Image from page 499 of "The Bell System technical journal" (1922)
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St ft-L 470 MICROWAVE RADAR TESTING 471 error that nia\- occur in measuring different values of SWR with various as-sumed directivities. With a simple two-hole coupler, the directivity deteriorates rapidly asthe frequency departs from that corresponding to quarter-wave spacing. I / / IRECTIVITY / / / / / / b -Dq:> -0 / ^ LIMITS FOdb DIRECT TS FOR 20 / / h V / / / ^ (\J ; 1 J ^/ / V / / / / / / / / / / / / / ^ / / / / / / X / / / / / / / / / / / / / V / / X / // v. y y A y. /y //// y — 2.2 2.6 MEASURED VS WR Fig. 20—Error in standing wave measurements caused by directivity of directionalcoupler. By providing additional couplings suitably spaced, the residuals fromdifferent sets of couplings can also be cancelled against one another and thedirectivity versus frequency characteristic can be materially broadened.With multiple hole couplings a minimum directivity of 26 to 30 db over afrequency band of 10 to 20% is readily practicable in quantity production,
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472 MICROWAVE RADAR TESTING 473 and much better values are obtained in the laboratory. Some of the nu-merous designs of directional couplers developed for association with operatingradars are shown in Fig. 21. Because they are more convenient than slotted lines and can be made moreaccurate, directional couplers have been extensively used for SWR measure-ment in the laboratory. A number of special arrangements have beendevised to improve both accuracy and convenience. A directional couplerarrangement which has been provided for field measurement of SWR in thevicinity of 25,000 mc is also illustrated in Fig. 21. In this the direct poweris brought to equality with the reflected power by an attenuator whose dialis calibrated directly in SWR. A wave guide switch facilitates the powercomparison. Auxiliaries and ComponentsRF Loads An RF load (or dummy antenna) which will absorb the radar power inan impedance which matches the transmission line is very useful in radarwork. Such a device permi
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