Coll 35/31 'Bahrain: application for W/T amateur transmitting licence for employee of Bahrain Petroleum Company' [‎47v] (94/250)

12
A GUIDE TO AMATEUR RADIO
20 metres) paths during the day, and C and D the
same paths at night.
There is for each frequency a limiting steepness
above which reflection is not possible, after 'which the
layer is penetrated, and so it will be seen that outside
the range of a score or so miles of the ground wave,
nothing can be received until such distance as the
reflected wave appears ; leaving a large area of
no signal.
Skip Distance
This area, which is annular in shape, is an area
of no signals and its width is called the " skip
distance.” The diagram shows why the skip is
greater at night than during the day, and why it is
greater during both periods for the higher frequency.
Weak signals may occur inside the skip distance
but these are due to a certain amount of scattering
at the place of reflection and are not true reflected
signals. It is often impossible to receive British
amateurs on the 14 Me band in Great Britain,
except when inside their ground-wave range, but
their signals are coming down in America, Asia or
even further afield.
On wavelengths greater than about 200 metres,
reflection is always possible at any angle and so the
skip area does not exist, although the change-over
from the Heaviside to the Appleton layer during
night is evident. On medium waves used for
broadcasting (200-600 metres), the more distant
European stations are only weak in this country as
a number of hops are involved to cover the distance,
with considerable loss at each earth or sky reflection.
After sunset, when the Heaviside layer gives place
to the Appleton layer greater strides are made and
the distant stations are more easily received.
As the wavelength is shortened or the frequency
increased the skip becomes greater until at some
limiting frequency the wave is not sufficiently bent
to follow round the curvature of the earth, and long
distance propagation is no longer possible. This
limit was until recently about 25 Me (12 metres),
but in the last two years, due to increased activity
on the part of the sun, the limit has moved above
30 Me and long distance working on the 28 Me
amateur band has become good at times. The
upper frequency limit shows great variation with
changes in the reflecting layers and offers an easy
method for the study of short-wave transmission.
The wavelength of 7 metres, chosen for Television
work, is normally beyond the upper limit and can
only be received in the range of the ground wave,
though authentic cases have occurred where
reflection apparently became possible for a while
and 7 or even 5 metre signals have been heard
over distances of several thousand miles. The 28 and
56 Me bands are now occupying the attention of
advanced amateur workers, though whether long
distance communication will ever become regular
on the latter frequency remains to be seen. It is
quite to be expected that in the years to come,
with decreasing solar activity, the upper limit will
recede to lower frequencies. The general variation
seems to follow the known eleven-year cycle of the
sun, but it will be necessary to watch over many
years before the general trend can be established.
The daytime signals on 7 and 14 Me are
usually not very strong because these are never
completely reflected, part of their energy passing
through the lower layer to be reflected by the upper.
as indicated by the dotted track E in Fig. 5.
Because of this loss, and the number of hops that
would be involved, the daytime range is not very
great. At night, reflection from the more intense
upper layer, and the longer stride, allow much
greater distances to be covered and world-wide
range is possible.
Long distance propagation takes place by means
of the upper layer, and the highest frequency tracks
appear to use this layer only. In the daytime the
waves must pass through the lower layer and suffer
considerable loss. In commercial communication
high power is used, but in amateur work effective
long-distance communication is only possible at
hours which favour the required direction.
Fading
It is usually possible to find more than one path
by which a signal can travel from one place to
another, and these paths differ in length, so that it is
possible for the two positive crests of the w-aves
to arrive together, or alternatively one positive
crest can arrive by one path and a negative crest
by the other. In the former case they will add to
strengthen the signal, whilst in the latter they will
oppose and weaken it. Now the reflecting layers
are seldom perfectly steady, and so as they move the
two or more waves can move in and out of step,
causing undulations in the strength of signal
received. This effect is known as ” fading,” and
sometimes a fade from maximum to minimum can
occupy several minutes, whilst at others, and
particularly on higher frequencies, the fading cycle
can be so rapid as to sound like an audio frequency
modulation.
Naturally the effect is very pronounced when some
part of the track from transmitter to receiver is in
the shadow edge of sunset or sunrise, involving
transition of the reflections from one layer to
another.
Amateur Bands
Amateurs are permitted by international agree
ment to transmit continuous wave telegraphy and
telephony in small channels of frequency which are
usually referred to as the 1 • 7 Me, 3 • 5 Me, 7 Me,
14 Me, 28 Me and 56 Me bands.
The actual frequencies are set out in detail in a
later chapter, but it should be mentioned here that
the 1 -7 Me band is just below the medium broadcast
band, and is useful for British and European
working. The 3-5 Me band falls between 75 and
90 metres and besides offering a useful channel for
local working, it permits long distance communica
tion during the winter months.
The 7 Me band is used extensively for both short
and long distance contacts, but is losing some of
its popularity for DX (long distance) work, owing
to the enormous amount of interference from local
stations. The skip effect on this band frequently
permits the reception of European and long distance
stations simultaneously, which as a consequence
demands the use of a highly selective receiver in
order to eliminate the unwanted local (European)
signal if DX stations are to be contacted.
For all-round long distance work the 14 Me band
has no equal, but in common with all other amateur
bands the best time, or best period must be chosen
if reliable contacts are to be established. As a
general rule East Coast American amateurs can be
worked from Great Britain on this band at most