Air Core BALUN
Air Core Balun - Feb 1992
Reading through a number of old BREAK-INs during the past month, I came across a number of articles which may be of interest to Club members. It seems fitting to follow the article from RADCOM in this issue on Ferrite Core BALUNs with an article titled "AIR CORE BALUN FOR 1.5 TO 30 MHz" by Grant Simpson, ZL2BK, from BREAK-IN of October 1982. The article discusses why BALUNs are needed for proper matching and reduction of stray RF, and that a BALUN is required to maintain antenna radiation pattern symmetry. ZL2BK writes:
"A simple and cheap BALUN has been in use at my QTH for six months, with satisfactory results on a trapped dipole. Two others have also been in use for some time with no problems showing up.
The BALUNs I have built maintained balance from about 1.2 MHz to 32 MHz. The design uses three 1.3 metre lengths of enamelled
copper wire (I used 18 swg because i t was available) and a piece of 34 mm OD plastic pipe 100 mm long (also used because it was there).
In the pipe, 25 mm from one end, cut a small slot to pass the three wires through side by side. Leave 50 mm on the inside of the pipe. Holding the three wires side by side, wind ten trifilar turns and pass the three wires through another slot cut so that the wires can be pulled tight allowing the turns to stay together. Identify the ends of each and connect the start of the middle winding to the end of one of the outer windings. The end of the middle winding is connected to the start of the remaining outer winding."
ZL2BK continues the article by describing how to connect the co-ax and antenna to the BALUN as shown in the accompanying figure opposite. He then states that whilst the end of the co-ax should be sealed to prevent moisture entering the co-ax, the BALUN does not have to be weather-proofed. He states his BALUNs have not suffered any change in performance due to weather. My calculations show that this is a 1:1 ratio BALUN, does anyone have a different opinion?
The second article which Club members should find interesting comes from BREAK-IN of July 1982 and is entitled "A WIDEBAND ANTENNA FOR THOSE SOLID-STATE RIGS" by A.O.J. Woodfield, ZL2PD. He writes:
"From Bill Orr's latest book on wire antennas comes this little
beauty. He claims that it goes from 3 to 30 MHz with a maximum VSWR of 2.5:1. It reminds me of a similar antenna used by an Australian antenna firm, and may have originally been inspired by this one."
The diagram accompanying ZL2PD's article is reproduced below; if the wideband BALUN above is modified to be a 4:1 ratio, the two could probably be put together. Any experimenters out there who are going to give it a go?