Eddystone Short Wave Manual page 16

14
EDDYSTONE SHORT WAVE MANUAL
SHORT WAVE AERIALS-continued
The length of. feeder_ lint_from the receiver
to the aerial should be not, less than .a quarter
of a wavelength, i.e., greater. than
(A+ B).
The inset, Fig. 4(a), shows the directional
property of the doublet, the aerial receiving
the most energy from a direction at right
angles to its own plane.
3.
" INVERTED V " AERIAL.
When used on wavelengths above 25
metres the physical dimensions of this aerial
become large and it has, therefore, become
popular on lower wavelengths.
It has the
advantage of being semi -resonant over a wide
waveband either side of the resonant frequency.
For example, an aerial designed for 15 metres
will give five times the performance of an
ordinary aerial- on this wavelength, on 20
metres
a gain of
three times,
falling
off
gradually until about 40 metres, when no gain
is -Obtained over the stand-S.tcratrial systems.
Figure
3
illustrates the aerial, which
consists of a wooden pole (bd) with the aerial
wires (ab) and (bc) forming two sides of an
ov
55
50
45
40
35.
CIS
16 17 18
19
20
2.1
22.
23
24
21
.-----j-
-
;
....0,----------------------
1.----------
I
B
)15
16
17 18
19
20
21 22.
23
24
2'
130
120
g no
100
t 90
ao
70
C
15
16
17
IS
19
20
21
22 23
4
21
WAVELENGTH IN METRES
Graph No. 1.
FIG. 3
.A1114
ciiiNM
A
111111F:4i
isosceles triangle (abc).
The length (abc) is one
continuous wire and the aerial is directional,
receiving signals which are arriving parallel
to the line joining the feeder and aerial in the
direction nearest the matching resistance R.
Resistance R should be about 400 ohms and
of
the non -inductive type.
A metallised
composition resistance is perfectly satisfactory
but should be enclosed in a glass tube or
waterproof box since it will be exposed to all
kinds of weather. The earth lead should he
fitted to the earth terminal of the receiver and
the far end of the feeder line also earthed to the
nearest point. Eddystone transposition blocks,
Cat. No. 1041, may be used. 16 gauge wire is
advised for the feeder line.
The length of wire (abc) should equal the
length ac plus 1 wavelength, i.e.:
abc = ac+ A
It is not advisable to make ab a wavelength
long as maximum signal to noise ratio occurs
when ab or bc are an odd number of quarter
wavelengths long. Graph 1 shows the correct
length of the aerial wires for ab=bc
=::A as
this enables the installation to be erected in
a reasonably small space.
MAXIMUM
REcernoN
A simple all wave aerial, which can be
coupled to the receiver without any form of
matching transformer,
is shown in
Fig.
5.
The dimensions given are for general short
wave use.
Both the " Inverted V " and Doublet
aerials are coupled magnetically to the receiver
by means of the coupling coil provided in
each range, and the earth terminal of the
receiver taken to earth proper. The lead-in
must be duplicated in order .to connect both
the feeder lines to the receiver.