Please visit VK2TIP's Book Shelf. My personal recommendations, thanks.
Wednesday, 18-Mar-2015 17:25:57 PDT
•NEW! ‣ - Amazon Electronic Component Packs. Check out the Amazon Electronic Component Packs page.
"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.
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.
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.
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.
ORDER - U.S.A. NOW Building and Using Baluns and Ununs : Practical Designs for the Experimenter - by Dr. Jerry Sevick, W2FMI.
If you are involved in electronics then consider joining our "electronics Questions and Answers" news group to ask your question there as well as sharing your thorny questions and answers. Help out your colleagues!.
The absolute fastest way to get your question answered and yes, I DO read most posts.
This is a mutual help group with a very professional air about it. I've learn't things. It is an excellent learning resource for lurkers as well as active contributors.
NEW! - How to link directly to this page
Want to create a page link to me from your site? It couldn't be easier. No HTML knowledge required; even the technophobes can do it. All you need to do is copy and paste, the following code. All links are greatly appreciated; I sincerely thank you for your support.
Copy and paste the following code for a text link:
and it should appear like this:
href="http://www.electronics-tutorials.com/basics/baluns.htm" target="_top">visit Ian Purdie VK2TIP's "Baluns" Page</a>
visit Ian Purdie VK2TIP's "Baluns" Page
and it should appear like this:
Looking for more? Visit my site map page:
This site is hosted at WebWizards.Net for better value.
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.
Commercial use prohibited without prior written permission from www.electronics-tutorials.com.
Copyright © 2000 - 2001, all rights reserved. URL - http://www.electronics-tutorials.com/basics/baluns.htm
Updated 30th September, 2001