A UHF corner reflector aerial
Some details
Expressed quite simply, the aerial consists of a 
dipole (where λ is the standard symbol for wavelength, making a ‘
dipole’ a half-wave dipole).
Nothing new in that, you might say. However, the interesting feature is the reflector, which is not the usual single element, but a 90° metal ‘corner’, acting rather like a parabolic dish as used for satellite signal reception. The wind resistance of this type of reflector makes it impractical so, to reduce the ‘windage’ quite significantly, we make the ‘corner’ from closely spaced rods, as illustrated in Figure 1.
The reflector consists of a number of 0.6λ rods, spaced from each other by 0.1λ. The aerial frame can be made of metal or wood, but wood is easier to work with, and mounting the elements to the frame is simpler. The prototype was made with wood of 20 mm by 15 mm cross-section, as Figure 2 shows. The wood was varnished for protection. The elements were made from 1.5 mm diameter copper wire, because a large reel of the wire happened to be available. The wire diameter is not critical; tubing could be used just as successfully. 14 SWG hard-drawn copper aerial wire would be even better than that used in the prototype.
Construction
This project is just as much a woodworking project as a radio project! Follow the instructions carefully, and you should have little trouble.
• Cut the booms for the reflectors, as shown in Figure 2. A mitre block is invaluable here in producing the 45° corners.
• Using the dimensions given on the diagram, mark the hole positions for the reflector elements, and then drill holes of a size which holds the elements firmly.
• Cut the driven element boom according to the diagram, and mark the point midway along the longer side, which will assist you later in positioning the driven element.
• Cut the back plate to size (about 120 mm by 80 mm). You may need to alter this size depending on the size of the U-bolt you will be using to clamp the aerial to the mast.
• If you want to be extra cautious in your construction, use the belt-and-braces approach, commonly known as ‘screw-and-glue’ to fix the booms to each other and to the back plate.
• Fix the reflector booms to the back plate first, then slide in the driven-element boom until it will go no further, then apply the wood glue and screw the two ends tightly to the reflector booms. Leave for the period prescribed by the glue manufacturers for the glue to harden.
• Varnish the whole structure.
• Cut the driven element to the correct size plus a couple of centimetres (the reason for this will be evident in the Testing section), and fix it to the centre of its boom (at the position you marked earlier) with a ‘chocolate block’ connector to which the coaxial feeder cable will eventually be connected.
• Cut and fix the reflector elements in place. If you find that these are a loose fit in the holes then, for each element, drill a pilot hole through the boom to intersect the hole for the element. File off the point of a woodscrew, and screw it gently into the pilot hole until it meets the element and grips it in place. You will now see why the point was filed off! Alternatively, you can glue the elements in place.
Testing
Place the aerial on a mast, clear of obstructions. Connect it to a transceiver with a length of coaxial cable, with a standing-wave-ratio (SWR) meter in circuit. Find a clear frequency, identify your transmission and ask if the frequency really is clear. If so, key the transmitter again and note the SWR. Do not stand in front of any aerial when it is radiating! The length of the driven element must be adjusted to obtain an SWR of less than 2. If you have to shorten the dipole, bend the ends over rather than cut them off. That way, if you go too far, your can lengthen them again! The dipole was initially cut too long intentionally, to allow for adjustment here! Bending the ends over also reduces the risk of physical damage to clothing, skin and eyes. You may like to consider applying the same technique to the reflector elements for that reason alone.
Moving on …
Once you have warmed to the idea of the corner reflector as an aerial, you might like to ring the changes regarding the reflector. How about replacing the 20 reflector elements with a wire mesh, such as garden centres sell as ‘chicken wire’? Choose the finest mesh if there is a choice. Some extra support may be needed around the edges of the mesh, but you could go on to make a comparison of aerial gain between the two types, using the UHF Field Strength Meter described elsewhere in this book.