300 ohms.

In earlier days extensive use was made of 300 ohm twin lead ribbon cable to feed the signals to the TV receiver. BTW you can use just a length of 300 ohm ribbon cable to make a folded dipole.

When colour (or color if you prefer) TV was introduced the ribbon cable often created problems which could be rectified by the use of co-axial cable. This is not strictly correct because coax had earlier uses in TV because of other problems such as ghosting which became intolerable with the introduction of colour.

Now 300 ohm coax can be and is made. However 50 and 75 ohm cable is preferred for a variety of reasons. Our folded dipole also exhibits a "balanced" feed characteristic whilst coaxial cable has an "unbalanced" characteristic. Two problems. Each solved by the use of the balun.

This image is copyright © by Ian C. Purdie VK2TIP - schematic of a balun transformer

Fig 1. - Schematic of a balun transformer

Perhaps we should have some clarification about this balanced versus unbalanced jazz. Mentally visualise it this way. If we have a plus 12V D.C. supply with ground return. That could be regarded for our illustrative purposes as the unbalanced 75 ohm input. On the other hand we could a symmetrical power supply referenced to ground which provides a + 12V D.C. AND a  - 12V D.C. voltage.  The only difference is we are dealing with RF which of course is very high A.C. Photo of Balun

A practical 300 / 75 ohm balun could consist of four pieces of 0.4mm wire wound 2 1/2 turns through a balun core (look a bit like binoculars). In so doing we have achieved our two goals, (a) the impedance transformation and (b) gone from unbalanced to balanced. Higher power levels require a different approach.

My good friend Glen Zook K9STH, has an excellent article on Improving "Boat Anchor" Receiver Performance using TV Baluns.

Ready made 300 / 75 ohm baluns can be purchased from many outlets. Here's an affiliate link to Amazon's Balun Page.

HERE ARE SOME USEFUL LINKS TO HIGHER POWER BALUNS

Usual disclaimers apply. Links above do not imply any particular endorsement of organisation, products or services but are provided as a further aid to information.

A VERY WORTHWHILE PUBLICATION ON THE TOPIC OF BALUNS

 This image is copyright © by Ian C. Purdie VK2TIP - Building and Using Baluns and Ununs : Practical Designs for the Experimenter   ORDER - U.S.A. NOW Building and Using Baluns and Ununs : Practical Designs for the Experimenter - by Dr. Jerry Sevick, W2FMI.

 

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the author Ian C. Purdie, VK2TIP of www.electronics-tutorials.com asserts the moral right to be identified as the author of this web site and all contents herein. Copyright © 2000, all rights reserved. See copying and links. These electronic tutorials are provided for individual private use and the author assumes no liability whatsoever for the application, use, misuse, of any of these projects or electronics tutorials that may result in the direct or indirect damage or loss that comes from these projects or tutorials. All materials are provided for free private and public use.
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Updated 30th September, 2001

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