70cm Moxon

Over the past few months, I've been experimenting a fair bit on 70cm.  I've bought a lovely helix for more serious, satellite work when I eventually put everything together on a alt/az rotator.  But, for now, messing about on terrestrial repeater work is fun enough.

70cm is attractive from this QTH as it has a line of sight over the Irish Sea to three, internet-linked repeaters on the Isle of Man, a good 100km away.  What's more, folk on the Isle of Man seem to be quite active, whereas my more local repeater is always empty.

The test timber-and-wire Moxon.  Not pretty, but at 1:1.2, why worry?

So, having started off with a slimjim, I thought I'd throw together a Moxon for 70cm, just to turn those handheld 5 Watts into something more easy to listen to on the other end.

Before I give you the measurements, remember I'm in the UK, and working repeaters.  So, I'm up at 434-point-something MHz.

A few internet-sourced plans came in with quite significant variations in element size and spacing.  After a bit of cursing, resoldering and cutting, I found the following brings home an SWR of about 1:1.2

Each half of the DRIVEN element (half of A) is 125mm, with B being 25mm.  Of course, you need to make two of these.

The REFLECTOR has A as 260mm, with D being 50mm.   A is slightly shorter for the driven side, because of the small gap for the feedpoint, which is made up by the short coax tails, which radiate, used to connect up.

The SPACING - C - is about 20mm.

I found that it is the length of the driven elements that has the biggest impact on SWR reduction - the already-published versions were too long by several mm - a result of most being designed for SSB frequencies.

A good line of sight, here during good Fata Morgana (superrefraction) conditions, to the Isle of Man.

You may need to bend, nibble or even extend on these measurements, but you should be fairly close on what I found to work.  The coax does tend to interact with the antenna a fair bit, so an arm to take away the coax at right angles to the beam direction is a good idea.

Used as a vertical, this Moxon works remarkably well.  MD6ICL/P reported my signal of 5W FM was very loud, and an easy QSO was had whilst Mike whirred away on some power tool at work.

So there you go - cost nothing - except for when I come to make a more durable, cosmetically appealing Moxon!