MICROWAVE RADAR TESTING 



463 



frequency are present under the envelope. In radar practice conditions 

 are, as a rule, not sufficiently stable for this to occur. 



Because of its bandwidth, an echo box cannot reproduce the ideal spec- 

 trum envelope of Fig. 15a. Instead the curve for a good spectrum may 

 resemble that of Fig. 15b, while spectrum irregularities detrimental to radar 

 performance may be revealed by curves such as those of Fig. 15c and 15d. 

 Broadening of the spectrum is undesirable because less energy falls within 

 the receiver band. Energy removed from the main concentration may 

 result from double moding or from the occurrence of a different frequency 

 during the rise or fall of the pulse. Frequency modulation due to a sloping 



a- IDEAL SPECTRUM 

 FROM RECTANGULAR PULSE 



b GOOD SPECTRUM 



c POOR SPECTRUM 

 SHOWING FREQUENCY JUMPING 



d POOR SPECTRUM 

 SHOWING WIDE FREQUENCY SPREAD 



Fig. 15 — Radar spectrum analysis with echo box. 



or spiked input pulse produces a non-symmetrical spectrum, not infre- 

 quently characterized by a high side lobe. Frequency jump in the middle 

 of the pulse, due to line reflection, may produce two distinct maxima. 



Another device widely used for this purpose is the "Spectrum Analyzer" 

 as developed by Radiation Laboratory, which provides an oscilloscope 

 presentation of spectrum component amplitudes versus frequency. 



Standing Wave Measurements 

 Theory 



The expression for the distribution of current or voltage along a mis- 

 terminated line of appreciable electrical length yields tw'o terms which 



