THE EQUIPMENT list
The arrival of the Army and the entry of the U.S. into World War II transformed Camp Evans into a major center for radar research and antenna production. Project Diana, however, introduced a whole new series of challenges beyond the reach of existing technology designed for (relatively) short-distance detection of enemy aircraft. As DeWitt knew from the start, only a very special combination of transmitter, receiver, and antenna would do the job. A major constraint in addressing these challenges was the lack of either time or budget to design and purchase new equipment; everything depended on modification of equipment already on hand.
Early on, Jack DeWitt decided to use as a starting point the crystal-controlled FM transmitter/receiver specially designed for the Signal Corps by radio pioneer Edwin Howard Armstrong. Armstrong’s transmitter was in turn based on the “Old Faithful” SCR-271, bedspring-type array antenna, which could operate at a wavelength of 2.7 meters—short enough, DeWitt and his team believed, to penetrate the ionosphere.
The Armstrong set had features that could be adapted to address two important problems:
First, due to the relative velocities of the earth and moon, the frequency of the returning echo differed from the transmitted signal (a phenomenon known as Doppler shift) by as much as 300 Hz, a number that was constantly changing depending on the earth’s rotation and the moon’s orbital path. To ensure that the outgoing signals would bounce back and then be detected by the receiver, recalculation of the earth’s position relative to that of the moon and the resulting Doppler frequency shift was required for each attempt; this work was carried out by the mathematician Walter McAfee of the Theoretical Studies Group. The modified Armstrong radio was capable of being fine-tuned to the exact frequency required to compensate for the Doppler shift at a given point in time.
Second, signals bounced off an object 240,000 miles from earth would take much longer to echo and be much too weak to be detected by receiving antennas then in use, a problem that had bedeviled previous attempts to shoot the moon. Several steps were taken to enhance the detectability of the echo. “We realized that the moon echoes would be very weak,” DeWitt later recalled, “so we had to use a very narrow receiver bandwidth to reduce thermal noise to tolerable levels.” In addition, the power of the transmitted signal was increased. To further augment the returning signal, it was decided to generate a much longer pulse that would be easier to detect. The Armstrong radio was one of the few existing sets capable of generating such a long pulse.
Echoes were received as blips on a nine-inch cathode-ray tube and as 180 Hz beeps.
The iconic bedspring antenna
To ensure successful reception of the echo, however, an even larger problem remained to be resolved, that of antenna sensitivity. No antenna on-site was up to the job. To come up with a solution, DeWitt called on two very senior specialists from the Antenna Design Section, one the Section head, who proposed a clever system using quarter-wave step-up transformers. The only problem: It didn’t work, even after extensive efforts to tweak the transmitter.
DeWitt then turned to his own little group, which came up with the inspired idea of positioning two SCR-271 stationary radars side-by-side to create an enormous (40x40-foot) double bedspring antenna consisting of an 8x8 array of 64 half-wavelength dipoles with reflectors that further enhanced the 111.5 MHz signals. Herbert Kauffman, writing in 1946, credits King Stodola, Harold Webb, and Jack Mofenson with developing this approach. Translating that idea into reality was undoubtedly easier said than done, but the Mechanical Design Section was able to implement this alternative plan and assemble the antenna that eventually became the lynchpin of the operation. The resulting array was 152 times (~22 decibels) more sensitive than a single dipole. Unfortunately, engineering specifications were destroyed by the Army in 1971, so the exact design details are unknown.
This whole assembly was mounted atop a 100-foot reinforced tower in the northeast corner of Camp Evans. The heavy and ungainly antenna could not be tilted, it could only be rotated in azimuth; so moonshots could only be attempted twice a day, usually at moonrise but occasionally at moonset, during the 40-minute window that opened when the moon passed through the 15-degrees-wide beam of the antenna pattern.