48 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



the reflection from the parabohc reflector and mouth is also finite low, 

 and this characterizes a good antenna installation. 



The extra reflected pulse on the right of the lower trace on Fig. 11 

 appeared when a fiberglas weatherproof cover was installed over the 

 open mouth of the horn. This cover by itself would normally produce a 

 troublesome reflection. However, in this antenna, it is a continuation of 

 one of the side walls of the horn. Consequently, outgoing signals strike 

 it at an oblique angle. Reflected energy from it is not focused by the 

 parabolic section back at the waveguide, so the overall reflected power 

 in the waveguide was found to be rather low. However, measuring it 

 with this equipment, we found that an extra reflection appeared to 

 come from a point 16 feet out in front of the mouth of the horn when the 

 cover was in place. This is accounted for by the fact that energy re- 

 flected obliquely from this cover bounces back and forth inside the 

 horn before getting back into the waveguide, thus traveling the extra 

 distance that makes the measurement seem to show that it comes from 

 16 feet out in front. 



7. SEPARATION OF MODES ON A TIME BASIS 



If a pulse of energy is introduced into a moderate length of round 

 waveguide to excite a number of modes which travel with different 

 group velocities, and then observed farther along the line, or reflected 

 from a piston at the end and observed at the beginning, separate pulses 

 will be seen corresponding to each mode that is sent. This is illustrated 



TE, 



•^^ " 



PROBE 3 DIAM ROUND GUIDE 



COUPLING (WILL SUPPORT 12 MODES) 



Fig. 12 — Arrangement for showing mode separation on a time basis in a multi- 

 mode waveguide. The pulses in the trace ])]io(o have all traveled to the iiisloii and 

 back. The earlier outgoing pulse due to direelional coupler unbalance is not shown. 



