This page contains construction details on a 2 metre 144MHz VHF Yagi beam antenna, designed for portable use. It has successfully activated the following SOTA summits: G/LD-004 (Skiddaw), G/LD-021 (Robinson), G/LD-003 (Helvellyn), G/LD-001 (Scafell Pike) and G/LD-020 (Dale Head).
Since this antenna was shown in the July 2011 edition of RadCom, a few people have contacted me asking for some information on how it was constructed.
I made this antenna in 2008 for our first SOTA activation in the Lake District. I had previously made some Yagi antennas for amateur satellite use and also for WiFi and these had all performed well. The challenge for this antenna was that it had to be compact enough to walk up a mountain and be quick and easy to assemble and disassemble. 5 elements was chosen, as the boom length is 1.5m (5ft) which keeps it portable.
With that chosen, the next problem was how to build it so it can be put together quickly. I used 32mm solvent weld waste pipe for the boom, with a screw access plug on the end, so that the end can be unscrewed and the elements can then all be stored inside the boom. The reflector and directors are made from 6mm aluminium tubing and pass through holes drilled in the boom, with a small cable tie super glued on the elements to provide an 'insert to' stop, in order to centralise the elements (see photos below). The most difficult part here is drilling the holes perfectly, so that the element is 90° to the boom and all elements are in line. A pillar drill is handy for this, if you have access to one.
Portable beam on mast for UKAC
The driven element was the most tricky part to design. Yagi antennas that I have built in the past have used folded dipoles and a 4:1 coax balun. Due to the need for this antenna to be portable, a folded dipole was not really an option. A gamma match might have been an option, but I decided to go with the basic open dipole driven element. The problem was that the driven element has to be two separate elements and therefore can't really be slid through the boom like the directors and reflector. This was done by cutting a slot through the boom and inserting a piece of plexiglass. A strip of copper was cut from some 15mm copper water pipe and brass connectors from a terminal block were soldered to it. The copper was then screwed and glued to the boom and plexiglass and the coax was connected to the copper via solder tags. It was tricky, but it worked well. More detail of this can be seen in the photos below. The driven elements are made from 4mm aluminium solid rod, so that they fit inside the terminal connectors.
A coax choke was made by winding 8 turns of coax around the boom between the feed point and BNC connector. 8 turns is more than enough for VHF! The antenna would probably perform fine without it, or with just a few turns.
The Yagi is a DL6WU design calculated with the excellent software by VK5DJ. The DL6WU design is well known and used mainly for long Yagi's with 10 elements or more, but small designs still work well. The software can be downloaded from http://www.vk5dj.com/yagi.html. The S.W.R. of this antenna is 1.2:1 around the design frequency and below 1.4:1 across the whole 2m band.
Here is a couple of screen shots of the software for the design of the Yagi described here:
Inputted data | Yagi results
Of course there are many ways to fabricate a Yagi antenna, but hopefully this page gives you some idea how to create yours. Below are some photos showing in detail how the antenna is put together.
Close up of the feed point, showing coax choke and BNC connector.
The feed point
The feed point
The feed point, showing soldered terminal connector and 4mm aluminium rod.
The reflector, showing glued cable tie to provide a stop to centre the element in the boom
The screw on end cap (32mm waste pipe solvent weld screw access point)
The mounting method (40mm tee, welded to 40mm waste pipe mast) The 32mm beam mast rests in here.
The finishing touch!
The Yagi beam on a tripod in the garden
Below are photos of the antenna in use...
Dave, M0TAZ using the beam in the 144MHz UK Activity Contest, August 2011. 40w providing about 240w E.R.P. QSO map is here.
John, M0UKD on the highest point in England, Scafell Pike.
Dave, M0TAZ on the summit of Skiddaw.
John, M0UKD with the beam on the summit of Dale Head.
Dave, M0TAZ with the beam on the summit of Dale Head.
John, M0UKD operating on FM with the beam orientated vertically. Summit is Robinson.
John, M0UKD on Skiddaw.