BALUNS by Dan N5AR
There is considerable confusion about the various types of baluns. There are 4 types in wide use.
1. Coax wound around a ferrite toroidal core.
2. Ferrite beads slipped over a short piece of 50 ohm coax.
3. Coax wound into a single layer solenoid on a plastic tube.
4. Coax wound into a scramble wound bundle.
5. A rule of thumb that has been used is that the impedance of the balun, at the operating frequency, should be 10x the coax impedance, 500 ohms. Recent work by K9YC and others indicates that we need 5000 ohms or so to guarantee that the balun will not be damaged by overload conditions which occur in ham applications.
6. These happen due to antenna unbalance conditions or variations in antenna impedance across the bands. The problem is particularly bad with ferrite core baluns. It is exacerbated when the ferrite cores are small and encapsulated in epoxy so heat cannot escape.
7. Commercially available baluns use type 73 or similar ferrite which is best suited to mid HF and VHF application. Fortunately a new ferrite, type 31, has become available, which is much better for use in our HF bands. It does not appear that any of the suppliers of baluns or balun kits are using type 31
8. K9YC has designed and measured, with high quality equipment, a number of baluns which can achieve 5000-10000 ohm impedance using type 31 ferrite. A report is available which describes how to build them at:
9. It is very possible that many of us have damaged baluns. This might show up as a change in SWR, F/B, or loss of the deep nulls off the ends of your antenna.
10. Have you ever applied power to the wrong antenna by mistake? If so there is a good chance you have a fried balun.
Fortunately for us, WA2SRQ took the time to make careful measurements of the last 3 types over the hf bands. He has posted the results several places on the internet including the K1TTT web page. He measured only one ferrite balun, the ferrite bead job which is very popular .The solenoid type baluns he measured look good for single band use if you could put them on precisely the right frequency. They would be affected greatly by capacity to any nearby metal. Possibly one could grid dip them in place to check or provide a small tuning capacitor capable of handling the voltage across them.
I might mention that there is a ton of good information on the K1TTT web page on other ham technical topics. Here are the measurements:
below is an addendum added 7/9/2013 by Dan
Addendum to Baluns
There are 3 basic types of Baluns
1. Solenoid inductor consisting of rolling a section of the coax feed line around a PVC form. There are pro’s and con’s to using these.
Pro. They are easy to install and require no connectors or protection from the elements. They are very cost effective. They are almost indestructible when the antenna impedance is not 50 ohms.
Con. They are most effective at a single frequency which is dependent on the resonance of the inductor and is vulnerable to coax line length, and its grounding as well as nearby objects such as cars, buildings etc. They are totally unsuited to multiband antennas like tri-band Yagis.
2. Small teflon coax passing through the center hole of a number of ferrite beads. These are usually small beads and the unit is frequently enclosed in a piece of plastic pipe around an inch or so in diameter filled with epoxy. These are widely used, particularly with tri band beams.
Pro. The impedance can be quite high and not particularly frequency sensitive. They are small and easy to mount to the feed point of the antenna.
Con. The single turn of wire through the center hole means many beads are needed. They do not get the N squared impedance increase available with multi turn coax through the core. The small size and encapsulation means they cannot easily cool and therefore are frequently overheated in high power use or high duty cycle applications like RTTY. Since most antennas cannot maintain a 50 ohm impedance over a complete band (and some nowhere in the band) they can be easily damaged due to cable spillover power at the antenna. A momentary shot of power for the wrong band could fry it.
3. One or more ferrite cores with multiple coax turns thru the center openings.
Pro. Using the proper type of ferrite, usually type 31, it is possible to build Baluns which have impedances as high as 20,000 ohms or more over a number of bands.
These can be designed to accommodate full size RG8 type coax. The cost of commercial Baluns in weatherproof boxes with legal limit power capabilities is well under a hundred dollars. Companies like Balun Designs and Array Solutions offer these. It is not difficult to build your own and save money if you prefer.
These utilize the principle of operating a low frequency ferrite in the upper part of its frequency range where the resistive component of its impedance dominates. This assures that the inductive part of its impedance does not make it frequency sensitive. Thus the spillover RF sees a very high resistance.
If small size teflon coax is used, like RG142, (rated 9 kw) a number of turns can be fitted into the center of one or two of the 2.4 OD cores to make legal limit Baluns.
Cons. If one wishes to use RG8 size coax, the number of turns thru the core is limited and you may need as many as 5 to 7 cores. The allowable bend radius for that coax results in a rather large choke.
This addendum was created to concentrate the latest information on Baluns in an easily understood format. The earlier article has additional information and links which are still useful
August 11, 2008