The Compact 20m 2-element Yagi

Living in a windy location, as I always go on endlessly about, makes for a difficult existence as a radio operator.

Over the past few months, and despite the atrocious operating practices often to be found there, the 20m band has become my favourite band for DX.  All of this has been with a simple 1-element delta loop for the band, which though simple, is exceptionally effective from this undeveloped hilltop, metal-saturated ground location.

KA3DRR's lovely 2-element yagi - yours will look much the same.

Recently, I decided to pursue a simple 2-element beam for 20m, to see whether the added complexity of  getting a beam several metres into the air is worthwhile.

I'd been looking at this easy-to-follow guide for some time, and had long ago built the coils, but never completed the antenna.  Inspired by some calm weather, out I went to add some wire and start tuning each element individually, rather than take a chance on the published measurements, build the whole thing and then find it way off resonance!

I also found this guide on a very similar antenna to be excellent and well worth downloading and keeping safe.

Although the peculiarities of the metal saturated ground make this site quite unique, I did find that the element lengths (using kevlar wire) needed to be considerably longer than published, and that's allowing for the fact I wanted my beam mainly for the SSB parts of the band.  Kevlar wire is thin strands of copper wound lightly around a kevlar core, and may well behave very differently from flexweave or drawn copper - you'll have to experiment with your own wire preference.  The big advantage of kevlar is that it's about three times lighter than flexweave, which is a consideration with saggy fishing poles.  The drawback is kevlar's higher cost.

VK2ZAY's image of a loading coil shows how simple it is - just some known length of wire wrapped neatly around  PVC tube.  Even I can make these!  Keep the coils neatly wound by wrapping firmly in weatherproof tape.

In the end, I found that each half of the dipole, without the coils, needed to be 114.5" (2.91m) long.  In other words, the length of each bit of wire either side of each coil (of which there are two per driven element) is 57.25" (1.455m).  I did try 3m poles, but they proved too flexible at the ends, so with only a modest penalty if increased weight, I used the upper few sections of an old set of 7m poles, with most of the thinnest, uppermost tip cut away so it doesn't sag.  Poles are very cheap at about £7 each from Paul's Angling Supplies (also on Ebay), who have superb service standards.

Detail of the boom to spreader joint.  A single screw goes through the spreader timber into the boom, which is then strengthened with varnished plywood screwed as shown.

 
The reflector is built a bit longer and in accordance with general design patterns for Yagi-Uda antennas - about 5% or so (you can spend all week reading up and chasing different web articles about precisely how long you may want to make it, but you have to start somewhere, so 5% is a good guide!)

The boom is varnished timber, with simple timber just strong and wide enough to accommodate the poles and to tie cable ties around to keep the poles in place (this is by far the lightest, cheapest way, but use quality ties).  I'll need to fashion a short stub from timber and plywood to attach the whole thing to the as-yet-unavailable push-up mast, but that's pretty easy. 

Here's an idea of the size of this antenna - note the garden fork for scale. 

In theory, the antenna should produce about 4dBd forward gain, with a modest 10dB front-to-back ratio.  In other words, if you take a standard rig at 100W and put this antenna at the end of the coax, you'll end up with about 250 watts effective radiated power going off to your target.  Signals to the rear will be reduced by about 2 'S' units.  Not bad for the price of some wire and fishing poles!

As soon as I get enough money to buy a simple aluminium push-up mast and get the beam at a decent height, I'll let you know whether all the effort is really worthwhile, relative to a simple delta loop propped up with one 8m fishing pole, and which needs no rotator or rotating!

One interesting point to note, well made by my friend John, ZL2JBR, is that you can often find signal 'sweet spots' at certain mounting heights, which you need to find for your location by experimentation (sticking the antenna up at maximum height, then watch your 'S' meter for any changes as you lower it bit by bit).  John, for whom I have great respect in understanding radio, reckons that the increase in signal strength can be as dramatic as 10dB.  Given John is pretty much the only ZL you will hear on short path most days, and that his antenna is several metres lower than usual theory dictates, his practice certainly matches his ideas!  For reasons of reducing mounting difficulties and wind loading, any reduction in height is a welcome thing, believe me.

In the meantime, if you don't want to build a beam, remember that you can make this in the 1-element dipole flavour - a 20m dipole made of super-lightweight fishing poles that's about 40% shorter than a full-sized version, all with very minimal reduction in performance. You can even collapse the poles for stowing out of view, or for taking portable.  You should also be able to turn this into a moxon rectangle following some experimentation.  Or you could, with stronger supports, fashion some end capacitance hats and shorten the elements further.  Plenty to keep you wasting miles of wire, there...


UPDATE:

I found this beam to be a poor performer at 8m.  It was 1-2 'S' points down on my 1-element delta loop for 20m.  I think the tuning of the elements could have been better, to be honest.  That said, it did get across to VK with no problem on long path, but that isn't actually very difficult.  Beams are simply too complex for me to bother with at the moment, because of the need to mount them so high and that they catch so much wind.  The delta easily wins again!