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Copyright © Austin
Hellier 12/02/2001
Diagram 5. Shown above are the Aerial and Earth wire arrangements.
AERIAL WIRES
A good Aerial wire should be as long and as high as possible. In
reality though, there will often be constraints on what you can
do. Where I live, in Shellharbour, NSW, I'm about 5 kilometres from
the nearest radio transmitters, and an Aerial of around 5 metres
in length brings several local AM radio stations in at quite good
volume. If I increase the height and length of my Aerial, i get
stronger signals, (this is called good 'sensitivity') but then the
radio stations start to 'run into each other'. This is because of
a factor, known as 'selectivity'.
Simple radio receivers like the Crystal Set have problems distinguishing
one radio station
from another with good signal strength. More complex tuning circuits
can be designed and built in order to compensate for this, but the
simplest solution is to have a shorter Aerial wire. Short Aerials
act in conjunction with the Earth wire system to form a small capacitor
of some 200pF in value, and together they form an 'Earth Return'
path fro signals from the desired radio transmitter.
You can anchor the Aerial wire on the side of the house, a tree
or the roof of a garden shed, or erect it between two long poles,
as shown above, by using plastic "egg" insulators. These
are obtainable at most electronics stores for a few dollars.
WARNING: Never erect an Aerial wire next to or near to power
lines or telephone cables, as these services carry high voltages
and currents, and any contact with them may result in fatal injuries.
EARTH WIRES
The safest form of Earth wire is one that is connected to a cold
water pipe or tap.
WARNING: Never connect Earth wires to hot water pipes, gas pipes,
or 240 volt earth connections that terminate in a separate Earth
pipe imbedded in the ground next to your house. Any of these arrangements
could cause shocks or fatal explosions, and are to be avoided at
all costs.
FINAL WARNING- Completely disconnect the Aerial this receiver
during thunderstorm, as lightning strikes may occur on your Aerial
wire.
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