Failed direction finding test in Lae

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Ric Gillespie, Finding Amelia, p. 74.
On July 1, before having the plane fueled for the flight to Howland, Amelia made a short test flight to confirm that everything was working. She was, at last, able to establish two-way voice communication with the ground, transmitting to Lae on her daytime frequency of 6210 kilocycles and receiving Balfour’s reply on Lae’s frequency of 6522 kilocycles. Balfour’s assessment of the aircraft’s transmitter was that the 'carrier wave on 6210 kc was very rough and I advised Miss Earhart to pitch her voice higher to overcome distortion caused by rough carrier wave, otherwise transmitter seemed to be working satisfactorily.' Earhart then asked him to send a 'long dash' while she attempted to take a bearing on the station, but this attempt to use her homing device was unsuccessful as well. ...
During the test flight, Earhart found she could receive Lae’s signal, but the intensity of the sound did not change when she rotated the loop. In the terminology of the time, she could not 'get a minimum,' and so could not get a bearing on the sending station. The problem was that although the radio receiver could pick up the signal and she could hear the tone in her headphones, the direction-finding aspect of the system could not respond to such a high frequency. Amelia, however, decided that the test had failed because the airplane was too close to the station and the signal was too strong.
It is not clear whether Balfour’s previous ground test of the receiver included taking a bearing using the direction finder, but it is known that he carried out his test on a signal of 500 kilocycles, a frequency well within the loop antenna’s 200 to 1500 kilocycle capability. In the flight test, Earhart tried to take a bearing on Lae’s 6522 kilocycle signal but could not get a minimum. Balfour accepted Earhart’s diagnosis of the problem, and so passed another opportunity to discover the flaw in her plan for finding Howland Island.


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