Omni 2M Vertical
An omni-directional 2M Vertical Antenna - March 1993
This idea was described in AR of May 1986 by Ian Keenan, VK3AYK. Your editor built one of these and it has proven to be a very useful and easy to make antenna. The electrical design is derived from a commercial design, it is a unity gain (= dipole) antenna.
Its advantage is that it is easy to tune, less than 1.5:1 VSWR is achievable over all the 2m band and in use, there are no awkward radials to deal with. This is particularly important when operating portable as radials seem to get caught in trees or end-up poking you (in the eye) or worse still, puncturing the Li-Lo. Its slick design is easy to tie onto a mast of almost anything available without deteriorating the electrical characteristics.
Fabricating the antenna is self explanatory from the diagram. A couple of points are worth noting. The braided section shown on the upper part of the co-ax inner, is a small piece of wire soldered to the end of the inner to give a margin for cutting to tune the antenna. 'Use white PVC conduit, not grey as the filler will deteriorate performance; I haven't tried "orange" PVC tube for antennae yet. If anybody has tried it, I would appreciate a report on its performance. Don't forget to put a hole in the bottom cap to drain condensation.
The coil construction is reasonably important. The coil must be tight and i t is best to cover i t with good tape or heat-shrink tube to hold it in place. I use both, using tape to hold it before covering with the heat shrink. I also seal the two holes with silastic for good measure.
To tune (or check the tune as it will normally be OK as is), tape the top of the inner to the tubeing; check VSWR over the band. If you want to, you can optimise i t for the section of the band you normally use (eg FM end of band, packet etc). I have found it excellent over the whole band. When tuned, trim the end as necessary and fit the end cap, bending the upper wire over the edge and holding it in place with the cap. If you have any problems, I'd can show you one already made, otherwise, you can go read VK3AYK's article in AR. A highly recommended antenna.