NR16020 antennas
|
| draft |
Ric Gillespie, "Propagation Analysis," November, 2000, TIGHAR Tracks.
The other part of the equation seems to be the changes that were made to Earhart’s transmitting antenna prior to her second World Flight attempt. Originally, Western Electric had set up the vee antenna that ran from a mast on top of the fuselage to each vertical fin on the tail to be an appropriate length for Earhart’s two primary communications frequencies, 3105 and 6210 Kcs. The much lower 500 Kcs frequency required a much longer antenna which was provided by a “trailing wire” that was played out into the slipstream after the aircraft was in flight and reeled back in before landing. The wreck in Hawaii that ended the first World Flight attempt also wiped out the mast on the belly from which the trailing wire was deployed. During repairs back in California the decision was made to eliminate the trailing wire and lengthen the vee antenna on top of the fuselage to accommodate all three frequencies on the one antenna. The mast that supported the point of the vee was moved forward several feet. It was a terrible compromise that provided no meaningful capability to transmit on 500 Kcs while greatly complicating the problem of putting out a decent signal on 3105 and 6210. There appears to have been, however, another consequence to lengthening the vee. The new length made an excellent antenna for the unintended harmonic frequencies.
Ric Gillespie, 26 April 2000 Forum.
- Earhart's transmitting antenna was the dorsal "vee" and was, in fact, somewhat directional.
Bob Brandenburg, 27 April 2000 Forum.
- At radiation angles below about 30 degrees above horizontal, the azimuthal pattern of AE's antenna was shaped rather like a figure eight with a large waist and fat lobes. The pattern was symmetrical with respect to the axis of the beam pattern, and each of the two lobes was about 80 degrees wide. The axis of the pattern was oriented about 30 degrees to starboard of the the aircraft's longitudinal axis - - an apparent consequence of the location of the antenna feedpoint - - but the width of the pattern lobes was such that there was no significant loss of signal strength in the fore and aft directions relative to aircraft heading. As the radiation angle increased above 30 degrees, the pattern became closer to circular, and above about 45 degrees was omnidirectional. It's worth noting that AE's antenna was shorter than a half wavelength at both of her frequencies, which accounts for its broad radiation pattern.
- When AE turned southeast on the LOP, Howland and the Itasca were very close to the peak of the aft lobe for low angles, but that factor didn't come into play. When AE was 100 miles from Howland, the vertical radiation angle was 70 degrees. At 150 miles, the angle was 45 degrees, and the radiation pattern was still omnidirectional. At 200 miles, the radiation angle was down to 30 degrees, but by then AE's signal strength at Howland and the Itasca had dropped well below the threshold for detection.