2 Meter Moxon (18 April 2015)SOTA is appealing for many reasons. One of the reasons is that you need to carefully think about your kit because you will have to carry everything up to a summit (so it must be light and portable) and when you get there, it must work, come rain, wind, hail, comets or whatever (so it must be reliable).
Being of an engineering bent, I tend to easily slip into the infinite loop of "further optimization" because - to take this antenna as an example - it could always be lighter, more compact, have more gain, be simpler to set up and take down, be more robust... And this does not even touch upon the further dimension of buying new components to suit a design, or designing to suit what you already have on hand. So although it's definitely not the best 2 meter SOTA Moxon "this side of Alpha Centauri", iteration 1 works really well. To read more about how it saw the light, dive in!
Why Introduce Directionality into a SOTA Antenna?Gain is introduced by focusing the radiation pattern. Focus reduces the beam width, and turns an omnidirectional antenna into a directional antenna, trading omnidirectionality for gain. A bit like turning a light bulb into a laser, not adding energy but concentrating what is already there. Although LASERs are cool, sometimes you rather need the diffused glow of a light bulb than the focused beam of a LASER. In the same way, there are arguments for and against focusing an antenna radiation pattern when building a SOTA antenna.
The case against it:
You never know where your callers will call from. Directionality may cost you a summit activation if someone calls from a location perpendicular to your antenna's beam, directly in your deaf spot (null). Or from behind you, if your antenna has a huge front to back ratio.
The case for it:
When you are activating a summit out in Woop Woop, especially in Australia, you most likely want that extra bit of gain, especially if you want to make Summit 2 Summit Contacts (something that you could plan for by monitoring alerts on SOTAWATCH before you go out). Furthermore, unless your beam is as thin as a pencil, you would have no dramas if you point your antenna to the nearest big population centre. Finally, some of the SOTA Summits host radio and mobile telephone equipment and sometimes the equipment can cause quite a bit of RF interference that one may want to null out by rotating your antenna to align the noise source and your antenna's null.
In my mind, the case for more directionality won out, so I steamed ahead.
RequirementsBefore I started out, I thought about what I wanted from it.
- A compact, light and portable 2 meter antenna with some directionality and gain.
- It must work with existing kit, utilise the squid pole as mast and play nice with the link dipole.
- Ideally it should require nothing that I do not have available.
- It should be robust enough to last for "a while". (To be specific, at least 50 summits.)
- It absolutely had to have a weird part or two that I could design and print out with my new shiny Rostock Max V2 3D Printer.
DesignI settled on the Moxon design pretty quickly, because I thought I could make something up with wire (junk box) and 9mm fibre glass rods (junk box) and shock cord (junk box). Besides, I have already built a Yagi (based on G0KSC's LFA design) and I thought that a Yagi would be hard pressed to match the Moxon in terms of weight, portability and cost. I also ruled out the Hentenna design, because it looked like the design may not work as well with floppy radiating elements. I also had a Hentenna 4NEC2 model that I did back in 2011 and I thought I may return to it if the Moxon design did not fare too well during simulation.
Modelling and Optimization
I calculated the rough dimensions for the reflector, director, short elements and the gap, starting off with a symmetric set-up where the reflector plus its short sides would be an exact mirror image of the driven element.
|Image courtesy of the Moxon Project (http://www.moxonantennaproject.com)|
Next, I modeled the Moxon in 4NEC2, my go-to Antenna Simulator. It is free and it works great. It integrates with other free tools such as GNUPlot and run models through the free NEC2 (Numerical Electromagnetics Code) engine. See the Downloads Section below for the NEC file.
I used 4NEC2's Genetic Algorithm Optimizer to find a solution that maximized gain and minimized VSWR. From memory I think I used a weighting of 50% Gain and 50% VSWR, I put the optimized model through the mill two more times, after I had a look in my junk box and found 4mm thick Green and Yellow insulated wire. Once the optimization was complete, I ran an average gain sanity check and then rounded off the all the fractions of millimeters to end with round numbers for all dimensions. This was the final model that I used to build my 2 meter Moxon. The results from 4NEC looked better than the old model from the Hentenna, so I dropped the Hentenna from the running.
|2 Meter Moxon average gain prediction of around 10 dBi and a F/B Ratio of around 5 dBi*.|
The 3 patterns are for the same antenna at 3 different frequencies: 144, 146 and 148MHz.
