With the warmer weather fast approaching and the magic 50 MHz band conditions expected to start improving, I decided to build an antenna for 6m. Living in a house with a small back yard I have limited places to mount an antenna and I didn’t want to install a new mast. My preference was to add a new antenna to my current 2m/70cm mast which currently holds a dual band Diamond X-30. I also didn’t want to build an antenna that required special matching devices.
Previously I had purchased a V2000 tri-band antenna covering 6m, 2m and 70cm with the aim of completely replacing my X-30. It seemed to be the perfect solution as I had a tri-band FM radio, however it came with some limitations. On 6m the V2000 antenna was tuned to the lower end of the band which was perfect for CW/SSB but at the upper end where the local FM repeaters are located, it had an SWR of nearly 4:1 . Adjusting the 6m counterpoise as per the instructions raised the resonant frequency only slightly with a significant deterioration of the SWR at resonance. Clearly the V2000 wasn’t going to be suitable for FM repeater usage on 6m.
This directed me towards easy home-built alternatives and I finally settled on two antenna designs – the ‘Flowerpot’ coaxial vertical and the Moxon beam. Both designs were easy to construct and both preform well with minimal parts and setup. After building both antennas I finally chose the Moxon antenna for its smaller size and directional gain. It could also be mounted vertically or horizontally making it suitable for both local FM repeaters and SSB/DX applications.
The Moxon Antenna
The Moxon antenna, developed by Les Moxon G6XN, is a derivative of the VK2ABQ square antenna developed by Fred Caton. It is basically a 2-element parasitic beam with the tips of the elements folded towards each other to form a rectangle about 30% smaller that the equivalent beam. However despite its smaller size, the Moxon has several advantages over the Yagi antenna. It greatly exceeds the Yagi in front-to-back ratio and it has a wider beam-width making it less critical when aiming. It also has almost equal gain to the Yagi.
As with the Yagi, the Moxon antenna can be mounted vertically or horizontally making it perfect for either FM or SSB/DX operation on 6m. It also has a 50 ohm feed point so you can connect your 50 Ohm coax directly to the antenna and the SWR curve is quite flat making it a good performer across the entire 6m band.
There are hundreds of websites dedicated to the construction of Moxon antennas. Some use an ‘X’ frame of fibreglass poles or a rectangular frame of PVC pipe with wire stretched around it while others use rigid aluminium tube. I chose the aluminium tube method.
All the parts for the Moxon are regular off-the-shelf items from Bunnings costing less than $35 (not including the coax and screws). The straight sections are built from 12mm aluminium tube while the corners are made from 10mm aluminium tube. The 10mm tube is able to slide inside the 12mm tube for adjustment of the final frequency. As it turned out, the calculated dimensions placed my antenna frequency just where I needed it and no further adjustment was required.
The boom is made from 20mm PVC pipe and is connected to the elements using two 20mm electrical conduit tee pieces. I attached the tee pieces to the main elements using rivets but stainless steel bolts and nuts will do just as well. The rest of the antenna is held together with 3mm self tapping screws.
The end-gaps between the driven and reflector elements are secured using two 300mm lengths of 13mm poly drip irrigation tube. The poly tube slides neatly over the 12mm aluminium tube and is secured with screws.
|Description||Qty||Bunnings Part No|
|12mm x 3m aluminium tube||2||I/N: 1079420|
|10mm x 1m aluminium tube||1||I/N: 1067742|
|20mm x 1m PVC pressure pipe||1||I/N: 4750047|
|Holman 13mm x 1m Drip Irrigation Tube||1||I/N: 3120562|
|20mm Inspection Tee Conduit||2||I/N: 4330800|
The dimensions for the antenna were calculated using the free Moxon Antenna Calculator program (available for download from here). I set my frequency to 52 MHz with a wire size of 12mm to represent the 12mm aluminium tubing I was using. The resulting calculation is shown in fig 2a below.
For the actual construction I simply rounded the numbers for convenience (since 0.5 mm has little affect at 6m) leaving me with the following dimensions:
Referring to Fig 2b above, the measurements shown in black are the overall assembled dimensions of the antenna (as measured between the centres of the tubing). The measurements in red show the actual size of each of the tubing sections.Cut the straight sections from 12mm tubing. The 12 mm tubing is available in 3m lengths and you should be able to cut one antenna from 2 x 3m lengths with a little left over.
For the corners, cut a 1m length of 10mm tube into four 250mm sections. Form each section into a right angle with a 50mm bend radius. This will leave two legs each about 100mm long. These legs will slide into the 12mm tube to allow adjustment during the final assembly.
As I didn’t have a suitable bending machine I fashioned a tool using a piece of wood and some heavy gauge wood screws which I used to manually bend the aluminium to shape.
Preparing the Aluminium
Once the aluminium parts are cut, sand the 10mm tube corner legs and remove the burs on the ends of the 12mm tube until they slide freely together. Depending on the tolerances of the aluminium stock you may need to use a 10mm drill to expand the inside dimensions of the 12mm tube slightly. When trial fitting, allow the aluminium to cool first and if they are a bit tight don’t let them jam as they can be difficult to separate when this happens (trust me).
