A mate of mine, Denis ZL4DB, works in some wonderful places. He has been to Macquarie twice and is very likely to be working from another sought-after entity or IOTA at some time. Therefore I built him a 6 meter band Moxon portable to take with him in the hope that the band opens up and he can offer a “new one” to the deserving.
The first thing to do is to visit the Moxon Antenna Project website and download the Moxgen program to obtain the necessary dimensions. Below is the results based on a Moxon for 50.110MHz using 2.5mmsq wire.
I used old bits I had laying around. But any of the following can be replaced with what ever you have available to do the job. Only the dimensions are critical.
10 x #8 12mm self-tappers
2 x 25mm galvanised half-saddles
2 x 800mm pieces of 25mm PVC conduit “inners”
4 x 800mm pieces of 20mm PVC conduit “outers”
1 x 20x20mm aluminium square tube
4 x 1.5m lengths of 2mm fishing braid or strong UV resistant cord
4 x 4mm tinned copper crimp sleeves
200mm x heavy nylon/weedeater line
90mm dia x 120mm PVC pipe
50 Ohm coax cable
4 x 200mm black UV resistant cable ties
1 x Vee-block clamp assembly
1 x roll insulation tape
4 x pegs (see text)
1. Cut the wires as per Moxgen as exactly as you can; a single piece (dimensions A + D + D) for the reflector and two short pieces (dimensions A + B) which make the driven element.
2. Solder the driven element wires directly onto the end of the coax, one on the centre conductor and one on the screen and tape them up well.
3. Prepare the driven element inner by marking the exact half way point (i.e. 400mm from the ends) and drill a 6mm hole 15mm either side (30mm apart). Slide the wires/coax assembly into the holes and out thru the ends.Mark the halfway point on the reflector element inner and thread the reflector wire through it leaving approx half hanging out each end.
4. Prepare the 4 x outers. One end will be the tip through which the wire will pass. The other end will slide into the inner. Drill a 4mm hole through one wall on the tip to which the braid will attach later.
5. Prepare the boom. Mark the exact centre of the boom and fit the V-block clamp.
6. Determine the exact spacing between the two elements (dimension E), using the boom centre mark as a guide. Drill a pilot hole two of the self-tappers about 15mm from the ends of the boom.
7. Place a conduit saddle over the exact centre of the inners and line up the inside edge of the conduits with your element spacing marks. Drill pilot holes and attach the saddles/conduits to the boom. Make sure that on the driven element inner, you place the coax feed point in between the boom and the saddle as per the photo below.
8. Use the Moxgen design to calculate how much of the outer should protrude out the ends of the inner (dimension A – 800mm divided by 2). Thread the wires through the outers and slide the latter about inside the inners to the correct length and as a temporary measure, tape the inner and outer together.
9. With the antenna on the ground, use pegs/rods/screwdrivers pushed into the ground to hold the outer tips in place. Then measure the distance between the pegs to match the Moxgen calcs. Adjust the position of the pegs to suit.
10. Crimp a 4mm sleeve halfway on to each of the wires, then use the Moxgen calculations (dimension C) and cut two nylon joiners to length and crimp them into the other half of the crimp sleeves.
11. Place the loops of the braids around the self tappers on the ends of the boom and thread through the hole in the outer tips. Make sure the arms are straight then pull tight and tie in place. This should take any slack out of the wires.
12. Wrap approx 6 turns of coax around the 90mm former as a coax choke to prevent common mode RFI from radiating on the coax shield. Use cable ties to hold the turns in place.
13. Once all adjustments are made, test the antenna for SWR. If all works satisfactorily, drill pilots and use 2 x self tappers in the end of each inner to hold the outers in place, and remove the temporary tape.