Radio equipment on NR16020

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The primary focus of this article is the kind of radio equipment carried aboard the Electra on the final flight. The question of the antennas used on the aircraft is treated in a separate article.

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Subject: Sad Story of Confusion Date: 5/12/00 From: Hue Miller [1] I have been rereading the radio logs at "logjam.html" site and the more I read, the more moved I am by how sad a story of confusion this is.

AE's Bendix, RDF adaptor apparently included an innovation included in the U.S. Navy's 'next generation' of df-adaptors ( series 'DU' ) : HF (shortwave) coverage. Unfortunately, she was under the impression this equipment was standard, including USCG ships. She had tried it out unsuccessfully at Lae.

William Donzelli tells me he read some Navy text stating that the HF-df feature of this later appearing model adaptor (DU), was found simply not to work, and was dropped in later production of this model. So all those RDF efforts were simply a waste of precious attention and time. I also wonder if this innovative feature was somehow linked with the talk about the 10E's RDF equipment being secret or classified.

AE could have homed successfully on Itasca, using the same RDF, but the Itasca would have had to transmit on the conventional DF bands- 200 kcs to 1600 or so. The ship certainly had equipment to do this, as all seagoing ships had equipment covering at least 400 -- 500 kcs with a few hundred watts power. If AE were to send df signals to the Itasca, on the other hand, she would have needed that trailing antenna to be able to use the lower frequency.

I wonder: if AE had all along been unaware that Itasca could not send voice on 7500, did she simply not hear the toneless rushing pulses of cw til she was quite close and the signals were stronger, quite strong? What I am wondering is, expecting voice on 7500, did she fail to have the receiver switched to CW on the CW/VOICE switch? Out a few hundred miles, with cabin noise, and electrical noise from the 10E engine, and atmospheric noise, it could be difficult to hear the the cw signals unless the receiver, via this switch, was set to reproduce them as sounds with an audio pitch. (The receiver is usually tuned so the "CW note" is somewhere in the range 500 -- 1000 cycles. ) BTW, the conventional wisdom was that CW was "10 times as effective" as voice, so IF the propagation & skip path allowed, you would certainly think the Itasca's 7500 kcs cw signals would have gone the distance, at least of a few hundred miles.

Also, I do not agree that AE at the last switching to 6210 was a fatal mistake. 20:00 hours is still early enough that this "day" frequency is still viable, and since you do not know the location of the aircraft, you do not know if the "skip zone" of either frequency favors or disfavors communication.

I am thinking that all along, there should have been more knob-twisting to arrive at the best frequency. I am also a little puzzled as to why a wide divergence in frequencies used by the 2 parties, over most of the flight: I would think the best one would be the best one both ways. Also, it kinda surprises me the USCG cutter didn't have equipment to specifically talk to aircraft -- if the Itasca's voice equipment topped out in the 6000 kcs band, that sounds like it was the usual medium wave ship voice radio, usable about 1600 to 6000 and some kcs. But this was 1937, still pretty early in the communications game.

The radio log and commentary are at Logjam. More "information" is also in an article in Insight magazine which summarizes the theories, including the far-out "Japanese capture" one, and the focuses of the different search organizations.

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Message: 6 Subject: Random Radio Ramblings Date: 9/18/00 From: Hue Miller [2]--double check

Vern writes:

> Past postings suggest that it was not uncommon to use a dedicated receiving > antenna, such as a belly wire, while an antenna on top, or a trailing wire, > was used for transmitting. It is my understanding that this was practical > with relatively low-power transmitters and vacuum tube receivers. Vacuum > tubes were much more tolerant of RF voltages from the transmitter than are > present day solid-state circuits.

Right, the WE receiver also has a neon tube in the front end to discharge too-high voltages, either static or signals, coming from the antenna.

> Conclusion: The belly antenna may well have been the receiving antenna. It > may also have been the sense antenna for the RDF. The topside "V" antenna > may have been use for transmitting only -- no need for wiring from the > receiver to the transmitter T/R relay terminal.

--That’s the way i see (guess) it, it was inherited from the trailing-antenna days of her plane, and still offered the advantage of avoiding longish inside antenna lead-in runs.

> > QUESTION: What about the the two antenna terminals (HF and LF) on the > Western Electric receiver that are switched depending on the frequency band > selected? > > If you have no "designated" LF antenna -- like if you’ve done away with the > trailing wire antenna, as had AE -- then you connect the two antenna > terminals together and to whatever you use for a receiving antenna, via the > T/R relay in the transmitter, or not. Now this antenna is functional on all > bands. > QUESTION: Was the Bendix coupler unit necessary whatever receiver was used > with the Bendix RDF loop? > > Yes, a coupling unit of some sort was pretty much necessary. It was > necessary to transform the balanced, high-impedance, of the loop to the > unbalanced, low-impedance, input of the receiver. Some amplification was > also desirable due to the inefficiency of the small diameter loop compared > to a more conventional, relatively long, wire antenna.

--I don’t think "coupler" is the best term for the Bendix RDF unit. "Coupler" better applies to the WE product, which performed this function between the WE untuned (no tuning, also limited "upper end" of frequency usability). The Bendix thing was more a tuning unit/ amplifier; "Adaptor" better suits its description, as it contained part of the tuning circuit, the rest being the loop itself; also it held the 2-tube (equivalent) pre-amp. Also, small point, Bendix sez the amplifier was to make up for losses between the adaptor and the receiver proper in the coaxial connecting cable.

