I needed an aerial to cover 10, 12 and 15m and one that would fit in a confined space. I considered a shortened LPY, hexbeam, Telerana and the quad. I chose the latter because of its small turning radius and relatively easy and cheap construction.
I turned to the NZART’s e-mail-based reflector for advice on all manner of related issues, from design, to matching and construction materials… and this is what I came up with.
For design measurements, I used the Cubical Quad Antenna Calculator, which is available at a number of web sites. This is simply a matter of entering the centre freqs and off you go! I selected 21.225, 24.95 and 28.25MHz.
For matching I used the 1/4 wave of 75 Ohm coax, simply a 1/4 wave length multiplied by the velocity factor. The coaxes are joined with PL259 plugs and connectors, then thru a coax balun (4 turns on a 100mm former, close-spaced) then into an Ameritron switch.
For spreaders, I opted for cheap, fibreglass fishing rod blanks, available from Kilwell NZ. They cost $10 each plus freight. Tell the good folk at Kilwell that you want them for a quad antenna, and they’ll know exactly what you’re talking about! The spreaders arrive “raw” and need painted. To appease “her indoors” as much as possible, the spreaders were spray-painted matt green -“drab olive” to give its correct title.
For the boom, I re-used 2.0m of 32mm aluminium pipe off an abandoned trap tribander beam, and used the 8-metre 50mm aluminium mast from the same to hoist the quad.
The boom was fixed to the mast with muffler clamps through an aluminium plate.
For wire, I dived into the junk box and came up with some 1.0mm insulated copper-coated steel. This may be a bit thin as the bandwidth is a tad narrow. In retrospect, I’d go with 2.5mm conduit wire.
As a rotator wasn’t initally in the plans, I attached a housed thrust bearing below the lowest wire to attach guys and provide an rotation point, this allows for the Armstrong rotor method. Guys are 6mm polynylon rope.
After assembling the quad spreaders etc, I used the quad calculator formula to mark where the wires should overlap, taking the measurement on the half-diagonal from the centre of the spredaer supports.
The reflector wires were attached over the measurement marks with cable ties, then bound with self-amalgamting tape, then a single wrap of insulation tape just to hold the self-amalgamating tape till it did its thing. I tied one wire end to one spreader and worked around the other spreaders, joining both ends with a jumper soldered across the wrapped wires.
To prevent the spreaders moving as they were threaded, I placed a long bolt against each of them.
Self-amalgamating tape is available from electrical wholesalers, usually a Scotch 3M product in 19mm or 25mm rolls. And while it may seem expensive, you’ll soon find a little goes a long was as you stretch it into place.
Knowing that the driven element would need some adjustment, I also held them with cable ties, but didn’t pull them firmly home, then a couple of layers of black insulating tape to keep everyhting in place. The wires finsihed in the centre of a span, separated by plastic insulators supplied by me old mate Bill, W0WOI, a similar electric fence insulator is available at stock agents.
I allowed for the possibility of lengthening the wires by leaving tails (as you’ll see above), the ends of which were tacked back onto the driven elements to avoid upsetting tuning. As it was, the tails were only needed on the 12m loop as the other two needed shortened.
I didn’t have a tilt-over tower or the likes to make adjustment easy… rather a temporary 4m pole at tuning height, and two sons! I don’t know how many times we lowered and raised the beastie, eventually we got 10 and 15m to work well across the bands.
At this point, I must confess that the12m loop was a real problem-child. I couldn’t get it below an SWR of 2:1 without it affecting the other two loops. Just too much mutual coupling so I decided to dispense with the 12m loop. There ain’t much DX up there at this part of the solar-cycle anyway…
OK… so how well did the quad work? Very well… my only other aerial for 10 and 15m is a 12.2m vertical (a 40-footer) with an SGC coupler. Without exception, the 2 element quad runs rings around the vertical. Average signal improvement is 3 – 5 S points, i.e. 20 – 30dB!
Acknowledgements; Thanks to all the guys on the NZDXR reflector for their suggestions, many of which were implemented, and to Morrie ZL2AAA and Lee ZL2AL for sending copies of useful articles for perusal.