-21 



the product is a round number (4 x 10 ) that is convenient to 



manipulate in both algebraic and logarithmic form (10 log 4 x 10' 

 -204dBw per Hertz bandwidth). 



21 



It is important to determine the reference temperature when comparing 

 specifications from different manufacturers. 



IV. ANTENNA/RECEIVER CONSIDERATIONS 



The noise temperature of a receiver is largely determined by the 

 input stage of the receiver. Typical noise figures and noise 

 temperatures of various commonly used input amplifiers in the 

 1.7 GHz frequency range are given below: 



Typical bipolar transistor 



Premium grade bipolar trans- 

 istor 



Gallium-Arsenide (GAS) FET 



Noncooled (ambient temp.) 

 parametric amplifier 



Cryogenic parametric amplifier 



0.69 



0.29 



50 



20 



$ 25,000.00 



$100,000.00 



(Note: These numbers are approximate as they neglect the noise 

 contribution of succeeding stages. This should be small if the gain 

 of the input amplifier is large, say lOdB or more. 



As an example, if a premium grade biopolar transistor amplifier 

 with a noise temperature of 289<^K is used, to which must be added 

 the antenna noise temperature of 750K, the result is a total system 

 noise temperature of approximately 364°K. The noise power in a 

 1-Hz bandwidth is given by: 



Noise power 



(joules per °K) x 

 X 10-2 1 watts/Hz 



= 1.38 X 10 ^^ 

 365°K = 5.02 



= -203dBw/Hz 



= -173dBm/Hz 



As the DCS information is 100 bits/s, the postdetection band- 

 width must be 100 Hz. The noise power in a 100-Hz bandwidth will 

 therefore be 100 times (or 20dB) greater than the noise power in a 

 1-Hz bandwidth. The total noise power at the receiver output will 

 be -153dBm/100 Hz. This is the natural noise against which the 

 signal must compete. 



54 



