60 



TRANSMISSION EXPERIMENTS 



with clear skies, as will be explained in Chapter 9. 

 The work undertaken in England on experimental 

 transmission paths of various types is quite extensive, 

 and the preceding description hardly gives an idea 

 of the variety of experiments made and results 

 obtained. Most of the experiments are of a smaller 

 size than the ones described here. 



82 EXPERIMENTS AT THE EASTERN 

 COAST OF THE U.S. 



In the early years of the war a transmission 

 experiment was undertaken by RCA Communica- 

 tions, Inc., between New York and two points on 

 Long Island. 131,155 The short path of 42 miles was 

 optical, but the long path of 70 miles was nonoptical, 

 the receiver being about 400 ft below the trans- 

 mitter's line of sight calculated on a % earth's radius 

 basis. Transmission was carried out on 45, 475, and 

 2,800 mc. The results show what has been confirmed 

 by later experiments, that the amplitude of fluctua- 

 tions is larger the higher the frequency. On the 

 optical path the range of fluctuations of the 45-mc 

 signal averages only +3 db, whereas over the same 

 path the 475-mc and 2,800-mc signals exhibited 

 fluctuations which were in excess of 40 db, so far as 

 they could be measured. As was to be expected, the 

 2,800-mc signal fluctuated more than the 475-mc 

 one. Over the nonoptical path all three signals show 

 very wide fluctuations of intensity, the rate and 

 amount again increasing with the frequency. 



In the course of these experiments a certain amount 

 of meteorological study was carried out and fore- 

 casting of propagation conditions was done on a 

 tentative basis. The general results again fore-shad- 

 owed the more complete data obtained by later 

 studies, and a description of the details will be 

 omitted here. 



Similar experiments were carried out simultane- 

 ously by the Bell Telephone Laboratories [BTL] on 

 optical paths near New York City. The wavelengths 

 employed were 10, 6, and 3 cm. 168,174 Here we find 

 clearly established the different signal or fading 

 types that are described in detail below. 



A very extensive program of transmission measure- 

 ments was carried out by the Radiation Laboratory 

 of Massachusetts Institute of Technology [MIT]. 

 The meteorological records were made in cooperation 

 with the U. S. Army Air Forces. The first measure- 

 ments were made in 1942, and experiments on a very 

 large scale were carried out in 1944. 3 ' 10, 12, 153 ' 183 Two 



optical transmission paths were operated in 1943, 

 a 22-mile path over the sea and a 45-mile path over 

 land. A 10-cm continuous signal was used, and the 

 strength was monitored by means of thermistors. The 

 antennas were dipoles with 30-in. parabolic reflec- 

 tors. The received signal was automatically recorded 

 on meters having a range of 60 db. The signals 

 received were correlated with meteorological observa- 

 tions, the results of which will be given below. 



In the spring of 1944 a new over-water transmission 

 path was installed which was operated simultaneously 

 with the 22-mile one. This path was nonoptical, 41 

 miles long, and crossed Massachusetts Bay from the 

 southern tip of Cape Ann to the northern tip of Cape 

 Cod near Provincetown. Transmission over this path 

 was carried on with 256-cm waves, 10-cm S band, 

 3-cm X band, and 1.25-cm K band. The 256-cm 

 equipment used Yagi antennas and operated with 

 continuous waves. The microwave transmitters used 

 pulses with a repetition frequency of 700 c and used 

 parabolic reflectors as antennas. 



The transmitter for the short path was about 120 

 ft above mean sea level, and the transmitters for the 

 long path were at a similar height. The two receivers 

 were about 136 and 30 ft above mean sea level. The 

 transmitter power was monitored and continuously 

 recorded during the experiments while the receivers 

 had automatic frequency control with apparatus 

 which searches for the signal if it is lost. The auto- 

 matic gain control of the receivers was arranged to 

 give a spread of the signal over 70 to 80 db. The 

 receivers were directly calibrated by means of signal 

 generators and a very close check was kept on their 

 performance throughout. The rectified output of all 

 receivers was fed directly into recording milliammeters. 



Coincident with the operation of these transmission 

 paths there was a very extensive meteorological 

 program determining sea and air temperatures and 

 atmospheric humidities by means of fixed installa- 

 tions, captive balloons, ships, and airplanes. The 

 distribution of the refractive index along the trans- 

 mission path was thus known in considerable detail 

 during practically the whole course of the experi- 

 ments. Concurrently with these measurements, a 

 program of forecasting the transmission conditions 

 was carried out. 



Results 



The results obtained on the various transmission 

 paths on the east coast of the United States are 



