Something does not add up

[Diego Vásquez]

The 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.

The 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.

[Colin Taylor]

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?

[Diego Vásquez]

If 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."

Is 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.

[Colin Taylor]

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

[Diego Vásquez]

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. 

[Martin X. Moleski, SJ]

Colin, I think this conversation came to an end a few posts ago when you wrote, "So, I am inclined to agree that there was only one receiver, hence only one ventral aerial and the loop control box was connected to the WE receiver."

I do not see what you see in the photos.

You have not shown the face of the control box in a reliably dated photo.

I am of the opinion that the loop antenna did not require a separate control box from the receiver.

The crank to turn it was above AE's head, directly under the loop.

The controls to tune the radio and to select antennas were elsewhere in the cabin.

If you agree that there was 1 receiver and that there were 2 different antennas, we are in agreement.


I find the sequence of pictures in Ric's article persuasive. Your photos do not persuade me that you are seeing the same box that Ric showed in the so-called "Miami Cockpit photo." He shows the position of the box in the cockpit and then shows a picture taken from the same angle later that shows that that box is missing.

The more modern Hooven system needed a control box because it did have its own receiver that needed to be tuned to the desire frequencies.

The obsolete Bendix system did not have its own receiver and was tuned through the normal radio controls, as explained in the Hooven Report.

[Colin Taylor]

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.

[Randy Jacobson]

The only time AE was listening on 7500, she was using the loop antenna for directional finding.  All other times, she was listening on 3105 or 6210.  The only time she reported hearing something was on the 7500 and loop antenna.

[Martin X. Moleski, SJ]

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

That has been well and thoroughly answered.

AE's one and only receiver could be turned across a full spectrum of usable frequencies.

You can click on this link to read more on this topic than I am quoting here: Radio Equipment on NR16020

Western Electric Model 20B

"The receiver aboard NR16020 was a Western Electric Model 20B. This receiver was designed for communications purposes. It contained no circuitry to enable its use as a navigation receiver.

"The tuning range was divided into four bands. Originally these were:

Band 1, 188-420 KHz (beacon and marine);

Band 2, 550-1500 KHz (standard broadcast);

Band 3, 1500-4000 KHz;

Band 4, 4000-10000 KHz.

"As the requirement for 500 KHz operation existed in Earhart's case, the Band 2 tuning range was factory modified to 485-1200 KHz, covering the lower frequencies at the expense of the upper part of the broadcast band. A 1939 source lists a Model 20BA receiver, with Band 2 covering 485-1200 KHz. Earhart's equipment may have been the prototype for this off-the-shelf model.

"The Model 20B receiver was a remote-control model, with tuning dial, band switch, volume control and other controls located in a Model 27A remote control head linked to the receiver by means of tach-shafts. The remote head was mounted in a center console below the instrument panel in NR16020; the receiver itself was mounted beneath the right seat in the cockpit."

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.

There is an amazing wealth of discussion in this Forum and on the website about the oddities of radio reception.

This is a link to a table of all of the transmissions logged during the last flight: Transmissions heard from NR16020

No transmissions were heard from AE on her daytime frequency for the first four hours of her flight.

Presumably she was transmitting at the times she had planned to do so.

Apparently, there was something odd about her daytime frequency that meant it was hard to receive it when she was close to the receiver.

It is not odd at all that the Itasca could hear her with increasing clarity and volume when she was on her nighttime frequency but heard nothing when she switched to her daytime frequency.

You can click on this link to learn more about the discussion: Radio propagation

So here again is the set of facts that we have covered:

1. There was only one receiver on board during the fatal flight.

2. The loop antenna did not have its own receiver.

3. The one and only receiver on board could be connected to the loop or to the belly antenna.

4. The only transmission AE heard was on 7500 kcs while using the loop antenna to attempt to find a null.

5. When her system was configured to hear voice on her daytime or nighttime frequencies, she never heard any transmissions.


Inference: the DF loop antenna received the Morse code on 7500 kcs. There was something wrong with the belly antenna itself or the circuit that was supposed to connect it to the receiver.

Which is essentially what Randy Jacobson said in his post.

Dr. Jacobson is one of the most remarkable people whom I have met through TIGHAR. Not only does he possess excellent technical credentials but he has spent countless hours transcribing every message about the fatal flight from government archives and designed a Monte Carlo simulation to test various theories about weather conditions and fuel consumption.

When Randy speaks, I listen.

This is a link to a page about him and his work: Randall S. Jacobson, Ph.D.

His work has been phenomenal.

No one has done more than he has to fill in the blanks about the history of the final flight.

[Randy Jacobson]

Oh my....Marty: your $1000 check is now in the mail! 
Thank you for your kind words.

[Martin X. Moleski, SJ]

Oh my....Marty: your $1000 check is now in the mail! 
Thank you for your kind words.

The kind of respect and gratitude that I feel toward you can't be bought.

You did outstanding work. We are all in your debt!