TD-2 MICROWAVE RADIO RELAY SYSTEM 1053 



position at the hybrid and this results in transmission of the total signal 

 energy through the fourth arm of the hybrid and into the receivi*ig converter. 

 The reflection filters are transparent to frequencies outside the desired 20- 

 megacycle band. These frequencies are recombined in the second hybrid 

 for transmission to the following channel separation networks. 



An image suppression filter is located in the waveguide just ahead of the 

 receiving converter. Its purpose is twofold: first, to provide discrimination 

 against interfering signals which are the intermediate frequency image of 

 the desired signal; and, second, to provide a critically spaced reflection of a 

 beating oscillator component from the converter for control of the converter 

 intermediate frequency output impedance. 



The receiving converter is of the balanced crystal type in which the two 

 crystals are mounted in a hybrid junction assembly. The signal input con- 

 nection is by waveguide and the oscillator input connection is by coaxial 

 line. An unbalanced output at intermediate frequency without the use of a 

 balanced to unbalanced transformer is obtained by reversal of the polarity 

 of one crystal relative to the other, permitting a parallel output connection. 

 The 405A varistor unit, having symmetrical terminal design, was developed 

 for this appHcation. The preamplifier utilizes two 41 7 A grounded grid 

 triodes and has a gain of approximately 12 db. Its transmission band is 

 centered at 70 megacycles and is flat to within 0.1 db over a 22-megacycle 

 bandwidth. The converter-preamplifier has a net gain of approximately 6 

 db. The output of the preamplifier is coaxially connected through a delay 

 equalizer to the input of the main IF amplifier. 



The main IF amplifier shown in Fig. 10 has input and output impedances 

 of 75 ohms and approximately 65 db gain. It consists of eight stages of 

 amplification, the first being a 41 7 A grounded grid triode followed by six 

 stages of 404A pentodes and a 418A tetrode output stage. The input, out- 

 put and interstage networks are all of the double-tuned impedance-matched 

 type except the network between the sixth 404A pentode and the 41 8A 

 output tetrode which is triple tuned and mismatched. Tuning of the triple- 

 tuned network provides for adjustment of the over-alt transmission band 

 shape. Automatic gain control operating upon the grids of the first five 

 pentode stages maintains the output power to within 1 db of a selected value 

 between -f 4 dbm and -flO dbm for a 30 db range in input power. A low 

 level bridging tap is available at the output of the main IF amplifier. 



D. Radio Transmitter 



The transmitter modulator consists of a 416A microwave triode mounted 

 in a structure which provides a resonant cavity between cathode and grid 

 and another between plate and grid. This cavity structure is used in both 



