ELECTRONICS
January 1946

By McGraw-Hill Staff
Page 92 - 97

The Army's first radar countermeasures laboratory
was in the attic of the Marconi Hotel (above)

tenuated only as the square of the distance. Consequently, a radar signal can be detected at distances far greater than the maximum range at which the radar can see a target. In the second place, the direction to the source of the radar signals may be observed by

the use of a directional antenna on the search receiver. Two or more bearings so obtained reveal the location of the radar. Radar reconnaissance can thus be carried out, with airborne search receivers for example, without fear of detection by the radar itself.

The same favorable discrepancy exists between radar range and jamming range, since the jamming signal competes only with a weak reflected echo on the radar screen. Consequently, a jamming power level in the tens of watts is sufficient to compete with a radar peak power in the hundreds of kilowatts. This advantage is reduced, however, by the necessity of jamming with a c-w signal, and so the average power of the jammer is often nearly as great as the average power of the radar.
The accompanying table illustrates the frequencies, power levels and bandwidths of the jamming transmitters. Power in the tens of watts, modulated over bands up to 10 mc, is obtained at frequencies up to about 700 mc using triode tubes, notably the door-knob types. Cavity resonators, using disk-seal (lighthouse) tubes, give about the same performance at frequencies up to 2000 mc. For higher power levels, especially at the highest frequency ranges, c-w magnetrons are used. One important example is the Broadloom jammer, which produces 150 watts

94 January 1946 - ELECTRONICS

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ELECTRONICS January 1946

ELECTRONICS
January 1946