*Note that dBi refers to the antenna's gain relative to an imaginary isotropic point source that emits radiation equally in all directions. To compare it to a dipole, subtract 2.15 dBi. In other words the Moxon is predicted to have a gain of about 7.85 dBd (dBd is gain with reference to a dipole. dBd = dBi-2.15 dB).
When I mentioned to Andrew VK1DA that I was busy building a 2 meter Moxon, he asked how it would stack up against a simple 3 element Yagi-Uda. I didn't know, so for a laugh I modeled it in 4NEC2 (and subsequently built it, but more about that later). Here I have included the modeled comparison between the Moxon and the Hentenna's radiation pattern (in elevation/from the side), and as a sneak preview, the Hentenna, Moxon and simple 3 element Yagi on the right.
|Left: Moxon vs Hentenna. Right: Hentenna, Moxon vs. a 3-ele Yagi.|
By running a frequency sweep in 4NEC2, I could get a feel for what the Moxon's VSWR would be across the 2 meter band:
|Predicted VSWR looks good, with 144 to 149MHz around 1.5.|
|Marked Up Moxon for dimensions below.|
Update on 24 April: I made a mistake when entering the diagonal spreader dimensions: they should be 380mm and not 572.5 as I originally had them. Sorry about that. ;-)
Building Tip: The antenna can start of as two pieces of wire, ABCD and EFGH. So measure ABCD and then EFGH, attach their ends to each other with elastic shock cord. Snip the driven element in half and solder in a BNC connector. That is it. Then you need to think about spreading the wire into a rectangular pattern according to the dimensions above. I used old tent poles and a 3D printed hub that slips over another tapered pole.
Building the AntennaI had the dimensions and all the materials, but was missing one final piece of the puzzle. The central hub that would hold it all together and mount on the Squid Pole Mast, out of the way of the Link Dipole. It needed at least four holes to hold the glass fiber rods that would give it all some stability and it needed to attach to the Squid Pole. I had a look at some of the hubs on The Moxon Project's website but none would do. In the end I knew I had to make some sort of sleeve that slips over the Squid Pole, like the hub of a bicycle, with spokes radiating from the central hub - a bit like what I did to 3D Print a guy rope mount for my Squid Pole.
|Guy Rope Mount for Squid Pole|
I settled on a flattish square block that could slip over the pole and had radiating holes to accept the spreaders. I had one last look on the interwebs and stumbled onto this great design for a "central hub" on SOTA Beams' website. I shamelessly adopted their round doughnut shape because it was simpler to combine with radial holes in FreeCAD, and it looked just right - so much better than having a square block. I also got the idea to add a fifth hole for a rod to support the BNC Connector and Coax from their Central Hub's design. I punched the dimensions into FreeCAD and this is how it came out:
|Moxon Doughnut with Spreader Holes at the correct angles.|
|Spreader Rod End Caps|
|The Little Fold-up Moxon on One Tree Hill, Ready to deploy|
|When disassembled it collapses into a tiny bundle of wire and rods|
It came together rather nicely and works a treat. The antenna has some gain (I am not sure how to accurately test real-world gain yet), slips nicely over the Squid Pole and folds up into a tiny bundle of wires and rods which I bind up with elastic bands before they go into the backpack. It also has a really nice and flat VSWR Curve, as can be seen from the MiniVNA output below and it weighs next to nothing.
|Measured VSWR (Green Line) is below 1.5 from 140 MHz to 150 MHz.|
I hope you found this useful and was able to duplicate or improve on this little antenna. If you have, please let me know. Happy building!
DownloadsThese downloads are free to use and change as long as you credit the source, don't use it for commercial purposes and license your improvements under the same terms:
To print out for yourself: Moxon Doughnut Hub STL File
To print out for yourself: Moxon End Cap STL File
To tweak the design and make new STL files: FreeCAD File for Doughnut Hub
To tweak the design and make new STL files: FreeCAD File for End Caps
To tweak the antenna design for this or other frequencies: Moxon NEC File