By Mike, G3JVL
The G3JVL 6M ground plane vertical is a compact antenna that is ideal for
portable operations. It packs away into a small bag only 1.3 metres long which
is an ideal size for hand-baggage on aircraft.
Although a little
engineering work is required, it well within the abilities of the home workshop
and all you newly licensed amateurs!
The vertical itself is constructed
out of four overlapping sections of aluminium tube whose sizes are given in the
diagram opposite. The four tubes are meant to telescope so wall thickness should
be chosen to achieve this. In practice, 16swg or 1.6mm might be OK but 18swg or
1.2mm will easily fit. The lengths are held in place by three stainless steel
self-tapping screws.
The vertical is bottom-loaded with a coil wound on
an insulated former (nylon or similar material - it is not too critical at
50MHz). - the former is 130mm long with the top 20mm turned so it can be
inserted into the bottom section of the vertical. The loading coil consists of
ten turns of 16swg (1.6mm) diameter copper wire with one end connected to the
bottom section of the vertical with a machine screw and the other end connected
to the ground plate. A tap at 5-turns is connected to the input PL-259
socket.
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The construction of the 6m vertical antenna |
The four ground plane elements are Constructed from 1250mm long, 9.5mm (3/8")
diameter aluminium tube. These are mounted to the base plate by the use of eight
stainless steel machine screws. This arrangement allows easy
disassembly.
The vertical itself is supported by a piece of 350mm by 50mm
x 4mm aluminium plate bent into a 'U' shape. The bottom section of the vertical
is insulated by a turned piece of nylon or similar tubing. If you do not have
access to a lathe any other insulating arrangement should suffice so long as it
is robust.
The overall length of the vertical is 3950mm including the
loading coil. The top section should be trimmed to set the centre frequency. If
the VSWR at resonance is not close to 1:1 then alter the position of the tap on
the loading coil (remember, changing this will alter the resonance of the
antenna!).
Once completed give the whole assembly several coats of
varnish to keep out the weather.
