Eddystone Short Wave Manual page 15

.EDDYSTONE SHORT WAVE ',MANUAL
13
SHORT WAVE AERIALS-continued
TYPES OF AERIALS.
There are two types of short wave aerials :
(1)
Non -resonant.
(2)
Resonant.
In the4lrst category there is the conventional
Inverted
.L and T aerials, which are quite
efficient, particularly in districts where little
interference is present. Small space is required
for their erection.
Where plenty of ground space is available
and interference is negligible, the " Inverted
V " aerial is suggested. This aerial has to be
resonant on the desired
wavelengths if
maximum efficiency is dpsired.
For town and general use the Crossfeeder
type
of
aerial with transposed lead-in
is
recommended since not only is an improved
short wave performance obtained, but the
interference due to man-made static is reduced,
as Figures 1 and 2 show.
This short discussion on aerials will help
the reader to decide which aerial best suits his
own conditions and the above types of aerials
will be dealt with in detail.
1.
" INVERTED L " OR " T " AERIAL.
This type of non -resonant aerial for general
short wave reception is made about 60 feet
long from the free end of the aerial to the set.
The down -lead is kept well away from buildings
and not allowed to run close to the wall of
the house.
In the case of the " T " aerial
the down -lead should be
taken from the
centre and soldered.
The best
method
of obtaining the down -lead for the " Inverted
L "
aerial
is
to continue the horizontal
portion
by securely
twisting it at the
insulator and so avoiding the necessity of
making a soldered joint.
2.
CROSSFEEDER DOUBLET AERIAL.
The Eddystone Crossfeeder Doublet is a
static -free, large signal to noise ratio aerial.
Basically, it is a form of Hertz aerial, and as
such, the two top sections can be cut to a
definite length to give maximum response at
certain frequencies.
The top section is cut
to the length corresponding to a half -wave-
length of the station which it is desired to
receive consistently.
To cut' the lengths A and B of Figure 4
for any given frequency or wavelength, their
combined length should be approximately
Figs. 4 and 4a.
one half of the wavelength of the desired
station.
The formula for working out the length
A -1-B for any given frequency is :
(A -}-B)
ft.
= 468,000
kc/s.
Remembering that
Frequency in
kc/s.
x Wavelength in
metres = 300,000,
it is easy to work out the length required for
any given wavelength.
For the convenience
of
readers not
wishing to work out these figures, the curve
of Graph No. 2 has been prepared. An aerial
designed for a frequency of 4,000 kc/s. (75
metres) will also resonate at its second har-
monic 8,000 kc/s.
(37.5 metres) and at
its
third harmonic on 16,000 kc/s. (18.75 metres).
Graph No. 2.