WAVEGUIDE TRANSMISSION 335 



1 1 1 



(6.5-5) 



V^' V^' 



VT^ 



Referring to Fig. 6.5-1 (a) we have indicated that the wave front 1 is 

 made up of lines of electric force directed through the plane of the illustra- 

 tion and hence away from the observer. There are, of course, lines of mag- 

 netic force and also other lines of electric force both ahead and behind the 

 wave front drawn, but these have purposely been omitted in order to 

 simplify the illustration. If we were to take the magnetic force into con- 

 sideration we would find as in Fig. 6.4-2 that, at the reflecting surface, a 

 tangential component only is present and its magnitude is twice that of 

 the magnetic component of the incident wave. 



In the discussion of reflection of plane waves in the previous section, it 

 was also pointed out that the act of reflecting a wave reverses the direction 

 of the electric force. Applying this principle to the case at hand, we see 

 that if the electric force is directed downward in the section of wavefront 

 1 of Fig. 6.5-1 (a), it will be directed upward in 2. Carrying this idea for- 

 ward to Fig. 6.5-1 (e) we find that in fronts 1, 2, 3, etc., which we rather 

 arbitrarily called crests, the electric vector alternates in direction as shown 

 by the open and solid circles. Likewise the direction of the electric vector 

 alternates in the fronts designated as 1', 2', and 3', but in this case they 

 are respectively opposite in direction to 1, 2, and 3. Continuing to fix 

 our attention on Fig. 6.5-1 (e), it will be observed that the direction of lines 

 of force is the same in 1' and 2, in 2' and 3, and in 3' and 4, indefinitely along 

 the entire length of the guide. Thus there are regularly spaced regions 

 along the length of the guide where the electric vector is directed toward 

 the observer alternating with other regions where the electric vector is 

 directed away from the observer. Between the two are still other regions 

 where the respective component vectors are oppositely directed and hence 

 their sum may be zero. 



Adding the foregoing effects, bearing in mind that there are lines of force 

 both ahead and behind the highly simplified wave fronts shown, we have 

 a new wave configuration moving parallel to the main axis of the guide 

 with a phase velocity Vz as suggested by Fig. 6.5-1 (f). Examining more 

 carefully the wave interference that is here taking place, it becomes evident 

 that if we pass laterally across the guide along the line x in Fig. 6.5-1 (e) 

 the instantaneous value of the resultant electric vector as shown is every- 

 where zero. On the other hand, if we cross the guide along a parallel line 

 x', the electric vector varies sinusoidally beginning at zero at either wall 

 and reaching a maximum in the middle of the guide. It will be observed 

 that if we pass along the major axis z of the guide the electric vector at 



