Hi Marti

I think the Bendix Loop Control Box was mounted above the pilots window and can be seen in the Caripito and Lae photos. It replaced the Hooven Remote Head in the Miami photo. As Diego pointed out, the Loop Control Box is not a receiver but is some kind of antenna adjusting device.

The point of this thread is to ask how she was able to receive a signal on 7500khz but not on 3105 or 6210.

I note that although she said in her final message that she would repeat on 6210khz, Itasca never heard anything on that frequency and never transmitted on that frequency.

.....camels in the clouds? 

https://share.google/dzGI81gHtHfvNnZN0

(Source:  NewsNation)

Fwiw, it's not universally accepted that the belly antenna was pulled off or broken at Lae. My colleague Gary LaPook believes that AE established two-way communication with Lae during a position or weather report, meaning she had reception capability after takeoff from Lae (can't recall his exact reasoning, but something about the wording reported by Lae suggested that AE must have answered them at some point). I'm guessing if you search Gary's posts here and the terms Lae, position report, etc., you might be able to ifnd it. 

I believe Elgen Long also looked at the possibility that it was Itasca's transmitter on 3105 that was the problem, not Earhart's reception capability. He may have been prompted in this by the fact that in the days before AE's (non) arrival at Howland, there was cable traffic between Itasca and San Fran Div in which San Fran noted problems in Itasca's transmitter on 3105 and ordered them to get it fixed. Just going from memory on this, can't give you any details. But I assume you can find something on it in Tighar's vast archives, maybe discussions here on the forum, and probably in one of Ric's books or Elgen Long's book. 

Thanks for the message. I want to read the information on the side view - I am sure that will tell about the tuning bands.

Quote from: Colin Taylor on March 22, 2026, 06:17:31 AMIf LOOP TUNING does not mean what it says, then I am lost!

I may not have explained it very well, so I cheated and asked chatgpt to explain what tuning the loop meant. Here is its reply (lightly edited for length):

"That instruction is referring to tuning the loop as a resonant circuit. A WWII Bendix loop like that isn't just a piece of wire—it's actually a tuned LC circuit (inductance + capacitance), very similar to the front-end of a radio. A loop antenna inherently has: Inductance (from the wire loop itself) and a variable capacitor (usually built into the loop assembly or its control box). Together, these form a resonant circuit based on the principle of Resonance.

"What 'tune loop for max signal' means: Even though the receiver is tuned to the station frequency, the loop must also be tuned to resonate at that same frequency (emphasis added). When you adjust the loop's tuning control: You're changing the capacitance, and this shifts the loop's resonant frequency. At resonance means: The circulating current in the loop is maximized; the induced voltage is highest; and signal transfer into the receiver is strongest.

"In practice, the operator would: Tune the receiver to the known beacon frequency, adjust the loop's tuning knob, watch/listen for peak signal (often via headphones or meter), then rotate the loop to find the null (minimum signal) for bearing.

"Even if the receiver is perfectly tuned: If the loop is not resonant, it behaves like a weak, broadband pickup, and the coupling between antenna and receiver becomes inefficient. (emphasis added)

"Think of it like two tuned circuits: The loop (antenna circuit) and the receiver input circuit.  Both need to be aligned for best performance."

QuoteIs there some instrument associated with this set-up? What is the ARROW, the POINTER, the MASK and the UNMASKED PORTION OF THE DIAL?

I also asked chatpgt about this. Here's its reply (again lightly edited for length):

'Arrow will point to station': The loop assembly has a mechanical pointer (arrow) linked to its rotation. Once you've found the peak,the physical orientation of the loop is aligned with the incoming wave. The pointer is therefore indicating the direction of arrival. In other words, the antenna itself becomes a compass for the signal.

'Adjust mask so that zero mark is over pointer': The 'mask' is basically a rotatable compass card or dial overlay. You're doing a quick calibration. While the loop is aimed for maximum signal, you rotate the dial (mask), you line up the 0° mark with the pointer, now the system is 'zeroed' to the station.  After that adjustment: The loop's current direction = 0° reference. Any further rotation gives you relative bearings from that station."

The pics I posted may be size limited, but if you go to the website on which I found the pics and look at the one with the side view of the loop, you can see two printed arrows on the directional control wheel that are parallel to the loop and what looks like possibly another one (not printed, but rather perhaps a physical attachment or cutout?) approximately perpendicular to the loop.

Hope this helps.

If LOOP TUNING does not mean what it says, then I am lost!

Is there some instrument associated with this set-up? What is the ARROW, the POINTER, the MASK and the UNMASKED PORTION OF THE DIAL?

Quote from: Colin Taylor on March 16, 2026, 05:00:12 AMThe 7500kcs signal could not be received through the loop antenna because the loop control box could not be tuned above 1500kcs. Therefore it must be received through a static aerial.

Quote from: Colin Taylor on February 16, 2026, 08:04:31 AMThe loop antenna could not be tuned above 1500kcs hence no directional signal. The fact that the receiver could tune 7500 tells that the static aerial was functional not the opposite!

Joe Gurr discussed this via his Q and A with Fred Goerner.

    Goerner: According to the Itasca (Coast Guard vessel) radio log, Earhart asked for homing signals to be sent to her on 7500. Did her DF have that range? Could she have taken a bearing on that frequency?

