TELEGRAPHY — HILLIS 199 



trate the broadcast frequencies of 720 kilocycles into a beam 6° in 

 width, it would take a parabolic reflector approximately 13,000 feet 

 in diameter. Now to concentrate the radiation of 4,000 megacycles 

 into the same field pattern, we would need a reflector only 30 inches in 

 diameter. 



With such a reflector, instead of letting the radio waves go off into 

 free space in all directions, we are able to concentrate them into a beam 

 only 6° in width, the angle being inversely proportional to the dia- 

 meter of the reflector. This results in a very small field of intense 

 radio energy at the point at which the beam is aimed. In fact, the 

 reflector increases the energy 30 decibels, which is a gain in power 

 of 1,000 to 1. For example, a nondirective antenna might require 1,000 

 watts to give the required field strength at a given point. With the 

 parabolic reflector, only 1 watt of power would be required. If the 

 receiving antenna is surrounded by an identical reflector, the receiver 

 will pick up 1,000 times as much power as it would if the antenna was 

 out in free space. 



The net practical result is that radio repeater stations may be spaced 

 about 50 miles apart and we will require only one-tenth of 1 watt of 

 radiated power to give dependable 24-hour service, where before we 

 would require 90 to 100 kilowatts. 



The ability to concentrate the microwaves in a small space makes 

 it possible to use the same frequency for sending or receiving from as 

 many as eight positions at one location, or one for every 45° of the 

 compass. 



All radio waves are propagated in straight lines, microwaves in- 

 cluded. The difference between the longer standard broadcast waves 

 and microwaves is that when the low-frequency waves enter the ionized 

 layers of the upper atmosphere, this refractive mediimi causes them 

 to be bent and some will return to the earth. Here they are reflected 

 upward from the ground and again refracted to appear a second time 

 on the earth. If the sending-signal strength is great enough and the 

 signals enter the upper atmosphere at the correct angle, they may be 

 reflected back and forth and eventually go around the earth. 



The height of the ionized layer that causes the low -frequency waves 

 to be bent back to the earth varies continuously. The F-layer ioniza- 

 tion decreases after sunset and reaches a minimum just before sunrise. 

 After sunrise it splits into two distinct layers of ionization, which 

 are termed F-1 and F-2. These layers remain separate during the day 

 but merge into one at sunset. The average height of the F layer is 

 about 185 miles, the F-1, 140-160 miles, and the F-2 150-250 miles. 

 A long-wave-length signal entering these ionized regions at a con- 

 stant angle will not always return to the earth at the same place as 

 the skip distance varies as the height of the ionized layers change dur- 



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