Chapter 8 

 TRANSMISSION EXPERIMENTS 



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BRITISH EXPERIMENTS 



In 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. 3,10 



The 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 rectified 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 are permanently 

 mounted inside the station buildings behind large 

 canvas-covered "windows." 



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 

 E and F, for the long path, in Scotland at two heights. 

 In units of the geometrical horizon distance the 

 lengths of the various transmission paths are as 

 follows. 



AC BC AD BD AE AF BE BF 

 0.89 1.21 1.40 2.40 3.82 4.92 5.63 8.45 



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 7. 



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. 



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