432 BELL SYSTEM TECHNICAL JOURNAL 



increments of cavity length will not produce uniform increments in fre- 

 quency. The mode chart indicates graphically that a straight-line relation- 



/iV 



ship exists between {/Y and ( y ) • Uniformly spaced markings on a dial 



reading directly in frequency can be realized by the use of such mechanisms 

 as an eccentric operating on a limited arc. Adjustments are customarily 

 provided to bring the cavity resonance and dial indication into agreement at 

 some frequency of test. Frequency departures of the drive mechanism 

 referred to this point are held commercially to about one part in 5000. 



A pplication of Similitude 



Echo box developments have often been undertaken at frequencies where 

 adequate test equipment was not available. This has been especially true 

 as the radar art progressed to higher and higher frequencies. 



The principle of similitude has been utilized in the construction and test 

 of models at the frequency of existing test facilities. The models have then 

 been scaled to the assigned frequency band. This has been found to be a 

 very practical expedient. 



Use of Cavities For Radar Testing 



The high Q resonant cavity when appropriately connected to a radar sys- 

 tem returns to it a signal which may be used to judge the over-all perform- 

 ance of the radar. Its operation is as follows: During the transmitted pulse, 

 microwave energy from the radar is stored in the cavity in the electromag- 

 netic field. The charge of the cavity increases exponentially during this 

 interval but fails to reach saturation for the cavity by a substantial margin 

 because the pulse is too short. At the end of the pulse, the decay of this 

 field supplies a signal of the same frequency as that of the radar transmitter 

 (when the echo box is in tune) which is returned to the radar receiver as a 

 continuous signal diminishing exponentially in amplitude. 



The time interval betw^een the end of the transmitted pulse and the point 

 where the signal on the radar disappears into the background noise is the 

 "ringtime." The term is used somewhat loosely since, in actual practice, 

 the ringtime is measured on indicators whose range markings are generally 

 in miles or yards referred to the beginning rather than the end of the pulse. 

 The difference, of course, is small. It is customary to include the pulse 

 length in all ringtime figures on operating radar systems. Typical ringtime 

 patterns on radar indicators and a schematic of a radar test with an echo 

 box are shown in Figs. 10 and 11. 



As the output power of the radar either increases or decreases correspond- 

 ing changes in the "charge" of the cavity will be reflected directly in ring- 

 time changes. Similarly as the noise level of the radar receiver varies the 



