Building the 2m Moxon Kit

Before we charge into the build, let's reflect on the Moxon. The Moxon antenna, originally developed by Les Moxon (G6XN), is a two-element, rectangular-shaped directional antenna that provides high performance in a compact and mechanically simple configuration.

Functionally, it is a folded variant of the two-element Yagi–Uda antenna, in which the tips of the driven and reflector elements are bent toward each other, forming a rectangle. This geometric modification reduces the overall boom length and element coupling while maintaining strong directional characteristics and efficient radiation patterns.

A typical Moxon antenna exhibits a forward gain of approximately 5–6 dBi and a front-to-back ratio in the range of 20–30 dB, depending on the construction accuracy and operating frequency. Its radiation pattern is primarily bidirectional in the horizontal plane, with low side lobes and excellent rear signal rejection, making it particularly effective for point-to-point communication. The antenna’s input impedance is typically close to 50 ohms, eliminating the need for complex impedance matching networks and allowing for direct coaxial feed.

Some really good features of a Moxon are its size, great front-to-back ratio, and 50-ohm feed point. Although the gain is not as high as other designs, Moxons tend to be smaller than higher gain antennas and therefore easier to carry.

We designed the 2m Moxon kit to get into building a directional 2m antenna that is suitable for further future development, is light, compact, and is suitable for location on a telescopic fiberglass pole. The idea was to create a kit specifically suited to the UK 145 alive on-air days.

Our kit comes with:

Our custom PCB (the flying squirrel)

4x6 mm fiberglass poles, 500 mm long.

ABS 3D printed parts: clamps, wire holder, element separator, and hub.

All fittings M3

BNC connector

Coax RG174

Center hub

Wire

Here is an intro to the kit

I would suggest you have a play with Moxgen before you start your build. Moxgen is a free program which will give dimensions for your antenna and will show the length of the wire you may need for your build. I say "may" because it does not allow you to enter the velocity factor of the wire you will use or the intended antenna height. But it's good fun. See below.

You can see Moxgen has given you suggested dimensions for the wire, for example, A+2xB is the driven element at 943.6mm. The reflected element is A+2xD or 1033mm.

However, I believe it's assuming bare copper wire. We are using insulated wire, which will shorten the dimensions. For the prototype, we had 880mm for A+2xB and 955mm for A+2xD, which is about 7.5% shorter. The reason for highlighting this is that we did use the dimensions from Moxgen for our initial wire cut, and it was too long when we tuned the antenna at a height of 2.5m. This was probably due to the height of the antenna and also the insulated wire.

So when you build your moxon, you can use Moxgen dimensions and cut shorter to tune, or use what we found to work. Remember you will need to add 10mm on each end to loop through the spacers, C in the Moxon design above.

So just to be clear, Moxgen is calculating a driven element of 943.6 or 471.8 per leg, with a reflector of 1033mm.

We found our driven element needed to be 880mm or 440mm per leg for the driven element and 955mm for the reflector element.

One good thing with Moxgen is it will allow you to export to EZNEC or NEC, where you can then look at how the antenna should perform via modeling. This is a far-field plot from EZNEC.

You can see a Moxon has great front to back and also some gain. Gain is quite useful, if I put into the antenna at the feed point 5w, then we get about 20w of ERP out. Anyway lets crack on.

The Build

Below is the antenna built, please study the pictures. Also read to the end there some important notes aout spreader length and wire length.

Bottom hub side

Top side

You can see there is a center hub, a PCB with holes for the spreaders and a BNC connector. There is a common mode choke and element spacers with wire.

The first step during the build is to add all the spreaders onto the PCB. Please watch the video below, which will explain how this is done.

Hopefully, the video has given you a good idea of how the components go onto the PCB.

Cutting the spreaders.

I just want to cover this subject a little. When I made the prototype the Moxgen calculations were out a little, this meant that I could not use them to decide how long to cut the spreaders. On the built prototype I cut the spreaders down after tuning to 36cm, I would suggest you keep them at full size tune, then cut. My dimentions should work, but its better to be safe and it only takes a few minites extra time to remove them one at a time and cut them to size after tuning.

Just be aware you get a little dust when cutting fibreglass, so use a fine blade and go slow.

Don't forget to put on the end caps after cutting.

The next thing to do is position your wire holder clips. We suggest you put them at approximately 33 cm from the middle of the center hub spreader clamp. It might be prudent to put them at 35 cm to start with if your wire cut is the length specified by Moxgen.

This is a zoomed in image of the wire holder and spacing. Please ignor the wire at this point in time, it is the last thing you will add to the build.

You will not need to make the CMC for the antenna, we have explained how to do this in this video.

By now you will have the frame of the Moxon built. You may have cut the fibreglass down to size. The BNC will be located in one of the holes and the choke will be built with the little dipole on it.

Solder the BNC Connector to the choke.

The next part is to run the wire (cut to size) around the frame using the wire guides set at approx 33cm out, see above. If you look at the pictures below (prototype) you will see how the wire goes into the element spacers.

If you look at the picture above you will see we have folded back the wire about 1cm and used a loose cable tie to hold it in place.

The reflector is a single length of wire. The Driven element is two pieces of wire joined at the choke feeder. This picture will be replaced showing the mini dipole PCB in the video above, but you will get the idea how the choke works.

Assuming you have everything physically built, we can now move on to tuning after we discuss the fiberglass dimensions.

The final dimensions of the prototype were as follows:

1 = 955mm, 880mm plus 20mm to fold over

2 = 440mm per leg plus 10mm to fold over

3 = 36cm per spreader

4 = 16cm

This gave me excellent tuning at about 2.5m high. I suggest you start a little longer than these dimensions and keep the spreaders at full length until you have tuned the antenna. Then cut them back. It's the safest way. Although I'm confident my dimentions are pretty close to what you will need.

Tuning

Assuming you go safe and a little long on the elements, you will need to set up the Moxon as a rectangle, as above.

You will then need to do a sweep with your antenna analyzer or VNA to check the state of tune. I suggest you set the sweep range from 140 MHz to 148 MHz to start with. Perform a sweep and then check the results.

If the antenna is too long, resonant at, say, 143 MHz, cut 10 mm off each end of the wire on the spacers and move the wire connectors in a little. It's really important you take an equal amount off each end. If you cut the director element and not the reflector element, you will get very odd readings. If it's too short, use the rest of the wire and start again.

This can be an iterative process, so be patient.

When you get to resonance, you should see an SWR something like this.

On the prototype, the resonance was also spot on, and we had a very usable antenna from 144-146 MHz with an SWR under 1.4:1. I think the higher the antenna is, the more likely it is to have a flatter, lower curve, but I'm only going to use it portably on a fiberglass pole, so I tuned mine at 2.5 m high.

I did notice that if I lowered the antenna to below 2m, it was not tuning very well. Anyway, I hope you enjoy the build.

Coax

The Common Mode Choke on the antenna uses RG174 which is quite lossy at these frequencies, but at under a meter in length we will only lose about .31db. However, we do not recommend using RG174 or RG316 as a cable run to your tranceiver. I use RB58 and have a 5 meter cable run to my tranceiver. This means my total loss is .31db plus 1.02db giving a total of 1.33db. Why is this important, with my 5w output from my handheld only 3.7w got to the antenna. So be sensible about the coax runs and what type of coax you use.