EXPERIMEXTAL TKAXSVERSAL EQUALIZER FOR TD-2 



1437 



hybrid coil. Power entering one port divides (not necessarily equally) 

 between two other ports, but none of it reaches the fourth, or conjugate 

 port. In Fig. 5, the power entering at 1 divides between 2 and 4, that 

 entering at 2 divides between 1 and 3, that entering at 3 divides between 

 2 and 4, and that entering at 4 divides between 1 and 3. Directional 

 couplers inherently provide an impedance match at all four ports. Thus, 

 such a coupler sets up no reflections in the main line. Its insertion loss 

 in this line may be kept small by having nearly all the power enter- 

 ing at 1 come out at 2; then only a small fraction is diverted to 4. Co- 

 axial directional couplers have been discussed in the literature ' ' and 

 will not be dealt with in detail here. 



INPUT 



DIRECTIONAL 

 COUPLER 



/9Ef-i 



COit-K^) 



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ja>(t+JJ 



Q 



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Fig. 5 — Diagram of directional coupler. Input .signal divides between Ports 

 2 and 4 with no output at Port 3. Termination Z at Port 4 reflects some signal to 

 Port 3, proportional to the reflection coefficient, p. 



The coupler used here (J68333C) is one originally developed to 

 measure reflections on IF transmission lines in the TD-2 system. The 

 directivity of a coupler is defined as the coupling loss between main line 

 and branch line in the undesired direction less the loss in the desired 

 direction (loss from 1 to 3 less the loss from 1 to 4, for example). In the 

 J68333C coupler, the directivity can be adjusted to exceed 45 db over 

 the band of interest. This can be done by adjusting two screws, shown on 

 model in Fig. 6, to obtaiit the optimimi spacing between the coupling 

 elements. The loss between the main line and the branch line in the 

 desired direction is about 23 db at mid-band (70 mc), and decreases 6 

 db per octave with increasing freciuency. The loss along one of the coupled 

 lines (1 to 2 or 3 to 4) is very small. 



Use has been made of the directional properties of the coupler in pro- 

 viding a simple means of controlling the amplitude and polarity of the 

 tapped signal. Referring to Fig. 5, and keeping in mind the properties 

 of the coupler, it will be noted that a small portion of the input signal 

 appears at Port 4 of the coupler, but none at Port 3. If the impedance Z 



