A Moxon Rectangle
For 2 Metres
The aim of the current project is to provide a lightweight loft antenna for 2 metre SSB that can be easily rotated by a homebrew driver and controller.
The late Les Moxon, G6XN, developed what has become known as the Moxon Rectangle from a design by Canadian amateur VK2ABQ. The Moxon is basically a two element Yagi with the ends bent in towards each other with a small gap separating them. The Moxon has three main characteristics: It is small, as can be seen from the MoxGen program below, has incredible front to back ratio and it's feed impedance is 50 ohms. The gain is around 5 db.
The MoxGen program is free and can be downloaded from the link left, it will calculate the dimensions and (optionally) generate an antenna model file for a 50 ohm Moxon Rectangle antenna, given the design frequency and wire size. The program is based on an algorithm developed by L. B. Cebik, W4RNL.
The pattern for this design (right) is derived using the 4nec2 program and is for free space. It suggests that the front-to-back ratio is 37 db and the three db beamwidth is 80 degrees with gain being 6 db. naturally things will be somewhat different in my loft but I understand that the Moxon is fairly tolerant of nearby objects. We shall see!
The Moxon can be made from wire , tubing or solid rod (brazing rod is a favourite) and I have used 4mm OD brass tubing which comes in 1 metre lengths from B&Q. Unfortunately, the passive element of the Moxon is some 20mm over 1 metre. Fortunately, the driven element is under 1 metre and the extra tube is easily soldered on using a 50 watt iron. (I used a 3 mm drill to line up the pieces of tubing and wound a few turns of copper wire over the joint area before soldering.)
The tubing is easily bent by hand with the aid of a bench vice and and square for checking. I used a spirit level to ensure the bends were aligned. Both elements were bent and then the driven element was cut by removing some 10mm from the centre. The elements will be mounted on a piece of plywood 800 x 400mm (approx 31.5 x 15.75in), this will result in a turning radius of around 425mm (approx 16.76in) which should be fine for the smallest of lofts! (By the way the background in the image is a dressmakers' marking out board.)
In fact the 4mm plywood was cut to 800 x 350 mm (approximately 31.5 x 13.75 ins). A 5mm hole was drilled at the centre for attaching the antenna to the rotator shaft and an 8mm hole was drilled to feed the co-axial cable through.
Next the reflector was positioned on the board, the long side being set 35mm from the edge and around 25mm from each end, the position was carefully marked with a pencil. The plastic heads of 4mm cable clips (from Maplin) fit neatly over the brass tube, these were then glued into position using a two-part epoxy resin.
After the epoxy had set the two halves of the active element were in fixed in a similar manner taking care to ensure that the two elements were parallel and the gap at the ends the correct size. Prior to fixing down the ends were tinned and when the resin had set the cable was passed up through the hole and soldered to the brass ends.
The SWR was checked at 144.3 MHz and found to be around 1.6:1 with some 4 metres of coax, it was noticed that handling the coax altered the SWR so there was RF on the outside of the coax outer. A coaxial choke was added as close to the aerial end of the coax as was practical. Fourteen turns of the coax feed were wound on a ferrite rod (from Rapid Electric) and the SWR was re-checked.
This time more coax was needed to reached the SWR meter so there was about 9 metres in all with an in-line connection at around 3 metres from the aerial, with a 50 ohm load at this point the SWR was 1.1 : 1. The SWR of the aerial now proved to be 1.3:1 at 145.300 MHz and was pretty flat from 144.100 to 145.500 rising to 1.4:1 at 144.800 MHz.
To be continued!