200 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



ing the day. The skip distance not only depends upon the time of day, 

 but the phase of the sunspot cycle, the geographical location of the 

 transmission path, and the season of the year. Maintaining communi- 

 cations by use of this medium could not be considered reliable enough 

 for the telegraph company's use. 



When the microwaves enter this ionized region of the upper atmos- 

 phere they pass through it, regardless of the angle at which they enter. 

 This makes it necessary to transmit them parallel to the surface of 

 the earth, catch them before they leave the earth at the horizon and 

 retransmit them to the next tower. While the retransmission from 

 tower to tower is more expensive than a standard broadcast which 

 uses the ionization refraction for retransmission, it gives us a 24:-hour- 

 a-day service that the longer-wave-length frequencies can never equal 

 in dependability. 



During March 1946 there were a number of magnetic storms, some 

 of such severity as to blank out commercial radio, halting trans- Atlan- 

 tic air travel, while the microwave beam circuit showed no interfer- 

 ence from the aurora borealis. 



At this high frequency we are not bothered by any man-made or 

 nature-made static; it is even above that of a lightning discharge. 



Tests were made by having an airplane fly directly in the path of 

 the beam. No serious deflections of the received power level were 

 noted until the plane was about 50 feet from the reflector. 



Several difficulties were encountered in developing the transmission 

 of the microwaves when the beam was transmitted parallel to the sur- 

 face of the earth. One is the absorption of energy by water vapor in 

 the air, but this is not serious for wave lengths greater than 5 centime- 

 ters. For the shorter wave lengths, such as at 1 centimeter, a heavy 

 rainfall will drop the receiving power level 5 decibels, which would 

 not be noticed in FM reception, but a cloudburst along a 1-mile path 

 of the beam will drop the received level to one-thousandth of the 

 normal value. 



A slow signal fading was noted throughout the year, but in the FM 

 receiver this change caused no harm. 



A more serious type of fading was noted in the latter part of June 

 1945, when in the early morning hours of a still night, especially dur- 

 ing periods when the humidity was high, the received signal strength 

 would fluctuate wildly. The transmitting power was increased but 

 the results were still unsatisfactory. An investigation of the field 

 strength at the receiving tower disclosed that the line-of -sight signal 

 was being canceled by an out-of -phase signal. By placing a second 

 receiving reflector between 25 and 27 feet below the upper one and 

 using the combined output from the two reflectors, a fairly steady 

 signal could be obtained as the fading did not appear simultaneously 



