Removal of trailing antenna

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Purpose of the trailing antenna

The trailing antenna on the Electra was installed to communicate on 500 kHz. "In 1937, this was the international distress frequency, employed aboard ships since the early days of radio and consequently adopted for aircraft."[1]

500 kHz is a low frequency and therefore has a long wavelength. Long wavelengths require long antennas for effective transmission.

The trailing antenna on the Electra could be extended 250 feet.

Design of the trailing antenna

As delivered

"For 500 KHz, a second antenna was originally employed. This antenna was the trailing-wire type. The antenna was reeled out from the belly of the aircraft and had a full length of 250 feet. This was still shorter than a resonant length at 500 KHz, but a much larger relative component of the wavelength than the 46-foot Vee. With the addition of a special loading coil, mounted inside the fuselage near the transmitter, connected in series with the antenna to electrically lengthen it, the trailing wire could be effectively matched to the transmitter."[2]

Exit point relocated

"A trailing wire antenna came standard with the Lockheed Model 10. Earhart's Model 10E Special was delivered with a trailing wire antenna installed in the tail, visible externally as a small white cone protruding just below the tail nav light. ... Sometime in January or February of 1937, as part of the preparations for the first World Flight attempt, that antenna was removed and was replaced by a different type of trailing wire unit mounted in the cabin. This antenna is visible externally as a mast sticking down from the belly just forward of the cabin door. On the end of the mast is a lump which is, in fact, the lead weight on the end of the wire. Oddly, the mast protrudes from the belly at a right angle rather than at the customary shallow angle which allows the wire to play out more directly into the slipstream."[3]

Potentially unreliable

Prior to the first attempt, "we got into a hassle about the trailing wire antenna installed in the airplane, and which was intended for use on 500 kilohertz. I had previous experience with trailing wires, and it is an efficient radiator. However they are not reliable mechanically. The wire has a rather heavy lead ball attached to its end. An airplane moving at cruising speed puts an enormous strain on this wire, and the wire is subject to breaking, especially in rough air, the lead ball can break off, and you have no antenna. Airlines used them in the early days when low frequencies were used, and they soon discarded them.[4]

Using a trailing antenna

General considerations

Depending on the communication needs of any particular aircraft, the trailing antenna might be extended for the whole flight and retracted before landing. In other situations, it would make sense to extend the antenna only when it was needed.

For those aircraft that had the capacity to transmit on multiple frequencies, the length of antenna extended could be varied so as to match the active length to the wavelength of the frequency being used at the time.

Since Earhart had only one low-frequency, long-wavelength band on her transmitter (500 kcs or 500 kHz), the optimum extension of the antenna was 246 feet--one eighth of the 500 kcs wavelength. The modifications by Joe Gurr were intended to allow some 500 kcs capacity on the dorsal antenna without extending the trailing antenna; when Gurr made the modifications to the radio system in Burbank before the first round-the-world attempt, he left the trailing antenna system intact so that the radio operator (Manning) could decide which antenna to use.

Aboard NR16020

"Manning was to have been the radio operator, and one of his jobs would be to manually reel out and in the trailing wire (as well as to throw the antenna selector switch, located in the aft section." [5]

"The trailing wire’s disadvantage was that it had to be reeled out after takeoff and in before landing. A manual switch was also needed to transfer the transmitter output from the H-F Vee to the L-F trailing wire. This switch, necessarily, was located close to the transmitter which was mounted in the aft section of the fuselage next to the navigator's table. Since the fuselage was almost filled with huge fuel tanks between the navigator’s station and the cockpit, it was very awkward to change antennas if the navigator was riding up front."[6]

Removal after Luke Field crash

Ric Gillespie, January 24, 1998, Forum.

When Earhart wrecked the airplane in Hawaii on March 20, 1937 the trailing wire installation was, of course, crushed. Photos of the airplane in Burbank on May 20, 1937 (the day after repairs were completed) show no trailing wire in the tail or on the belly. A letter written to Earhart author Fred Goerner in 1982 by Joseph Gurr who claimed to be Earhart's radio consultant alleged that the trailing wire installation was left aboard at this time but not hooked up. All we can say for sure is that there is no evidence of the antenna visible externally when the airplane emerged from the repair shop.

It has often been alleged that the trailing wire was removed in Miami prior to Earharts's departure from there on June 1, 1937. Others have it removed at Darwin, Australia on June 28. As far as I know, all such accounts lack any real documentation. It appears more likely that the unit was never removed per se, but was simply not reinstalled after the accident in Hawaii. The reasoning behind such a move is, of course, speculative. It may be that, because Harry Manning's departure from the endeavor left the flight without anyone who knew Morse code, and because the trailing wire was seen as most useful on the 500 kcs frequency which was primarily a code frequency, Earhart decided that its potential usefulness did not outweigh its considerable weight.

Amelia's attitude toward radio has been roundly criticized, and perhaps rightly so. She saw it as a luxury, not a necessity. In fact, reading her own account of the world flight, as published in the heavily edited Last Flight, it's hard to find any occasion when she successfully used the aircraft's radio equipment for either voice or DF.

References

  1. Electra Radios
  2. Mike Everette, A Technical Analysis of the Western Electric Radio Communications Equipment Installed on Board Lockheed Electra NR16020.
  3. Ric Gillespie, January 24, 1998, Forum.
  4. Gurr to Goerner, 3 May 1982.
  5. Mike Everette, 7 September 2000 Forum.
  6. Mike Everette, A Technical Analysis of the Western Electric Radio Communications Equipment Installed on Board Lockheed Electra NR16020.
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