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[Ric Gillespie]

In support of the "second receiver" theory, the response to C.B. Allen's concerns about marine frequency reception may explain these otherwise anomalous Associated Press reports:

If these are all AP reports they probably come from a single reporter.  None of the reports quotes a source such as Earhart or "so-and-so spokesperson for Pan American."  I don't know what a crossed-loop DF antenna looks like but the only DF antenna on the airplane when it left Miami was the same Bendix loop that was installed back in march.  I have a hard time believing that there was a second DF on the airplane that:
1. Was not externally visible.
2. Was not mentioned by Earhart in the Karachi interview in which she described all the plane's radio equipment and its location.
3. Was not mentioned by Sgt. Rose in Darwin when he fixed the fuse on her direction finder (singular)
4. Was not mentioned in Chater's description of the July 1st test flight in Lae.

[Daniel R. Brown]

There's also this, a direct attribution from a different reporter:

"Minor Repairs to Amelia's Plane" (Dateline Miami FL 5/24/37) "Her husband, George P. Putnam said workmen will make minor repairs in the plane's radio set and install a radio range finder." Source: 5/24/37 San Mateo Times (United Press)

Based on these reports it sure seems that Pan Am technicians installed some sort of radio "finder" between May 24 and May 30. A Pan Am (PAMSCO) DF receiver might have been connectable to the Bendix loop via the loop coupler as easily as the WE20B receiver was, with no changes externally or in the "dash" necessary. Rose fixed the generator, not the receiver, and neither Rose nor Chater had any reason to expect a particular type of DF receiver was aboard.

I know, "seems" and "might have". FWIW

Dan Brown, #2408

[Ric Gillespie]

This is interesting.  Let's review the sources in chronological order.

5/23/37
“EARHART IN MIAMI ON TEST FLIGHT: Radio Direction Finder Installed in Plane”. [Dateline 5/23/37 Miami FL] “Pan-American Airways loaned her two technicians to install in her plane a radio direction finder similar to those used in South American and Pacific flights.” Source: 5/24/37 Atlanta Constitution

5/24/37
"Minor Repairs to Amelia's Plane" (Dateline Miami FL 5/24/37) "Her husband, George P. Putnam said workmen will make minor repairs in the plane's radio set and install a radio range finder." Source: 5/24/37 San Mateo Times (United Press)

5/30/37
“A radio direction finder has been installed by Pan American technicians.” Source: 5/30/37 New York Times

6/1/37
“Their own mechanics installed the radio direction finder which they developed for their clipper ships…” Source: 6/1/37 New York Times

6/26/37
GP still referred to the DF as a "Bendix beam-finder" in a late June interview. Source: 6/26/37 Atlanta Constitution (United Press)

On 5/23/37 it's a "radio direction finder similar to those used in South American and Pacific flights.”
On 5/24/37 Putnam calls it "a radio range finder."
On 5/30/37 it's a "radio direction finder."
On 6/1/37 "the radio direction finder which [PanAm] developed for their clipper ships"
On 6/26/37 Putnam calls it a "Bendix beam-finder"

A radio direction finder is not the same thing as a "range finder" or a "beam finder."  In 1937, U.S. commercial airliners navigated primarily by means of the "radio range" system. The radio range was an aural system in which the pilot listened for either an "N" or an "A" in code to tell he was to the right or to the left of one of four directional "beams" that would lead him to the station.  When he was "on the beam" he heard a steady tone.  The range did not require a loop antenna.

A radio direction finder was a visual and aural system that was used to home in on a non-directional beacon. The beacon put out a morse code identifier (for example, SYR for Syracuse). The pilot rotated a loop antenna to get a "minimum" signal and a needle on the instrument panel indicated the relative bearing to the beacon.

None of this makes any sense.  It's hard to imagine why any special installation would be required for Earhart to use the radio range and it was a uniquely American system, of no use on a world flight.  She already had a radio direction finder. Why would she need another one?  I think one of the problem we're dealing with is that neither Putnam nor the newspaper reporters understood the terminology.

"The standard receiver used in PAA Clippers was "distinguished by its wide frequency range (250 kc to 25 mc), its light weight (6 pounds) and its simplicity of design...". The Clippers were equipped with crossed-loop DF antennas. Source: "Flying the Pacific by Radio" April, 1936 Electronics, p. 7-10."

We need to find out what a "crossed-loop DF antenna" system looks like and whether it could be installed inside the airplane.

[Martin X. Moleski, SJ]

We need to find out what a "crossed-loop DF antenna" system looks like and whether it could be installed inside the airplane.

Wikipedia, "Direction Finding": "The crossed-loops DF antenna atop the mast of a tug boat."

