I highly recommed this book:

http://www.arrl.org/catalog/?category=Antennas%2C+Transmission+Lines+%26+Propagation&words=

You will find other web references on Baluns and Transmission Lines as well.

Also, you can get a lot of information from the
'rec.radio.amateur.antenna' newsgroup.  If you don't know of any way to subscribe to the newsgroups, then you can access the newsgroup archives through www.google.com.

Here is a pre-canned search of that newsgroup archive for the word balun:

http://groups.google.com/groups?q=balun&meta=group%3Drec.radio.amateur.antenna

Lots of good info.

By the way, what is your application ?  What band ?

Hello again,

  I was rushed in my previous reply and gave a website for books on every aspect of antenna and balun design.

  To answer your question more specifically, the impedance of an an antenna is intrinsic to the design (or a function of the design) of the antenna.  In the case a folded dipole with a separation between the two quarter wave elements and the halfwave element representing a small fraction of a wavelength the impedance is roughly 150 ohms.  If the antenna is reshaped to resemble a diamond or a square the impedance drops to about 80 ohms.

  The equipment and math required to actually measure the impedance of an antenna is best answered by some of the references cited.

  A balun is a transformer used for impedance matching between a balanced antenna such as a dipole and an unbalanced transmitter input/output.  It is usaully in the form of an air-core or ferrite core auto-transformer.  Transmission line transformers are also used for impedance matching but don't provide quite the same degree of balanced/unbalanced transformation.

  Your best bet is to search appropriate sites on the web for more detailed information.

PS.  Hi Jeff.  How's everything in Fall River?

Bruce

Also, for HF antennas, it is often difficult to get the dipole antenna sufficiently high that the ground will not affect (lower) the impedance.  And if the folded dipole is part of a VHF Yagi, then the impedance will be affected by the other elements and will drop.

For HF, it is usually simpler to tune the impedance at the bottom end of the feedline (using an antenna tuner).  The extra VSWR on the feedline might cause a slight increase in loss, but usually not worth worrying about (for HF).

Jeff

PS: Ottawa temporarily.  VE1BLL/VE3 for a while longer.

Thanks guys.

     It's been a while since I looked at the ARRL book.  If I remember correctly they liked using lengths of coax as baluns, but I never quite got how they figured the feed point impedance.  I'll look at the sources you've suggested.  

     Yeah, I have this fantasy of hiking and camping with an all mode 6-meter rig making SSB contacts.  I've played once in a while with the idea of a collapsible yagi for this.   

Hi longeron,

  I'm trying to find my book on impedance matching using a combination of different lengths of mismatched coax but I've not found it yet.

  A 4:1 coax balun will serve to connect a folded dipole to a 50 ohm receiver with a reasonable impedance match.  To make one take a half wavelength of coax (allowing for the velocity constant for the particular coax) and loop it so you can solder the shield at both ends to the feedline shield.  Solder the feedline centre conductor to that of one end of the loop.  This junction feeds one half of your dipole and the other centre conductor feeds the other.

  This may be what you were looking for.  If you want to know how it works just ask.

Bruce

Suggestions:
1. Building and Using Baluns and Ununs
-- Practical Designs For The Experimenter
By Jerry Sevick, W2FMI.
The latest information on transmission line transformer theory, design, and construction. Includes hundreds of examples for dipoles, Yagis, log periodics, beverages, multi-band antennas, antenna tuners, and more.
136 pages. Second printing 1995, © 1994, CQ Communications, Inc. (ISBN: 0-943016-09-6) #7644 -- $19.95
2.
http://www.bytemark.com/products/content6.htm
3.
http://w4cox.hypermart.net/page4.html
etc. for more info

longeron,
Folding a dipole increases it's feedpoint impedance 4x.  In free space a half wave dipole has a nominal 72 ohm feed Z and is increased to 300 ohm by folding it.  

When you place the antenna closer than 1 wavelength from the ground you could expect the feedpoint Z of a half wave dipole to fall to 40-50 ohms, giving 160-200 ohms when it is a folded dipole.

A balun or BALanced to UNbalanced transformer is easily made by coax for narrow band work or a broadband transformer for a wide bandwidth.

Using a 90deg length of 75 ohm coax in 50 ohm system will give a 4:1 transformation ratio over a narrow band (refer to the ARRL handbook for details).  

A transformer to match 50 to 200 ohms needs a 2:1 secondary to primary turns ratio which gives a 4:1 impedance ratio.  The actual number of turns is dependant on the core material and the operating frequency.  There needs to be enough inductance to generate the flux density at the lowest frequency and the windings must be few enough so as not to approach resonance at the highest frequency.

Hope this rather low level explanation helps.

Works for me.  Thanks again for all of the great resources.  I just looked through the AES catalogue and came away pretty disappointed.  I remember that Yeasu had  battery/mains powered all mode rigs for 6 meters, 2 meters, and 70 cm... The only 6 meter all mode radio listed was made by a company called Ranger.  I'll have to look around some more.
-73-
KB8WYA