OCFD BALUN SIDEBAR
Note: this page is intended to compliment the page on "What's Wrong With OCFD Antennas?"
In order to understand what is wrong with the current portfolio of commercially available baluns, it is important to review what the primary task of a balun is in hf antenna applications.
Contrary to what many people think, the balun’s job is not to balance the antenna. In fact it has nothing at all to do with the antenna.
The balun is a transmission line accessory whose purpose is to help maintain the normally inherent balance of “differential current” flowing inside of a coaxial feedline. It achieves this by providing the return current a high impedance path to ground, thus forcing all of the current to return through and along the inside surface of the coax shield, rather than on its outside surface. For more on this topic, please see the excellent article by Roy LeWallen, W7EL entitled
“Baluns: What They Do And How They Do It.”1
In this paper, Roy was the first one to coin the terms “Voltage Balun” and “Current Balun.” It is a “must read” for every ham radio operator.
The ability of any balun to perform this task depends upon its Common Mode Impedance (CMI) at the frequency of operation. Therefore it is frequency dependant. The amount of CMI any particular balun has depends on several factors:
- Its circuit design (e.g., Guanella, Maxwell, Ruthroff, Hybrid, etc.).
- Its toroid(s) or rod(s) (e.g. Ferrite or Iron-Powder Mix, size, number used in the balun).
- The number of turns of wire (or transmission line) wound onto the toroid(s) or rod(s).
With all of these variables, it is apparent that there can be huge differences between one balun and the next, even if they are of the same circuit design.
In order to understand just how much difference there is between these different balun types, one should read Steve Hunt’s (G3TXQ) excellent paper on “Basic Baluns.” 2
Steve clearly points out that voltage baluns on their own do not have very much CMI but more importantly that the most popular 4:1 “balun” of all, the Single-Core 4:1 Guanella, does not have any CMI at all.
At this point, the lights should go on and alarm bells should start ringing. The Single-Core 4:1 Guanella balun is the [so called] “balun” almost all commercial balun OEMs recommend for use with OCFD antennas and is also the most commonly used balun by OCFD Antenna OEMs.
To state it clearly, in the antenna most notorious of them all for common mode current problems, the commercial OEMs are suggesting to use a balun with no ability to do the balun’s primary job of preventing CMC from flowing on the outside of the coax.
It is this author’s opinion that these devices
do not even deserve the name balun.
More information on this can be found on the author’s web page, “Bad Baluns.”3
At the bottom of that page you will find links to supporting proof by W8JI, G3TXQ, VK1OD, and the author’s own field tests.
When a balun has enough CMI, RF current is impeded from flowing on the outside of the coax shield and takes the intended return path, inside the coax. However, when the CMI is insufficient to stop all current flow, a second problem occurs. RF current flows through the balun which is by definition wound on a “lossy” core. This loss is converted to heat and dissipated. This results in a loss in power and efficiency of the antenna. This can be the reason some people believe these antennas to be dummy loads. If the core heats too much, its Curie point is reached and it can crack or even explode, thus destroying the balun.
Unfortunately the commercial balun and OCFD OEMs have apparently failed to understand the magnitude of the CMC problem in installations that are less than optimum. Otherwise they would not be offering the balun portfolio that they offer.
Balun guidelines for OCFD antennas can be found on the author’s web page
“OCFD Balun Selection.” 4
IN SUMMARY, most balun and OCFD OEMs are offering products which are of the wrong design and even when they get the basic design type correct, they use the wrong ferrite material and the wrong number of turns of wire. The antennas will still perform in many installations but will be troubled with Common Mode Current problems in many other installations.
1 “Baluns: What They Do And How They Do It”by Roy LeWallen, W7EL. Originally published in ARRL’s Antenna Compendium Volume One, and available directly from Roy’s web site, here: https://www.eznec.com/Amateur/Articles/Baluns.pdf
2 “Basic Baluns” by Steve Hunt, G3TXQ, here: http://www.karinya.net/g3txq/baluns/basic/index.php
3 “Bad Baluns” by Rick Westerman, DJ0IP, here:
4 “OCFD Balun Selection” by Rick Westerman, DJ0IP, here: