Wednesday, 26 May 2010

Moxon beam for 2m (144MHz)


I was reading Practical Wireless the other day and found a piece about the PW 2m QRP contest on Sunday June 13 2010. I've never taken part in this, but thought that I might make the effort this year.

I have a Yaesu FT-817 and a couple of SLA batteries, but I haven't got a 2m beam. I used to own a 4el quad by Jaybeam which was brilliant, but sold it as I thought I would never use it again – duh! I looked at a few manufacturers' websites, but £50 for a 5el Yagi seemed a bit steep for one-day's operating.

So I decided to make my own. I wasn't interested in outright gain, but wanted something which was a) small and light, b) cheap and c) offered a little gain and some front to back.

The answer tuned out to be a Moxon rectangle beam. This is a two-element beam offering about 3dBd gain and massive 30dB front to back

If you head over to http://www.moxonantennaproject.com/wb5cxc/wb5cxc.htm you can find out all about it.

There is even a PC-based calculator that you can download.

So I headed off to B&Q and came back with 2m of white PVC trunking, two connector unions, and some PVC glue. I added some 2m PVC coated copper wire that I had, an S0239 socket and some 3.5mm heatshrink tubing – that's it. About £10 all in.

I modelled the beam for 145MHz and came up with the dimensions shown below. I then cut the PVC pipe accordingly and soldered/glued it all together. I used the heatshrink tubing to connect the ends of the elements. If I were making another one I would use a drinking straw first to strengthen them, although they seem OK with the tubing on its own.

Using my MFJ analyser I found that it resonated at about 136MHz – damn! I think this is because the velocity factor of PVC coated wire is about 94-95% and the computer simulation assumes bare wire.

So out came the junior hacksaw and I cut everything down to 94% of the original. This gave me a beam with about a 1:1.2 – 1:1.5 SWR across 2m. I fitted the beam to a 10m fishing pole (about 2m down from the top) using a couple of cable ties and hauled it up. I used 20m of Mini8 coax, but later that day found that it had a measured loss of about 2.4dB on 144MHz. Switching to RG213 reduced this to about 1dB.

So does it work? Yes, and quite well. I was able to hear the GB3VHF beacon at about S1-2 (148.20 km (92 miles) away). Swinging the beam around made the signal vanish into the noise. I could also hear the PI7CIS beacon in the Netherlands (JO22DC) - 221.11 km (137.4 miles). Swinging the beam vertically brought in repeaters from up to 60 miles away too. This was under flat conditions and I was unable to raise anyone on 144.300MHz SSB, more's the pity.

So for a total cost of about £10 a great little lightweight beam.

Friday, 7 May 2010

HF Propagation Podcast - May 2010


Still very few sunspots at the moment, but lots of coronal hole activity! This month I also take a look at WinCap Wizard from Tabersoft. You can listen by clicking on the headline or search for G0KYA on iTunes.