NR16020 antennas

draft

The design and function of the trailing antenna and loop antenna are discussed separately. This article is concerned primarily with the question of the location and function of the various antenna systems.

There are many kinds of antennas in this discussion:

• Dorsal antenna: a Vee mounted on the top side of the aircraft.
• Ventral antenna: a straight wire mounted on the underside of the aircraft.
• Loop antenna: interior or exterior direction finding loop.
• Trailing wire antenna: exited from the tail or from the underside of the plane.

Bell Labs installation: dorsal Vee

"The high-frequency antenna installed aboard the Electra in March 1937 was a "Vee" type running from the tip of each of the twin vertical stabilizers to a mast atop the fuselage, located at Station 176. The antenna was, therefore, a total of 46 feet, doubled back onto itself. This length did not include the lead-in wire which exited the aft fuselage through a feed-through insulator and connected to one leg of the Vee at a point a few feet from one of the vertical stabilizers. This antenna was, then, already 15% longer than optimum; but since the radio equipment had been installed by Bell Labs, it can be fairly assumed that it was tuned properly at that time.

"The length of 46 feet was greater than 1/8 wavelength at 3105 KHz (approximately 38 feet) and greater than 1/4 wavelength at 6210 KHz (again, approximately 38 feet; this relationship is due to the fact that 6210 KHz is the exact second harmonic of 3105 and the wavelength at the higher frequency is half that of the lower); or, a non-resonant length at either frequency.

"This antenna was unsuitable for low-frequency 500 KHz operation, as the total length would be miniscule compared to the wavelength at this frequency. The wavelength of 3105 KHz is about 97 meters (315 feet), and about 48.5 meters (156 feet) at 6210. At 500 KHz, the wavelength is 600 meters (1950 feet)."

Variant installations

Abbreviations

TX = "used for transmission"

RX = "used for receiver"

DF = "used for direction finding"

This is very much in draft stage. I'm just trying to develop a scorecard so folks can keep score at home. MXM, SJ.

History of antenna configurations

	July 24, 1936	after Bell Labs	Hooven RDF unit	after Gurr	March 17, 1937	July 2, 1937
dorsal vee		TX (46')	TX	TX (+500 kcs)	TX	TX (54')
starboard ventral wire	TX & RX?	RX	RX	RX	RX	RX
port ventral wire			DF	DF	DF	DF
trailing wire	TX 500 kcs	TX 500 kcs	TX 500 kcs	TX 500 kcs	TX 500 kcs	removed
belly pod?			DF
exterior loop				DF	DF	DF

Gurr: "We have seen other pictures showing the 'cigar shaped' DF unit installed on the underside of the airplane."

unsorted material

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.

Ric Gillespie, 24 October 2000 Forum.

The antenna feed point was changed. Prior to the Luke Field wreck the dorsal vee fed into the fuselage at a point on the top of the fuselage just above and forward of the starboard side cabin window. The wire then ran down the interior cabin wall to the transmitter. When the airplane came out of the repair shop in Burbank in May the feed point had been altered so that the wire came down from the antenna and fed into the fuselage way down on the starboard side of the airplane just opposite where the transmitter was installed on the cabin floor.

There are lots and lots of good photos of the airplane after it left Miami and the insulators on the dorsal vee are easy to see. There are two insulators right up close to the forward mast and others right up close to the attach points on the vertical fins. There are no insulators elsewhere on the wire.

Related articles

• Frequency, wavelength, and antenna tuning.
• WE-13C Transmitter Harmonic Power Output.
• Radio equipment on NR16020.
• Removal of trailing antenna.
• The Lost Antenna.