K---' 



Lobe ^ 



Fig. 1-10 Simple Transmitting System. 



17 



The generating device may be visualized as producing a sine wave out- 

 put of constant amplitude and frequency. 



E{t) = ^cos (a;o/+ <i>). (1-14) 



If this power is in turn applied to an antenna which radiates a portion 

 equally in all directions (omnidirectional), we have the simplest sort of 

 radar transmitter. We may proceed to refine the system by modulating 

 the radiation in different ways. 



Space Modulation. The radiated energy may be space-modulated by 

 an antenna possessing directivity. Such a characteristic is shown in Fig. 

 1-10; the radiated energy is concentrated into a lobe by means of a parabolic 

 reflector. 



Three other types of modulation — amplitude ^ frequency , and phase — 

 may be introduced by suitable operations upon the power generator. 



Amplitude Modulation. If the output of the transmitter is ampli- 

 tude-modulated at an angular frequency coi with fractional modulation m^ 

 it then has the form 



E{t) = Aq {\ -\- m cos coi/) cos {ui4 + (^) 



= A^ cos (wo/ + <^) H 2~^ cos [(wo 



, mA u 

 ^ 2 



OJl) / + 0] 



cos [(coo + wi) / + 0]. (1-15) 



Note that this type of modulation produces sidebands in the generated 

 voltage; i.e., the generated voltage has frequency components which 

 differ from the carrier angular frequency wo by plus-or-minus the modulat- 

 ing angular frequency wi. The transmitted spectrum for the case of 100 

 per cent modulation {in = 1) has the form shown in Fig. 1-11. The voltage 

 amplitude of each sideband in this case is one-half that of the carrier, and 

 the power in each sideband is one-quarter of the carrier power. Obviously 



