The aerial element and the coil are made from a single piece of 1.5mm welding (brazing) rod, and the dimensions are given in Figure . Wind the coil around a 4mm rod or the shank of a twist drill. The lower end is filed to a point and then soldered into the centre conductor of a 4-hole panel- mounting BNC socket. (Try to obtain a good-quality BNC socket with
PTFE insulation – the insulation of cheaper sockets is easily damaged.) Trim the element to 427mm (top of element to top of coil) after the wire has been soldered to the socket.
The base coil causes the aerial to be rather ‘whippy’, so a piece of 5mm plastic knitting needle can be cut to the length of the coil and then forced into it. The radials are made from four lengths of 3mm welding rod. These are bent and soldered into the four mounting holes of the socket, and then cut to the lengths shown in Figure .
The aerial is made waterproof by enclosing it in a 22mm diameter PVC waste water pipe. It is ‘weldable’, and available at plumbers’ merchants, usually by the metre. A coupler is slotted to take the radials (Figure ). Filethe BNC socket as required, so that it slides inside the coupler until the radials poke out of the slots. Cut a length of plastic tubing which is 30mm longer than the aerial, and push it into the coupler. You will then need a plastic bung or screw-top to waterproof the top end.
If you end feed this antenna as shown you are only base loading a 1/4 wave 2m antenna effectually making the antenna a 3/4 wave 70cm antenna. While this wile work and you will get a good SWR and improved signal over the 1/4 wave design, the design can be significantly improved. Connect the end of the coil to the ground and tap the center of the antenna feed to the second or third winding of the coil until the SWR is low. This effectually shunts the coil and turns the antenna into a 5/8 wave antenna. Now you will have a significantly improved radiation pattern.
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