Yagi vs. Delta Shoot Out!

Well, having come up with a lightweight, strong design for a 2-element loaded yagi, up it went, by moonlight, for tests the following early morning.

A 2-ele beam like mine.  Large, and needs to go up high.

Now, I was not able to put the beam up at anything like a good height - just 6m, though the ground does slope away sharply in front of it.  So what follows isn't an assessment of what the beam could do if it were at greater heights.

I turned the beam towards the long path signals from the Pacific.  Running at the other end of our large garden, away from any interaction between them, was my trusty 20m full wave delta loop which uses vertical polarisation.

Wow!  A SteppIR in a very macho scene.  It gives a good impression of the sheer size of even a simple 2-ele beam.  But the sad reality is that experienced beach operators will tell you that a simple vertical antenna easily outperforms a yagi from the seaside.  So 'beam' doesn't necessarily mean 'best antenna' for all locations.

Luckily, VK3VCE and VK5PAS were in a group that were only too willing to report on the two antennas.  The result?  The beam was between 1 and 2 S units down on the delta loop.  This was confirmed later in the day, when a station on Prince Edward Island also came in 2 S points down.

Propped up by an 8m fishing pole and made of any old wire, the delta, with its base at just 1.2m, beat the far more complex beam by anything up to 12dB.

So, in terms of decibels, that's a beam, at this height, returning 12dB less in and out firepower than a simple delta loop.

Now, again, the beam wasn't being given a fair chance, and it is a centre-loaded, shortened version.  BUT!  Let's look at this from the perspective of ease of deployment, use, and cost.

The beam is complex to get up into the air.  Books and magazines suggest it's easy, which it is if you have lots of money, plenty of room for guy wires, and permission to put a mast of some sort up.  For real hams, it's really very difficult; have a look at how wrong it can go if you're not prepared:



Then you need to rotate the beam.  This adds considerably to the expense, and necessarily means you'll have to twist your antenna around endlessly to get the best signals.

Then you'll need to make the whole thing windproof at, for 20m antennas, anything from 8-15m high, depending on whether you have any ground reflection sweet spots.  For me, with regular gusts to 80mph or more, this is a big ask, or a recipe for endless upping and downing the beam.

Big antennas can mean big disasters and high repair (or insurance) costs. 

Given that my 2-element beam would appear to have to reach at least 10m before it broke even with the delta loop, with an apex at 8m, supported by a single, $15 fishing pole and which resists winds to about 50mph quite happily, the beam loses the competition, hands down so far as my essential check list is concerned.

Low angle patterns typical of low-mounted, vertically polarised deltas.  This one's at 5m; at lower base heights, the upper lobe vanishes.  Low angle patterns are very useful for DX, and the delta excels in this respect.

Now, my QTH does have a clear aspect, is elevated at about 65m around the surrounding land, has some sea views, and has the best terrestrial ground you could hope for.  So a delta may not work quite so well from a cluttered environment, where a yagi placed above the neighbourhood would yield real benefits.

Just keep in mind that, whilst a horizontal beam sounds like a good idea, the reality isn't necessarily so.  

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!