As the HF bands drop off, I have decided it is time to up my portable capabilities on VHF. Having already tried out 6m with some enjoyable success, the option is to move to 2m or 70cm, the latter having shorter antennas but more critical dimensions, and the former being larger but more tolerant of poor construction (much like me).
The other requirement was for a short boom length, so 12 elements on 2m wasn’t going to fly. I went looking for designs and settled on a 4 element 28 ohm DK7ZB antenna, listed as very easy for beginners to build. I concur. The total length would be 76cm, and the widest element was 1030mm, which was roughly in the ball park of what I wanted (longest dimension 1m).
I ducked down to Bunnings, and grabbed some 10mm aluminium tube, and a 20mm square section aluminium boom. Had I still had my old car, I would have bought a 3m piece of 10mm tube plus another 1m section, but instead grabbed four 1m sections, as hacksawing the tube down in the car park to fit in the new car doesn’t quite gel with my style.
The dimensions for 10mm tubes are:
- Reflector: 1030mm
- Driven Element: 990mm
- Director 1: 937mm
- Director 2: 857mm
The elements were cut to length using a tube cutter, before a 30mm section was cut from the offcut of one of the directors, and stuck onto the reflector. To do so, I took an 8mm wooden dowel, chiselled it down to fit, and jammed it into the two pieces of tube so they wouldn’t move, then wrapped aluminium tape around the join for good measure.
Once the elements were cut to length, I used 3/8″ P-clips to attach the non-driven elements to the boom in the right locations. Two p-clips at each position, held in place with M4 bolts through the beam.
The driven element was marked in place using a 3-way junction box ($1.26 at Bunnings) and I used a 13mm barbed poly tube joiner to act as a spacer. I chose the Holman brand rather than the really cheap home brand version as it had a bit more clearance and the 10mm tube slid into it perfectly. I predrilled holes for screws to hold the elements in place, put in a BNC connector I had handy, and connected up the DK7ZB match.
The DK7ZB match is a parallel section of 75 ohm line to act as a 28-ohm to 50-ohm match. I used RG-59, again from Bunnings, at 70c for a single metre, cut to 42.5mm lengths (quarter-wave for 145MHz at a Vf of 0.82). Once I had everything in place, I fed through the driven elements and connected them to the other end of the match.
Gratuitously large amounts of silicone (and a wedge of paper) were used to hold the driven elements in the right place in the junction box. Saddle clips should probably have been used, but it is unlikely I’d break down and reassemble the driven elements on a summit anyway; I’d run out of saddle clips, and I had gratuitously large amounts of silicone handy at that point. This is a portable antenna, but not a bush-bashing one.
The junction box was attached in the right spot with two M4 bolts. The match points forward out the third hole in the junction box. It is held parallel with some heat shrink. The BNC connector is accessible from the rear of the antenna.
I connected up the antenna to test it out and SWR was fairly flat across the band – a little higher at the 146MHz end. My FT-857 is Japanese spec, so I can’t transmit outside of 144-146MHz without desoldering some links on the main board. I’d say it’s resonant around 144 MHz or perhaps a little below. SWR is barely noticeable on transmit between 144 and 145.
I intend to get out and test the antenna in the wild sometime over the next two weeks; keep an eye out for alerts.