70cm Yagi

I was very pleased with yesterday's 70cm Moxon, made for nothing more than the cost of my time and putting together some bits of wire and wood.

So, let's be clear and say that the 70cm Moxon is very successful, yielding strong signals to a repeater over 100km away on just under 5 Watts.  But a little more gain is always useful.

Unfortunately, fairly modest 70cm beams aren't all that easy to find.  The smallest I could see online was a 10 element - far too much for my situation - and costing £50.  That's a lot of money for a simple antenna.

Last night, reading some antenna books in bed as one tends to do far too often, I came across the 'Cheap Yagis' section in the ARRL Antenna Handbook.

Brilliant performance across to the Isle of Man repeaters on 5W. 

Now, those outside the UK will recognise the infuriating adherence in the US to English measurements, rather than metric.  And it was with that resigned sort of attitude I decided to try one of the cheap Yagi designs for 70cm.  I was so resigned to it all that I worked in inches!

I didn't have any spare wood, save for some very thin square section 8x 8mm (what's that in inches?)  So I used epoxy glue to stick two of these on top of one another, yielding a stronger structure and one that was ideally sized for the elements, especially the folded driven element, to stick together.

Untreated soft timber doesn't last forever, but from experience in the wind, salt and rain-lashed coast of Wales, varnishing in hot weather that lets the coating dry thoroughly means you can expect several years before rot sets in.  A better idea is to visit your local timber merchant and ask if you can have the pressure-treated ('tannelised') wood strips that separate the stacks of larger wood that come on pallets.  These last for a very long time, and don't need varnish.  Normally, they are just treated as waste, to see if you can get some!


So, not wanting something to communicate with deep space, I opted for a four element Yagi.  Following the measurements below, and being only moderately careful with lengths, the design came in at a good SWR of 1.1.2 or so - more than adequate for it to go straight into service.  Even just placed horizontally on top of a woodshed roof, it was easily opening the Isle of Man repeaters 100km away, so an instant success!

The element wire - 2mm craft wire in my case - is simply pushed through tight-fitting holes drilled in the timber, and then stuck in place using epoxy glue. A good dollop of exterior varnish to the whole antenna reduces the impact of salt-laden air and rain.

Test feedpoint detail.  Use plenty of heat as the element absorbs lots of it!  When properly installed, you can use non-conductive grease or varnish to seal the connection.

For posterity, here are the measurements I used for 434MHz (the Yagi seems less sensitive to small measurement errors than the Moxon, which was very fussy.) 

Reflector:

Position 0 inches (mm), length = 13.5 inches (34.29mm)

Driven Element:

Position 2.5 inches (63.5mm), total length (including bend to give gap of about 0.5 inches (13mm)) and the folded part of the element =  20.25 inches (514mm)

Director 1:

Position 5.5 inches (51.43mm), length = 12.5inches (318mm)

Director 2:

Position 11.25 inches (28.58mm), length = 305mm