Preparing the Tee Pieces
The two tee pieces form the main mechanical structure for the antenna. Unscrew and remove the access plates from the tee pieces and put them aside for later. The inside of the tee pieces have a circular ridge which needs to be removed with a hobby knife or file so the elements can sit flat along the inside-back of the tee piece.
Measure along the length of each tee piece and draw a mark inside to identify the centre point. Using the photo below as a reference, drill four holes through the back of each tee piece. The hole diameters should suit the rivets, screws or bolts you are intending to use. The hole positions don’t need to be exactly positioned as long as they are spread over the approximate positions shown in the photo below. Once drilled, these will be used to drill matching holes into the associated aluminium elements.
Measure along the length of the 1955mm reflector element and mark the centre point. Slide one of the tee pieces along the reflector element until the centre of the element aligns with the centre of the tee piece and clamp the two together. Use the pre-drilled holes in the tee piece as a template to drill four holes into the aluminium. Secure the reflector element to the inside-back of the tee piece using rivets or bolts.
Position the ends of the two 995mm driven elements inside the other tee piece so the ends are spaced 6mm apart and centred in the tee piece. Drill and fix the elements to the inside-back of the tee piece with rivets or bolts as per the reflector element.
When placed side-by-side the driven and reflector element assemblies should both be the same length.
Once all the parts have been prepared, lay them out on a flat surface and loosely assemble the antenna to make sure everything fits together.
The open ends of the tee pieces should point inwards towards each other.
Slide the four corner pieces into the ends of the reflector and driven element assemblies. Slide the two 274mm lengths onto the corner pieces attached to the driven elements and the two 398mm lengths onto the corner pieces attached to the reflector element.
Adjust the assembly by sliding the elements along the corner pieces until the antenna matches the dimensions shown in black in the mechanical drawing. The dimensions should be measured from the centre of the tubes. Ensure there is a 98mm gap separating the ends of the folded elements on each side of the antenna.
Lay the two 300mm lengths of irrigation tube alongside these gaps and roughly centre them so that an equal amount of aluminium tube will extend inside each end of the poly tub when assembled. Mark the locations of the ends of the irrigation tube on the aluminium, ensuring that the spacing between the elements is still 98mm. Now slide the aluminium elements inside the poly tube up to the marks and temporarily secure them with cable ties (it may help to cut small slits in the ends of the irrigation tube to allow the cable ties to tighten more securely).
Readjust the antenna assembly as necessary to make sure the outer dimensions are still correct and the ends of the irrigation tubes still align with the marks on the aluminium tube. Once all is OK, temporarily secure all joints with tape to hold them steady then drill and secure everything using 3mm self taping screws. Secure all the aluminium parts first then secure the irrigation tubes as shown below, being sure to maintain the 98mm gap. When the assembly is complete, re-measure the outside dimensions to confirm nothing moved then remove the temporary tape.
Mounting Boom and Coax Connection
The mounting boom is made from a length of 20mm PVC pipe that links the two tee pieces together. To get the correct length for cutting the PVC pipe, lay the pipe alongside the tee pieces. Adjust the antenna so the space between the reflector and driven elements is 770mm at the tube centres then mark and cut the pipe to the desired length. Cut a 15mm long slit in each end of the pipe to allow it to spread over the tee pieces. Drill a 7mm hole about 300mm from one end to allow the coax cable to exit the boom (the hole position is offset from the centre of the boom to allow clearance for the mounting hardware). Elongate the hole to minimise stress to the coax.
Fit the boom to the antenna by sliding the ends of the pipe over the tee pieces. The coax exit-hole should be closest to the driven element. Confirm a spacing of 770mm between the driven and reflector elements then drill and secure the boom with self tapping screws.
Insert the coax through the hole in the boom and feed it towards the driven element. Remove 15mm of the outer casing and attach solder lugs to the braid and centre of the coax using solder. Drill a hole into each element near the end and secure the lugs with 3mm self tapping screws as shown. Waterproof the connections with silicon.
Before continuing I suggest you mark the element that connects to the centre of the coax, with some tape or a cable tie. This will make it easier to identify the active element from the ground element once the tee piece covers are installed – important if you plan to vertically mount the antenna.
You can now refit the two tee piece covers
As you can see from the sweep, my antenna was centered around 53 MHz with an SWR of less that 1.1:1. This was slightly higher in frequency than I had planned for, but since I’m initially targeting the local FM repeater frequencies in the 53 MHz range I’ve left it where it is for the moment.
If your preference is for SSB/CW/DX applications at the lower end of the 6m band, the sliding-tube construction makes it easy to adjust the dimensions for about 51 MHz.
For full coverage of the 6m band, adjusting the antenna for 52 MHz should result in an SWR of better than 1.6:1 across the entire 6m band.
Based on the practical results from my antenna build, the following dimensions should allow you to re-adjust your antenna to suit your application.
|Dimension||SSB (51 MHz)||Full Band (52 MHz)||FM (53 MHz)|
|A||2155 mm||2113 mm||2072 mm|
|B||285 mm||280 mm||274 mm|
|C||101 mm||99 mm||98 mm|
|D||414 mm||406 mm||398 mm|
|E||800 mm||785 mm||770 mm|
Moxon Antenna calculator program – Download Moxon Antenna calculator as a Zip file