Short Wave Aerials - Eddystone Short Wave Manual

12
EDDYSTONE SHORT WAVE MANUAL
SHORT WAVE AERIALS
An -efficient short wave aerial should have
the following characteristics :-
(a)
Good pick-up
(b)
High signal to noise ratio.
(c)
Resonate on certain
desired fre-
quencies and be semi-aperiodic on
other frequencies.
(d)
Its impedance must be matched to
the input impedance of the receiver.
PICK-UP.
The first condition
is
easily attainable
provided high conductivity copper wire
is
used in the installation and the aerial is erected
in a position where dielectric losses are at a
minimum,
well away from buildings and
trees and particularly metal objects, such as
drainpipes; gutters, metal roofs, and telephone
or power lines. Since the current induced in
an aerial: is- directly
proportional to the
effective height of the latter it is essential to
erect the aerial
as
high as - circumstances
permit.
HIGH SIGNAL/NOISE RATIO.
The signal to noise ratio is one of the most
important factors to be considered in
the
design of an aerial. Due to thermal agitation,
shot
and Johnson noise, there is always a
considerable amount of noise present in
a
radio receiver, and it is a problem to reduce
this to a minimum-.
It is in the first stage
that these effects are troublesome since the
noise level developed is amplified by each
succeeding valve. Therefore, no radio signal
of less intensity than this noise level will be
reproduced in the loud speaker and if
the
strength of the weak signals can be increased
before they reach the receiver input then many
more stations will be heard.
In practice the strength of the weak signals
is increased by the use of resonant aerials,
and the, man-made static present in densely
populated areas is reduced by using transposed
lead-in wires.
Figure 1 shows the detector input of a
radio receiver during a period of short wave
listening.
The set is being used in a densely
populated area where the interference noise
is high, approximately 30 db, above the noise
level of the receiver. A weak station giving
20 db input is therefore not heard in the
speaker since it is 10 db below the level of
the interference.
so
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To
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TIME
FIG.1
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By using a resonant aerial adjusted to the
station being received, the strength of the
weak station may be increased to say 40 db as
shown in
Figure
2.
A crossfeeder lead-in
system will reduce both the intensity and level
of the man-made interference to 20 db. It
will be seen that the radio signal is now heard
clearly above the interference level which
previously drowned the weak signal.
RESONANT AERIALS.
The use of resonant aerials on short waves
is a practical proposition since the wavelengths
are small and aerials whose physical dimensions
are f, or a full wavelength long, do not
occupy too much space. For example,
a
half -wave doublet tuned to 30 metres only
requires a top span of 45 feet.
IMPEDANCE MATCH.
If an aerial of 400 ohms impedance is
connected to a feeder line of equal value, no
voltages will be impressed in the feeders. The
maximum output
from any generator
is
obtained when the impedance of the load
equals the internal impedance of the generator.
If an inverted " V " aerial has a terminating
impedance of 400 ohms, the feeder line should
have a corresponding value. Half -wave aerials
have an extremely high impedance when
measured across their ends, but by feeding the
line near the centre the impedance is reduced
to a few
hundred
ohms and maximum
transference of energy will take place.
The
sets described in this Manual are so arranged
that
the aerials
mentioned here
can
be
successful ly used.