After a number of successful SOTA expeditions around Snowdonia with my portable multiband vertical and IC706 it was decided to look for a lighter weight alternative. As 2 metres is the most popular SOTA band used, a lightweight antenna for use with a handheld transciever and providing some useful gain was sought.
A small 2 element Yagi (Plate 1) seemed to fit the bill quite well and could be easily mounted on my telescopic trekking pole that I use for walking, thus eliminating the need to carry a separate mast. The design problem however was how to keep simplicity, lightness, compactness, speed of assembly and a good match to coax all together. My complete ‘Pocket Beam’ antenna is just 20” long and 2” wide when dismantled and can be assembled in less than a minute. It weighs in at just 6oz complete with feeder! and is optimised for 5Dbd gain and a high front to back ratio. Most importantly SWR was measured at 1.1 to 1 when mounted either horizontally or vertically.
The critical dimensions are-
Driven Element – 94cm
Reflector – 104cm
Spacing – 26cm
Boom – 30cm long 15mm diameter plastic tube and plastic end cap
Feeder 50? coaxial cable (9ft of RG174)
Inducto-match – 2 turns of 14SWG ½ inch dia. by ¼ inch long (Equates to 8 turns per inch spacing)
1m x 6mm diameter hollow aluminium tube plus 2” 8mm diameter hollow aluminium tube
Self tapping screws, Cable ties and PVC tape
Plastic clamp and wing nuts
In view of the short length of coax needed and the desire to keep the system low in weight and as compact as possible I decided to use RG174. RG58 could be used instead and would reduce losses but may require elimination/alternative design for the choke balun. The driven (split dipole) element
I used was actually a telescopic (adjustable) set top portable TV antenna that usually come supplied by the manufacturer (also available from Sandpiper Communications for £2) and was a good push fit into the 15mm diameter plastic tube (Boom). The folding whips are attached at the base, by small machine screws and here the coax is connected on one side and the inducto-match on the other using ring connectors. Six turns of RG174 are made close spaced around the boom near to the feedpoint to act as a choke balun thus preventing stray RF affecting SWR and distortion of the radiation pattern.
The feeder and inducto-match are secured with cable ties to the boom (Plate 2). The feeder cable is then run along to the back of the boom and secured with more cable ties as this must exit away from behind the reflector element to avoid SWR and radiation pattern distortion problems. The reflector is made from 2 pieces of 0.5m long 6mm diameter hollow aluminium tube, into which a 3/16” Dowel screw has been inserted to provide a threaded end and make up the length required.
A small 2? inch length of 8mm diameter hollow aluminium tube (6mm internal dia) is used, inserted through the plastic tube and secured in position with a self tapping screw (Plate 3). The elements are a sliding fit into this, but to make them secure the threaded part needs a thread. To make the thread I used a small amount of chemical filler (fast setting exothermic stuff).
The threaded element makes the thread by first smearing with Vaseline to prevent adhesion and then after filling the tube with some filler pushing the elements in each end until they meet in the middle. Once the filler has sufficiently set (20-30mins) the elements can be carefully unscrewed and ‘Voila’!
Mount the antenna from the back of the boom with whatever clamps you like (I used bike accessory clamps with wing nuts) with one clamp on my trekking pole. My pole (Leki Makalu) has a basket on the base so once it is pushed into soft ground it stays upright even with the beam attached and can be easily turned around. The handle is also detachable to allow a camera to be mounted on the top and provides a suitable guying point for use in windy conditions.
Antenna tuning is done by extending the telescopic elements to the given dimensions followed by tweaking for lowest SWR. Once the best position is found the elements can be marked with a file. On air testing revealed the expected cardioid radiation pattern with two deep nulls and a minor lobe to the back. Signals were often readable/workable where they were not on my ¾ wave vertical. This is now my favourite portable antenna for SOTA operations. As the yanks say ‘It gives the biggest bang for your buck’.