[Ric Gillespie]

Assuming you got that by Googling "crossed-loop antenna", that one is not installed inside the boat.

[Martin X. Moleski, SJ]

Assuming you got that by Googling "crossed-loop antenna", that one is not installed inside the boat.

Yes, I got that by Googling "crossed-loop antenna."  It seemed to me to be a reasonable search term to use to find out the meaning of the phrase.

I've just inserted the caption for the photo.

I agree that it is not "inside the boat."   

[Harbert William Davenport]

Ric, I think you have raised an important question here:  "She already had a radio direction finder.  Why would she need another one?"

Let me follow up on that one with another:
Given the RDF equipment that we suppose they had on board when they departed Burbank or Oakland, do we have any reason to think that any attempts at RDF, during the flights en route to Miami, would have succeeded? 
     My present understanding is that successful RDF would have been difficult or impossible, using a manually rotated loop antenna to get a null at frequencies much higher than say 500 kc/kh, using the original Western Electric receiver they were left with after the Bendix receiver was removed.
     So let’s suppose, just hypothetically, that they had tried and failed with RDF en route to Miami.
Would not that explain their concern to add RDF equipment from Pan Am, if that were feasible?

[Ric Gillespie]

According to Elgen Long (The Mystery Solved, Simon & Schuster, 1999) the two radio technicians sent from Pan Am's radio shop at Dinner Key to work on Earhart's radios were Louis Michelfelder and Robert H. Thibert.  Long interviewed Thibert on November 2, 1991.  Thibert told Long that on May 29 he checked the direction finder on NR1020 by taking bearings on local broadcast radio station WQAM and found "The direction finder gave good bearings with a definite minimum when the loop was swung to point at the station."

Citing his interview with Thibert, Long goes on at great length about problems the Pan Am radio techs had with Earhart's transmitter and what they did to fix it, but there is no mention a problem with the DF, let alone replacing it with a Pan Am unit.

So whom do we believe?  Contemporary press reports or the 54 year-old recollections of the technician who actually did the work?  Press reports, as we all know, are often wrong, but so are anecdotal recollections.  Just as with the patch that replaced the lavatory window, the one type of evidence we have that we can rely upon is photography and photos of the airplane show no change in the DF antenna.

[Matt Revington]

"The standard receiver used in PAA Clippers was "distinguished by its wide frequency range (250 kc to 25 mc), its light weight (6 pounds) and its simplicity of design...". The Clippers were equipped with crossed-loop DF antennas. Source: "Flying the Pacific by Radio" April, 1936 Electronics, p. 7-10."

We need to find out what a "crossed-loop DF antenna" system looks like and whether it could be installed inside the airplane.

I have attached a figure from the Aug 1935 issue of Popular Aviation, the radio direction finding antenna pictured here uses a tall exterior mast

[Daniel R. Brown]

That's not it. I found several photos of various Clippers as early as 1935 with the DF loop antenna enclosed in a football-shaped exterior fairing over the cockpit. Will post next week if current Wx hold over ATL ever clears...
Dan Brown, #2408

[Daniel R. Brown]

Examples of a football-shaped exterior fairing over the cockpits of Clippers, presumed to enclose a DF loop antenna.

Dan Brown, #2408

[Ric Gillespie]

Examples of a football-shaped exterior fairing over the cockpits of Clippers, presumed to enclose a DF loop antenna.

Yep, those are all Boeing 314s, first flight June 7, 1938.  The football-shaped fairing over the DF loop seems to have appeared about that time. Idon;t think we'll find a football on any airplane much before then. Clearly, mounted a Df loop inside the airplane was not an option.

[Daniel R. Brown]

The photo I labeled "1935 Clipper" is alleged by the source (found on Pinterest) to be from a family album documenting their summer vacation in 1935. Looks more like a Sikorsky S-42 than a B-314 to me.

Dan Brown, #2408

[Ric Gillespie]

The photo I labeled "1935 Clipper" is alleged by the source (found on Pinterest) to be from a family album documenting their summer vacation in 1935. Looks more like a Sikorsky S-42 than a B-314 to me.

I didn't see that one before.  Definitely an S-42.

[Ric Gillespie]

I just looked at dozens of photos of S-42s and Martin M-130s in "Pan American's Pacific Pioneers" and found only two photos showing a football-type antenna. Both are S-42s.

The photo of Hong Kong Clipper was taken April 23, 1937 and so would be roughly contemporary with Earhart's time in Miami in May.
The photo of Samoan Clipper was taken December 24, 1937.
Both were survey flights, suggesting that Pan Am only equipped their aircraft with DF loops when they were operating outside their own route system.

In any case, NR16020 obviously never had one of these systems.