To promote and explore the hobby of amateur radio. Providing quality training for all amateurs and in all aspect of the hobby.
Tue, 05/06/2012 - 18:18 -- M0RBY

This week, having got some time off work, I decided to build a 2m Yagi. I've recently put together a Moxon for 2m which was OK but I wanted a proper beam and I needed it by today so that I could enter the 2m Activity Night contest. On to the web I went and found a generic Yagi calculator, banged in a frequency of 144.300 and got the dimensions. Nothing exciting about that. The only ally tube I had was from an old broken monster of a beam. What the heck - it'll do - 22mm for the elements and 33mm for the boom. So an hour or so and some tweaking and twiddling later the beam was born. Next came the fun and games of matching it. Somehow I'd managed to select a length of coax which appeared to give me an SWR across the whole of the SSB section of 2m of about 1.5:1 - that'll do for me for the moment. Hoist the thing into the air and lo, an SWR of Skyhigh:1 - erm, what's happened. Well, after consulting those more learned than me in the club there appeared to be a number of issues, not least of which was the relatively low feedpoint impedance of the design that I had chosen. Dark mutterings of such things as analysers and VNAs were uttered. Not having anything more complex than Google to hand I took the sage advice offered and found me a design for a 4:1 coaxial balun (http://n-lemma.com/calcs/dipole/balun.htm). Having a lump of RG213 with  a velocity factor of 0.66, numbers duly scribbled and a length of 27" was deemed to be adequate. I cut a lump about 32" to give me a bit to play with. Popped it on with the rest of the coax that I was to be using and found a very good SWR (1.2 - 1.3 across the whole band) I started to hear stuff out of the gloom like the GB3ANG beacon which was pretty exciting for a newbie on 2m.

So, for not much effort, a lot of fun and loads learnt, I'm hoping for a decent run in the contest tonight!