Sleeve Balun

The sleeve balun does not give any impedance transformation; it is a 1:1 balun. This is fairly easy to achieve at VHF. All that is required is a tube that is coupled to the outer of the coax at approximately 0.93 X l/4 from the antenna feed point.

Sleeve balun

The ratio D/d should be around 2.5 to 4. The open end of the tube facing the antenna element should be as close as possible. In effect this tube is a shorted l/4, at it open end the impedance looking back down the coax is high, thus preventing RF current developing on the outer of the coax.

I have made sleeve baluns using 15 mm copper water pipe and soldering a 'Free socket N' type connector onto it. The l/4 may be less than that of free space l/4, because of the close proximity of the outer tube to the coaxial cable. I found that 468 mm seems to work OK at 2 m, when using RG213 or URM 67.

You will have to use good quality silver-plated ' N types ' to make soldering easier, and I used a plumbers blowlamp to make the soldered joint.

First I cut the 15 mm copper pipe with a pipe cutter which gives a perfect clean cut end to the tube. Then I disassembled the in-line socket; the threaded nut where the cable enters the plug has to be soldered to the 15 mm tube. Make sure the copper tube and the nut are very clean, I use fine wire wool to do this and then I apply a thin layer of flux. Next line up the tube on top of the nut, this is made easier if you can clamp the tube vertically and seat it up against the back of the cable-retaining nut. I have a drill stand and found that this did the job for me. I have a piece of fireproof material under the nut and tube assembly. All you have to do then is apply some heat and run in a bead of solder around the nut and the end of the tube.

Soldering the N type connector to 15 mm TubeThe N type socket

Componant parts of N type socket

When it's cooled down, clean off any flux and you are ready to assemble the free socket in the normal way feeding the coax down the 15 mm tube. The free end of coax will connect to the antenna, so dependent on the box arrangement the antenna feed point has, determines the way you weather proof the connection and the open 15 mm tube.

Flexible version of the sleeve balun

(As shown in The UHF Compendium Part 1 and 2)

Flexible Sleeve balun

The Open Balun

The Pawsey Stub balun

This method of forming a 1:1 balun is sometimes easier to make although it is not quite as good as the tube sleeve balun.

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