Antenna
of the Month 4.
March 2003
The
20 metres Groundplane.
I
know from some M3 calls that they are having trouble making contact with that
elusive DX station on 20 metres. Most are using the G5RV either in full size or
half size version. The
problem is that the angle of radiation from the G5RV is no conducive to long
distance working with lower power levels, what is needed is an antenna that
gives a good angle of radiation which will give a better chance for that elusive
contact. Such an antenna is the groundplane, this is a vertical, which will not
take up much space.
All
that is required to construct the antenna is: -
An
old CB antenna which can be cut to the correct length or some aluminium tubing,
to make up the length of 16’6”.
52’
of stranded copper wire or enamelled copper antenna wire to
Construct
the radials.
1
piece of hardwood 8” X 4” X 1” to mount the antenna.
4
U bolts to fit both the antenna and the Mast.
SO
239 socket and scrap of aluminium plate to mount the socket .
A
15 to 20’ mast.
Firstly
remove the coil from the base of the CB antenna, then cut the antenna to
16’6” in length. Secure the
antenna to the Hardwood block (having painted the block with a suitable
preservative).
Fit
the mast to the block about two inches below the antenna section.
Fit
the SO239 between the mast and antenna, Solder a wire between the centre pin of
the socket and the antenna and secure the other end to the antenna with a
suitable solder tag and screw.
Next,
cut the 52’ of radial wire into three equal lengths of 17 feet and secure one
end of each to the aluminium plate holding the SO239 socket so that they can be
equally spaced around the mast. Next
fit a coaxial feeder of 52 ohms impedance with a PL59 plug to the socket and
waterproof this joint well.
Now
raise the antenna and mast to a vertical position and secure it well in the
ground with a stub mast or other arrangement.
Now take each radial away from the antenna at an angle of 45 degrees and
secure to a suitable ground post (these radials could be used as guys, if
sufficiently strong enough, but if you do, ensure that an insulator is placed at
the ground end of each).
Connect
the antenna to the Transmitter via an SWR bridge and check for resonance, this
should be about 14.1 mHz .
Why
not build one for 21 mHz, the formula is quite simple length = 234/f
for antenna, radial 239/f = length
Now try again for that elusive piece of DX .
With thanks to The ARRL Antenna Handbook, CQ Antenna Roundup,
Radcom and Practical wireless for the inspiration.