Well, for those of you who look things up on the internet, you will have great fun with searching Google for the word HAARP as you will be confronted by a huge American research antenna farm up in Alaska. My use of the word HAARP isn’t tied up with the mysteries of world Super Powers; it is simply an acronym for Ham Amateur Antenna for Radio Propagation.
After my first attempts of building a 6m antenna to the designs by the late Les Moxon, G6XN from a computer program called MOXGEN which I stumbled upon whilst researching small beam antennas, I started to wonder if there could be more gain to be had by adding additional elements, well gain back to my old friend the internet and after following many links and dead ends, I stumbled upon an article in the July 2010 edition of Radcom penned by Ron Stone GW3YDX (some may know of him through his Company Vine Antennas). Here he presented something he called the “Super Moxon”.
Intrigued, I proceeded to model it with the 4NEC2 antenna modelling software, which I had recently downloaded from the internet and had been playing with. Well from the measurements provided it showed some potential, although some of the build materials where a little light weight for my liking, but by adopting some of the quality materials advocated by both Justin G0KSC (of Innovantennas) and Steve G0UIH/VK2IAY/3D2FI (of Vortex Antennas), I was able to produce an antenna of good performance, measuring up to Ron Stones predications, yet with the build quality of a more robust antenna. Well this quite well covered my needs for the magic band.
Now my thoughts turned to a compact beam antenna for the SSB portion of 2m, again my thoughts went back to the Moxon, which as a two element antenna from experience would perform moderately well, but without the punch that I felt maybe needed on 2m, so revisiting the article on the Super Moxon, I again modelled the antenna, but this time although moderately happy I took the antenna through another step by adding a further pair of elements. This is where modelling on the computer really comes into its own, gone are the days of wasted aluminium and lots of off-cuts of wire, just a couple of clicks of the mouse and a tap on the keyboard and voila, another antenna.
Well after a few hours of adjusting spacing’s, lengths etc, I had in front of me an antenna which appeared to offer the gain I wanted and to boot in a very small footprint. On the following pages I will present screen shots of everything, from the computer design, through to actual photos taken during the build and testing phase to the final step, the on air testing. I hope I will demonstrate that this build was not as daunting as it first may appear, just remember the old adage, “measure twice and cut once”, (yes I fell into that trap too, but fortunately it was only that I measured something too long, so in this case no wasted materials).
Sit back and relax, as here I show various imaged from the computer design phase:
Fig 1. After all the hard work had been done of measuring out each individual part of the antenna taking into account event the dimensions of the tube I was intending to use.
Fig 2. So after all that effort, this is a 3 dimensional model, note the “purple” blob, that is the feed point.
Fig 3. So now having run the first real calculation I was presented by a series of figures that appeared to represent a successful antenna. (note the design frequency was 144,300Mhz).
Fig 4. A more traditional horizontal pattern, which gave a peak of 15.2dBi and a beam width of just short of 60 degrees.
Fig 7. Now a plot detailing the SWR over the frequency range of 144 to 144.6Mhz, it is at just below 1 : 1.01 at the target of 144.3Mhz, the Reflected Co-efficient is -47db at the target.
Fig 8. Next a graph showing the gain and Front/Back gain, here the gain is shown as 15.22dBi with an F/B of 13.98dBi (remember just subtract 2.15dB to get an idea if dBd).
Fig 9. Nearly there with the design pictures, this one gives you the Impedance (which is 50.4Ω for our target frequency), you will note that on the lower graph the Phase angle is only -0.199 degrees.
Fig 10. Finally for the last of the design, the ubiquitous Smith Chart, which I think is self-explanatory.
Ok you’ve probably got bored, so here comes some real photos of the build stages:
Fig 11. Everything starts with a drawing and this is no different.
Fig 12. An interesting tool and device, this is a tool for putting “inserts” i.e. little treaded bushes into sheet material (this is for a 6mm insert).
Fig 13. After drilling oversized holes here we can see some inserts fitted.
Fig 14. Yet another view, I even use a scalpel to scribe the aluminium to make sure I have everything “just so”.
Fig 15. From the Rule, you can just about start to see the scale of things.
Fig 16. When it came to the feed point, this was made from a piece of insulated material, but in my case I put a thick piece of heat shrink around the middle for the tubes to “butt” against.
Fig 23. Finally for the last bit of mechanical strength, the whole insulator is covered with an adhesive lined heavy walled heat shrink. (no moisture will every get in).
Fig 24. The larger ½” tubes are slotted to allow for miniature stainless hose clamps to tighten the corners to the main elements.
Fig 25. The 3 turn balun in place near the feed point. You can also see the “Stauff” pipe clamps used throughout the construction.
Fig 26. A close up of the direct connection of the feed point. The coax is sealed with self-amalgam tape and wrapped with insulating tape for UV resistance. Ultimately the terminals will be “painted” with rubber glue to act as a further weatherproof seal.
Fig 27. The completed antenna. Just a fraction over 1m long and about 700mm wide. Not bad for 6 elements.
Fig 28. The antenna mounted on a short post (about ½ wavelength above ground) pointed skywards for testing.
Table 1. Shows the predicted results against the actual measured results at the test location.
So, the final question, was it worth it?, well, all I can say is definitely yes, as I sit here finishing off this article, all I hear on my rig is the dulcet tones of the musical GB3VHF beacon located near Gravesend echoing throughout the shack (just remember it only puts out 30w and that beacon is just south of Gravesend some 250km South of my QTH). Not bad for 2 metres.