    Gurr: Yes.... The DF loop on the airplane was designed for low frequencies .... It would be much less efficient on 7500, and then the signal would have to be quite strong to overcome the greatly detuned (from resonance) condition. (emphasis added)

This seems to be at the heart of the matter.  The loop antenna was designed for use as a DF on low frequencies (up to around 1500 kcs), but it would still function as an antenna, albeit not an ideal one, for receiving on higher frequencies.  When Earhart switched the antenna input on her WE receiver from the belly antenna (which may have been damaged on takeoff) to the loop antenna to listen for Itasca's A's on the hour, she was close to Howland (within 100 miles if you're a crash and sinker like me, 225 miles if you're a Tighar true believer), and the antenna would still function, as suggested by Gurr, albeit degradedly.  She then set the WE receiver to 7500 and listened through her headphones that were connected to the WE receiver.  The antenna was a mismatch to the receiver, so the A's came in as detuned and weak, and, accordingly, they were audible but could not be heard not well enough for AE to get a minimum.

Relatedly, you seem to take the existence of a "Loop Tuning" dial on the Bendix MN-5 coupler to mean that the coupler must be tuned to a particular radio frequency (with its max being 1500) and that it therefore couldn't receive a signal sent on 7500.  Based on much clearer photos below of a similar Bendix coupler, one can see that the "Loop Tuning" dial on a Bendix coupler is marked 1-100, which would suggest that it is not a means of tuning the antenna to a particular frequency.  Rather, when I read the instructions on the control box in the pic I attached, it suggests to me that you turn the Loop Tuning dial while trying to listen for a max signal, and that perhaps the turning of the dial makes some kind of internal tuning adjustments to the antenna while you zero in on the max signal (i.e. "tuning it").  Or perhaps it just sets something on the compass dials that help to read you a bearing? I didn't entirely understand the directions, but it seems pretty clear that they don't refer to setting a radio frequency with the Loop Tuning dial. Take a look at the directions on the coupler pics below and see what you think.

Just fyi, although these pics are of a slightly later model (1942), you can see that the Loop Tuning dial and its markings appear to be the same as the one you have posted. They are found here.

I am referring to the cockpit photo in my March 16th post which shows the loop aerial above the cockpit and the loop control box above the starboard pilots window. You are saying that this loop control box was removed along with the Bendix RA 1 receiver before the second attempt. Why do you think that the loop had a control box for use with the Bendix receiver but not with the Western Electric receiver? I think the loop control box was part of the loop aerial and did not count as a separate receiver.

The Miami Photo article which you linked to shows a photo of Earhart and the Electra attributed to Carapito Venezuela June 2nd or 3rd 1937 and the caption says no 'Box' is apparent. The unattributed picture attached here shows Earhart and Noonan boarding the plane and a 'Box' is visible above the pilots window. Earhart is wearing the same clothes in both pictures so I think this shows the loop control box fitted for the second attempt. The third picture shows Earhart and Noonan boarding the plane at Lae. The corner of the loop control box is just visible through the side window. 

Quote from: Colin Taylor on March 17, 2026, 11:53:05 AMThe loop control box has LOOP TUNING printed on it. It has a tuning dial and a band selector. I guess it needed to be connected to a receiver for signal amplification, audio output and BFO function. The receiver would need to be tuned to the same frequency but would be limited to the frequency range of the loop control box. The loop alone will give two nulls. Combining the signals from the loop and the static antenna does not prevent the user finding the null but it gives a single null giving an unambiguous direction.

I am not sure what picture you are looking at.

If you mean the control box that was removed from the aircraft after the crash at Luke Field, the labels on it are meaningless.

Let's examine some more excerpts of the research paper written by Frederick J. Hooven, the designer of the more sophisticated direction finder that had been originally installed on the aircraft.

QuoteA much improved type of radio direction finder had recently been developed and Miss Earhart had had one of them installed on her plane, but had removed it to save its extra weight of about 30 lb., the weight of five gallons of fuel. The improved device was then called a radio compass, although it was later automated and was called the "adf". It incorporated a superheterodyne receiver of the highest sensitivity, and receivers then were just as sensitive as the best receivers are today. It used a regular antenna in addition to its loop, so that the operator always heard the signal that was being tracked. Most importantly the combination of loop and antenna made it possible to provide a visual left-right indicator that gave a single, unambiguous direction for the signal.

 The improved sensitivity made it possible to use a much smaller loop than the older device, and in Fig. 2 may be seen the smaller, streamlined housing of the newer loop as it appeared when installed on the Earhart plane in October, 1936. Miss Earhart did not realize that this streamlined loop reduced air resistance to an extent that would have saved several times the five gallons of fuel that she had added by removing the radio compass and replacing it with the older device.

1. The Hooven system had its own receiver.

2. The Hooven system had a "much smaller loop antenna" under a streamlined cover.

3. The Hooven system used two antennas: "a regular antenna in addition to its loop."

4. The Hooven system receiver may or may not have had a limit of 1500 kcs.

5. The control box for the Hooven system was removed when the Hooven system was removed. If your picture is of the control box for the Hooven system, it is utterly irrelevant to the Bendix loop in use on the fatal flight.

6. The Hooven system did not require the user to "find a null" in the transmission. The system found the null, eliminated the ambiguity, and provided "a visual left-right indicator" that showed the direction the plane need to turn to fly toward the transmitter.

7. AE had intended to detect the null by ear as she turned the loop over her head. That would have been difficult even if she had given a frequency that would work with the Bendix loop due to the stress on her hearing after so many hours sitting so close to the tips of the propellor, which were very noisy.


Original Hooven system: two receivers, two antennas, and a control box. The null was found by the system and displayed visually.

Obsolete Bendix system: one receiver, tuned by the band selector and the dial that were used for voice reception, and one loop antenna in use for direction finding. The null was to be found by the operator listening to the strength of the transmission.



Here again is a link to the research bulletin that discusses the removal of the control box before taking off for Miami to begin the second attempt to fly around the world: