AMA/) winiii \\n TKwsM issi<)\ rr:i<ii)i<\i Asch: 567 



and tlu' tx'lali\ely meager i)r()i)agali()ii e.\])erience now available. The hij^h 

 cost of the lowers, jiower facilities and access roads involved in repeaters 

 as we know them points to the dcsirahihty of few repeaters and long spans. 

 ro])ography usually permits spans of a few tens of miles without recjuiring 

 towers of excessive height. \'ery much longer spans can rarely be had 

 without tremendous towers and are questionable because of the increase of 

 fading depth with distance. 



In wave guide (or other metallic conductor) transmission entirely dilTerent 

 considerations apply and we will discuss some electrical relationships which 

 seem significant in this case. 



Let us consider a microwave repeater having a noise hgure XI'' and a 

 j)()wcr ca])acity l'C\ The overload characteristic, together with the am<junt 

 of nonlinear distortion that the signal can stand, determines the maximum 

 output power. This maximum power is the power capacity. These two 

 characteristics, PC and NF, thus determine the amount of attenuation that 

 may be introduced between the transmitting half of a repeater regarded now 

 as a transmitting terminal and the receiving half regarded as a receiving 

 terminal. This amount of attenuation expressed in decibels, wdiich we will 

 designate as M, is available to be used up by the loss of one span plus 

 accumulation of noise from // repeaters and may be regarded as a figure of 

 merit of the repeater. 



Five different relationships apply as follows: 



AM Systems: M = span loss,/,, + 20 log n (1) 



FM Systems: M = span loss,//, + 10 log n » (2) 



PPM-AM Systems wdth 



reshaping repeaters: M = span \osSdb + 5 log ;/ (3) 



Band increased -\/ii referred to (1) 



FM Systems with 



limiting repeaters: M = span loss,//, -j- 3..^-? log ;/ (4) 



Hand increased ^\/ii referred to (2) 



PCM Transmission with 



regenerative repeaters: M = span loss,//, + zero (5) 



In (1) the 20 log )i term includes 10 log // for noise accumulation {)lus 

 10 log II for cumulative compression. In microwave am{)lifiers only odd- 

 order terms contribute to the distortion and the third order term predomi- 

 nates for moderate degrees of ()\-erl()ad. This results in the well-known 

 comi)ression characteristic such as appears in l'"igs. 24 and 25 previously 

 discussed. A significant approximation for the over-all compression when 



