- WHAT A BALUN IS
- WHAT A BALUN DOES
- WHAT A BALUN IS NOT (and what it does not do)
- PROBLEMS WITH BALUNS THAT ARE NOT 1:1
- WORK-AROUNDS FOR THE PROBLEM WITH BALUNS THAT ARE NOT 1:1
- WHY OVER 50% OF THE BALUN INFORMATION ON THE WEB IS WRONG
WHAT A BALUN IS:
SPECIFICALLY: A BALUN IS A TRANSMISSION LINE ACCESSORY
- It's purpose is to shield the transmission line from external influences which would disturb the transmission line's inherently balanced flow of
- In the case of antennas, it is protecting the flow of RF current [through the transmission line] back and forth between the transmitter and the antenna.
- It is also one of the most misunderstood accessories in amateur radio, plagued with many false myths.
In most amateur radio antenna applications, a balun is an RF choke, or to be more accurate, it is a Common Mode Current (CMC) choke. It's primary purpose is to help prevent common mode current from flowing on the feedline.
In directional arrays such as Yagi or Quad antennas, it helps assure equal amounts of RF to each side of the driven element - thus preventing skewing of the directional pattern. This only really works when the antenna is high and in the clear of surrounding objects.
In some cases (some configurations) it can also be an impedance transformer, but this is an option which, although convenient, also has serious drawbacks under certain common circumstances. These are also referred to as a "Transmission Line Transformer" (TLT). The most common baluns of this type are 4:, 6:1, and 9:1, but in theory you could build baluns with practically any ratio - even fractions (i.e. 2.5:1).
The ratio quoted in the name refers to the impedance transformation ratio, not the voltage or current ratio.
There are other definitions and other uses for baluns, but for amateur radio antennas,
its main job is preventing common mode current from flowing back down the feedline,
thus keeping the normal flow of RF current ballanced within the feedline.
WHAT A BALUN DOES:
In amateur radio antenna applications, the primary purpose of a balun is to impede (choke) the flow of Common Mode Current (CMC) on the transmission line.
- For coax feedlines, CMC only flows on the outside of the coax shield. It does not flow inside the coax.
- For openwire feedlines, CMC can flow on either or both of the lines.
Although a CMC choke for openwire looks different than a CMC choke for coax, both do the same thing (impede the flow of CMC).
This helps avoid the disadvantages caused by having too much CMC on the feedline.
For information on these problems, see: Common Mode Chaos
If you don't know what Common Mode Current is, NOW is the time to find out, because it is very important to understand this when trying to optimise the performance of your station.
For more information on this, again see: Common Mode Chaos
A BALUN IS ALSO sometimes used as an impedance transformer. Normally a balun has a 1:1 impedance ratio between its input and output sides. By modifying the design of the balun, it is possible to have other impedance ratios. Examples: 4:1, 6:1, 9:1, etc. In these cases, we also refer to these at Transmission Line Transformers (TLT).
WHAT A BALUN IS NOT (and what it does not do):
A balun is NOT an antenna ballancing device. It does not ballance the antenna!
Many people wrongly think it will ballance the antenna. IT DOES NOT.
Besides, even a balanced antenna can cause common mode current on the transmission line.
The RF current flowing in a transmission line between the transmitter and the antenna flows in two directions: to and from the antenna. Normally (and ideally) the current flow in each of the two wires is equal, with current flowing from the transmitter to the antenna on one wire, and current flowing from the antenna back to the transmitter on the other wire - regardless of whether it is of openwire or coax.
As long as these currents remain equal, we say the current flow is balanced.
This is what we want.
An unballance of current flowing in a transmission line can cause big problems problems.
Many things can influence the the flow of current on the transmission line:
- using a poor or bad balun causes more problems than it solves*
- feeding a balanced antenna with an unbalanced transmission line
- any objects in closer proximity to one side of the antenna than the other (i.e., buildings, cars, trucks, etc.)
- other antennas in close proximity of the antenna
- Unwanted RF striking the feedline - in this case, the feedline itself is acting like an antenna
- Electro Magnetic Interference (EMI) being radiated from consumer products
- defective power lines
- and many other things
*I placed this one first to highlight it, because it is a common occurance, yet little known fact.
A balun helps maintain the balance of current flowing in the transmission line.
In other words it is helping prevent current flow from becoming unbalanced.
It is not the device balancing it!
PROBLEMS WITH BALUNS WHICH ARE NOT 1:1 RATIOS:
Many people think baluns with a high impedance ratio (e.g., 4:1, 6:1, etc.) are a good solution to many problems. Although they sometimes are, THIS IS OFTEN VERY WRONG! In fact they are often the cause of many serious problems.
