OBSERVATIONS FROM SCOTLAND

I needed a small aerial for 1298 and decided that a quagi might do the job. I've built quagis before, using K5OE's design, for 435Mhz so I'm not a stranger to them. I wanted some gain with a fairly wide main lobe so eventually settled on the following design.

element	length	spacing from reflector
reflector loop	240 (60mm per side)	0
driven loop	234.9 (58.725mm per side)	60.3
Director 1	99.3	111.1
Director 2	98.5	204.3
Director 3	98.0	254.1
Director 4	97.3	328.3
Director 5	96.5	402.5

All dimensions are in millimetres. On the top of the drawing are the element spacings, referenced to the Reflector. At the bottom are the lengths of the five director elements. The reflector and director element details are given below. The boom must be non-conducting ..of plastic or wood

At 1296 MHz, you need to measure and cut accurately... certainly better than 1mm if possible. The quagi was built on a length of plastic (PVC) electrical conduit. I measured and drilled the holes for the elements using a cheap electric vertical drilling machine. The easiest way to make the elements is to cut them slightly oversize then carefully file them to the finished length, checking with a vernier caliper set to the desired length, as you go.

Here's a tip. Drill the first hole in the plastic boom and push a piece of scrap element material through it. Take a small square of scrap wood and drill a hole in it and fit the element material into it. Space it the height of the base of the drilling machine and use it as a position guide to drill all the other element holes so they are dead square. Nothing looks worse to my eye than a DIY yagi or quagi with elements out of true!

Alternatively, rather than a PVC tube, you could use a strip of thick perspex as a boom. 6MM perspex will work as a boom material. PVC sink waste could be used for indoor use, but is probably too flimsy for outdoor use.

The reflector loop is a square formed from 3/32inch brass rod. Remember when bending the loop that the breaks are in the middle of the loop, and not at the edges. This makes soldering together the ends of the loop an easier job.

For the reflector, cut a length of rod to an exact 240mm and file or mark it off at....

30,90,150,210,mm. The material can be bent more easily if you file shallow notches at the points where it is to be bent.

Bend the marked parts to form a square. The bottom part should be formed by the two 30mm sections at each end of the wire. This forms a square of total length 240mm. The ends of the loop are butted together and soldered up. I wrapped a layer of thin copper foil over the joint after soldering it and tinned it to give it a little extra strength.

Both the reflector and driven elements are supported on pieces of 1cm diameter solid plastic (from plastic knitting-needles!) with notches filed in the ends. The holes in the boom for the reflector and driven element are drilled at 10mm diameter and the plastic rods glued into place. Once the glue has set, the reflector and director can be sprung into place in the notches and epoxied in place.

For the driven element, the same layout is employed but the wire should be cut to an overall length (exact) of 234.9mm and marked out as follows....

With the driven element, you should file the ends of the rod *before* bending it so it fits into the spill and fixing hole of the BNC you plan to use. Trust me, it will make life easier. Bend the rod into a square as with the reflector taking as much care as you can to be accurate. The actual finished shape of the reflector and director are not critical but they absolutely must be of the correct overall lengths.

The ends of the driven element are bent downwards at 90 degrees in such a way as to preserve the shape of the square and to connect to the BNC chassis socket with the socket facing downwards. When soldering the BNC socket onto the driven loop, the connection to the centre spill should be pushed fully home and soldered while the connection to the body of the connector with the rod pushed into the hole and soldered flush (no rod sticking out!)

The overall length of the driven element is important and the design relies on the ends being correctly fitted to the BNC socket. A normal small iron is inadequate for this job. Use a big iron or a small gas flame and work quickly.

The reflector loop and driven elements are supported on lengths of plastic 'dowel' that run through the boom in line with the other elements. for the 'dowels' I used material cut from number 10 (10mm) plastic knitting-needles. The ends are filed (slotted) to take the brass rods. These large diameter knitting-needles are available in many craft stores. Supporting the loop and driven element is probably the most difficult part of building quagis. It's a good idea to smear epoxy resin or some varnish over the plastic material where the elements attach since many plastics are hygroscopic.

Why use 3/32 diameter brass rod? Well it was available at a good price on EBay. The dimensions above are scaled to suit 3/32 rod. If you use a different diameter of rod, you are on your own!

The remaining elements are simply cut and filed to to the lengths required and pushed into the correct holes in the boom. The element lengths are 99.31, 98.55, 98.04 97.28 & 96.52mm. It's easiest to saw them a little bit oversize then carefully file to the desired length taking care to measure regularly. Accuracy pays off at these frequencies.

The elements taper towards the front of the aerial, just like a normal TV aerial. To lock the elements in place and avoid having wind noises, I wrapped one end of the boom with electrical tape and taped an old poly bag on the other. I injected some PU foam of the sort sold in cans in DIY stores for sealing large gaps. Once set... the foam is VERY sticky and messy.... I used a knife to cut off the excess foam that oozed out and unwrapped the electrical tape to leave a clean 'potted' assembly, with the waste PU neatly caught in the poly bag. The tape stops the foam adhering to teh plastic boom and leaves a clean and neat finish.

If you don't foam the interior of the boom, you should make sure the elements are secured with epoxy or similar and that the ends of the boom are plugged. If you leave the ends open, it will 'sing' in the wind and detune when it rains! Don't forget to drill a few holes on the underside for any water present to escape. Foam is a lot simpler. You may even wish to spray the finished aerial with water-repellant spray if you have any at home.

I smeared quick setting epoxy resin over the exposed back end of the BNC socket and the soldered joint on the reflector to help protect from oxidation and weather. A cotton bud can be used for this job.