Chapter 4. 
TRANSMISSION EXPERIMENTS 
BRITISH EXPERIMENTS 
N THE DEVELOPMENT of short and microwave 
communication and radar, the British were first 
to make systematic transmission experiments on a 
large scale. A number of such experiments were 
carried out at wavelengths below 50 cm, beginning 
about 1936 with some transmission paths over land, 
some over sea; and experiments in the 10-cm band 
were undertaken in the early years of the war. These 
experiments will not be reported individually because 
the earlier results are reproduced and verified in the 
later and more elaborate trials. Instead, attention 
will be confined to two major experiments, one over 
the sea and one over land.**!° 
Tue Irish SEA EXPERIMENT 
This transmission experiment represents a 
cooperative enterprise undertaken jointly by the 
Radio Division of the National Physical Laboratory, 
the Telecommunications Research Establishment, 
Signal Research and Development Establishment, 
The Ministry of Supply, The Naval Meteorological 
Service, The Meteorological Office, and the General 
Electric Company, Ltd. One-way transmission with 
stationary apparatus was carried on in the winter 
of 1943 to 1944 and continued in operation until 
the end of the war. 
Practically all the transmission is over the sea at 
wavelengths of about 9, 6, and 3 cm. At each fre- 
quency the transmitted signal consists of square 
pulses, with equal on-off periods and a repetition 
frequency of 1,000. The 1,000 cycle component of the 
modulation is reetified in the receivers to operate 
the recording milliammeters, and provision is made 
for monitoring the transmitter power and the sensi- 
tivity of the receivers in terms of a suitable standard. 
Parabolic mirrors 48 in. in diameter are used for all 
transmitters and receivers and aie permanently 
mounted inside the station buildings behind large 
canvas-covered ‘‘witidows.”’ 
There are two transmission paths, 57 and 200 
miles in length, which run roughly from south to 
north, but diverge from each other by about 17 
degrees and have the transmitting station in common 
at the southern tip in South Wales. There are trans- 
mitting stations A and B at 540 and 90 ft above sea 
level respectively. The receivers, C and D, for the 
short path are in North Wales at two heights, and 
Eand F, for the long path, in Scotland at two heights! 
In units of the geometrical horizon distance the 
25 
lengths of the various transmission paths are as 
follows. 
AC BC AD BD AE AF BE BF 
0.89 1.21 
It has not been found possible to utilize all these 
paths at the same time, because the amount of 
records accumulated proved too great for evaluation, 
but selected runs at various frequencies and for 
several paths have been made. 
There is an elaborate setup for measuring meteoro- 
logical conditions simultaneously with the intensity 
of the transmitted signal. A weather station is 
located at each of the three terminals, but the main 
meteorological program is carried out from ships 
which ply along the transmission paths. The Admiralty 
has detailed three ships for the sole purpose of making 
these measurements so that the transmission path is 
continuously covered by at least one ship on duty. 
The ships are provided with elaborate meteorological 
equipment of the type described in Chapter 3. 
RESULTS 
The following is a qualitative summary of some 
of the results obtained thus far. 
1. There is general agreement between signal 
variations over the two paths, though the short 
period variations often differ. 
2. Signals are obtained over the long path only 
when the signal strength over the short path BD is 
high. But if the latter condition is fulfilled, the former 
does not always follow. 
3. There is a marked diurnal variation when the 
general signal level is low or moderate with strong 
signals in the late afternoon or evening and a 
minimum between 6 a.m. and 9 a.m. 
4. There is evidence of an appreciable seasonal 
variation with high level for a greater fraction of 
the time in summer than in winter or spring. 
5. Low level occurs commonly, but not always in 
conditions of fog or low visibility. 
6. Low signal level is usually observed at the 
passage of warm fronts and high level at the passage 
of cold fronts. 
7. Generally speaking, high signal level tends to 
occur in periods of anticyclonic weather. 
A typical record of signal strength for 9-em waves, 
representing hourly mean values for a month, is 
shown in Figure 1. These records are from two 
links of the short path, both nonoptical. Important 
meteorological phenomena, especially passage of 