The fact is, they are only a good solution whenever the impedance of the antenna is not too different from the balun's output impedance. If the SWR on the antenna side is more than 2:1, you begin to get problems. If the SWR is a lot higher than 2:1, you may get some very serious problems! As a rule of thumb, expect an SWR above 3:1 to cause CMC problems with these baluns.
In order to understand this, I will remind you of the primary purpose of the balun in our antenna applications: "to impede the flow of common mode current on the transmission line."
Whenever a balun with a high impedance ratio does not see a good match,
its ability to perform its primary function (impede CMC) is seriously affected.
Although it continues to transform the impedance, it fails to do its job as a balun and impede CMC. Another solution is required here.
WORK-AROUND FOR PROBLEMS WITH BALUNS WHICH ARE NOT 1:1 RATIOS:
The work around for the problem described above with high impedance baluns is to cascade two baluns. Use a 4:1 or 6:1 balun attached directly to the antenna legs and use a second "1:1" balun immediately attached to the first. In doing so, the first balun will provide the impedance match and the second balun will perform the primary function of impeding CMC.
Sounds too easy? Unfortunately it isn't quite that easy. That will work at low power levels but at higher power levels we get additional problems. High impedance baluns used with high SWR applications will heat up much quicker than baluns used in matched environments. As a result, the core will have additional losses, and possibly saturate, which can lead to total breakdown.
Of course there is a fix for this second problem. Use a larger Toroid, or use 2 or 3 Toroids. Again it sounds easy, but unfortunately this introduces more cost and more weight - often too much weight for the intended applicaton.
At this point we are caught between a rock and a hard place. The solution is to add an additional support (pole or mast) at the feedpoint, capable of supporting the additional weight, or perhaps to change the type of feedline from coax to openwire and not use any balun at all at that point.
This topic will be covered more under the page on "Baluns for Tuners".
WHY OVER 50% OF THE BALUN INFORMATION ON THE WEB IS WRONG:
As you will read elsewhere in this section, there are many types of baluns. You'll see terms like voltage baluns, current baluns, transformer baluns, Maxwell baluns, Reisert baluns, Guanella baluns, Ruthroff baluns, etc.
It is indeed very difficult to understand what all this means, and why there are so many. It is even more difficult to understand when to use which one.
Although Maxwell was one of the first to write about baluns (the current balun), for some reason Ruthroff's voltage balun (which came later) gained popularity much quicker and soon became pervasive in amateur radio antennas.
As it turns out, the voltage balun really has no place in our antennas unless it is used together with a current balun, in a so called "Hybrid Balun".
I can only speculate why the voltage balun won the intital market share. I see two reasons: it is simpler and cheaper.
But it is lousy at impeding common mode current and after all, that is the primary task we want our antenna baluns to perform.
Initially articles were published in ham magizines showing how to build these, but with the coming of the Internet, web pages were created in vast numbers, continuing to promote this wrong technology.
In the meantime, the current balun has emerged as the balun of choice for our antennas. Most antenna tuner manufacturers have upgraded their products from voltage baluns to current baluns, and several OEMs are promoting current baluns.
Unfortunately many (perhaps most) are proposing a product which does not work! They are offering a 4:1 Guanella (which requires 2 transmission lines), wrapped on a single ferrite toroid. This actually forces an imballance!
In order for the 4:1 Guanella (current) balun to work properly, it must have two ferrite toroids and each transmission line must be wound on its own dedicated core. Failure to do this will result in a balun which fails to impede common mode current effectively!
As could be expected, most of the web sites showing how to build 4:1 Guanella baluns are doing so using a single toroid for the balun. These are all wrong and very bad advice.
Luckily the 1:1 Guanella (current) balun does not have this problem, so generally articles you read on these will be fundamentally correct, though many highly underestimate the amount of ferrite (size and number of toroids) required for running high power in the presence of high SWR.
Dependable sources of GOOD information on baluns:
*Unfortunately Owen's excellent web site has been taken down and is no longer available.
DJ0IP is NOT an expert! My contribution is in showing practical solutions that work
- that really do what they are supposed to do -
and pointing out when and where they are good. AND I will tell you who I think the industry's real experts are and point you to their work.
LAST BUT NOT LEAST:
Unfortunately hams tend to believe that a good balun anywhere is a good balun everywhere. NOT!
A good balun for one type of application may not be a good balun for another type of application.
The people publishing information about their own "great balun" typically fail to point out that it is not necessarily great for everything else.
Like always: DO YOUR OWN DUE DILLIGENCE!