Antenna
of the Month May
A
21mhz rotatable dipole.
Since
the HF bands are becoming very lively now in particular 21mhz, a dedicated
antenna for that band would be very desirable, and if it were
rotatable
even better. Such an antenna is described below.
Parts
list:
2
pieces 10 feet 1/2 inch aluminium tubing
1
SO239 coaxial socket
a
small length of 1/8th inch copper tubing for the inductor.
4
stand off insulators for the antenna mount.
2"x2"
4 foot long.
To
construct the antenna see accompanying picture this is very simple in its form.
Take the 2 10' lengths of tubing and flatten one end on each about 1 1/2"
long. This is to mount the coil. Drill a 5/8" hole in one of the flattened
ends ( to take the SO239 socket, and mount the SO239. Drill a hole in the other
piece of 10' tubing , to take the other end of the coil.
Using
the stand off insulators mount the two lengths of tubing on the length of 2x2,
leaving a gap in the middle to mount the coil .
The
coil, wind 5 turns of coil on a 1" inside diameter former leaving
sufficient length to fit to the coax plug and the the other element.
Solder
the coil to the SO239 and the other end to a solder tag and secure in to the
antenna element.
Antenna
adjustment, using either a GDO (dip meter) or an SWR analyser, adjust the coil
by squeezing or opening the coils, to the centre of your frequency of operation.
After
adjustment check for lowest possible SWR and hoist the antenna on a suitable
mast into the air, having made sure that the coaxial plug and socket are
waterproofed.
There
you have it a rotatable dipole for 21 Mhz
and excellent DX band, you can rotate the dipole in any direction, remembering
that the signal will radiate like a wire antenna in that its signal is strongest
broadside on..
Have fun.
with thanks to the ARRL Antenna Hand book 1964.