Baluns – by Dan, N5AR
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:
HTTP://AUDIOSYSTEMSGROUP.COM/NCDXACoaxChokesPPT.pdf
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:
http://www.k1ttt.net/technote/airbalun.html
73,
Dan, N5AR
August 11, 2008