> QUESTION: Is a sense antenna necessary for operation of the Bendix RDF loop > and coupler unit? > > No. The sense antenna serves to resolve the 180 degree ambiguity problem of > the loop. There are other ways (non-electronic) to resolve this ambiguity. > If one chose not to use a sense antenna, he would simply ground the sense > antenna terminal of the coupling unit. Or leave it unconnected.

--Makes *no* difference in the "B" (bearing, i.e. null, position of the RDF’s function switch. Only comes into play in the "D" (direction position). I think if you left off the antenna, and tried to "D", you’d get the same response as in the "B" position (if infact the bearing, null thing was working.)

> probably wouldn’t make much difference inside a metal airplane. Now the > loop exhibits the typical two-null response of a simple loop antenna -- the > figure "8" sort of response. This avoids some problems in getting > everything properly adjusted so the bearing obtained is a correct bearing. > (Phasing and amplitude of the two signals must be right.) The simple loop > is subject to fewer problems once it has been calibrated (bearing-wise) for > the particular aircraft and the particular installation. > > QUESTION: Was the Bendix RDF loop coupling unit on AE’s plane similar to > the RDF-1 for which we have a schematic diagram and description? > > There is little doubt that it was similar but, if we assume it was the unit > described in the Aero-Digest article, there were certainly differences. > This does appear to be the unit seen in some of the photos of AE with the > loop in her hands. To my knowledge, we do not have a schematic of the > Bendix unit, nor do we have photos good enough to do much educated guessing > about it. > > The RDF-1 is described as being designed to simplify the switiching between > the various functions available. AE may not have had benefit of this more > simplified switching. We simply do not know what all she had to do to > change over from normal communication receiving (which wasn’t working) and > RDFing.

--??? the RDF unit has positions for R, B, D. In "R" it uses the wire antenna, amplifies it with modest amplification, and sends the signals along to the receiver. With the RDF installed, i don’t see any need for additional receive antenna switching....

I suggest the circuit was quite similar to the RDF-2, for which we do have the schematic. No reason to suppose the circuits were substantially different. The chief area of uncertainty, and the killer, is the actual tuning range of the unit installed for AE.

> Whatever she did, she did hear the Itasca signal on 7500 kc. She reported > that she could not get a null. Might she have been able to get a null on > that frequency? Did she just not try hard enough? AE seemed to have the > idea that a radio bearing could be determined rather quickly.

--Well, it’s not magic, nor brain surgery. If you can get a clean null, one turn around of the loop antenna will see it. As i see it, if she could not find a null in short order: shortwave propagation was unsettled, via skywave, especially in those hours, or the RDF was somehow hooked up wrong . And i’m momentarily stumped as to what the "wrong" configuration might be like.

>In her > repeated requests for the Itasca to take a bearing on her, she was never on > long enough for a bearing to be taken. I can imagine her giving her loop a > rapid turn one way then the other and concluding that she couldn’t get a null.

--Under those conditions i don’t think she would have made one "rapid" sweep and given up. One "deliberate" turn around and you know if you’re gonna get a null, on LF. On HF, via skywave, depending on conditions and distance, you can get a mushy not real distinct null one minute, no null the next, no null at all ever, or maybe even, less likely, a real null. As i see it.

> QUESTION: Could AE have expected to get a null on 7500 kc? > > Frequencies above the AM broadcast band (up to about 1500 kc in that time) > were not generably considered usable for RDFing. Since she did hear the > Itasca, apparently via the loop, would the loop have exhibited its normal > directional characteristics? Should she have been able to get a null? > > There is one instance, that I know of, when a person familiar with radio and > DF loops of that time was asked that question. It’s purely anecdote and a > rather off-hand response. When asked whether a loop such as AE had would > give a null at 7500 kc, his response was: "Sure it would."

--And I’d say, "Sure it maybe would". Depends.

> One thing that has long bothered me is the matter of the automatic gain > control switch on the receiver and its remote control unit. Did she get > that turned off before trying to get a null? If the Itaska signal was > strong, she would not have got a null with the automatic gain control > functioning. (It’s labeled AVC on the receiver for "automatic volume > control." AGC is the modern, more generic term.)

--I think you should rest your mind regarding the AVC. It doesn’t work like some computer controlled effect. I do not believe it being on, would totally mask the null. Your concern, IMO, considers the AVC feature too effective.

> I wonder if that might be the genesis of AE’s idea that she couldn’t get a > null when too close to the transmitting location -- such as at Lae? Maybe > nobody had ever got through to her that she needed to turn off the AVC switch.

--However, she apparently had successfully used the thing in the past, right? My opinion (only) is that at Lae, she would have had to stay right over the Lae station antenna for this effect to have spoiled the experiment. ( IF i could get organized, i would hook up my DU unit to my RA-1 and try to replicate the condition, leaving AVC on... project # 1001-B )

Ric writes:

> Very nice summary and I agree with most of it (which is why I think it’s very > nice). > > I will point out that Amelia DOES say where the Bendix direction finder is. > She lists it as being on the instrument panel among her "navigation > instruments" such as "compasses, directional gyros, the Bendix direction > finder, and various radio equipment."

--This puzzles me. how do you mount such an instrument on the instrument panel? Look at the photo (Longs’ book). This photo shows the mounting plate with its fasteners, atop the unit, for attaching to a superior surface. ( opposite of deployment of Navy models of the RDF )

Hue Miller From Ric

There is an Al Bresnick photo, taken shortly after the Bendix loop appears on the airplane, that shows AE standing on the wing beside the open cockpit hatch. Bresnick was standing, possibly kneeling, just in front of the port engine and shooting up at a steep angle. A rectangular object can be seen that appears to be the size and shape of the Bendix adaptor installed on the eyebrow panel in about the same location as the much larger box that appears in Long’s photo.

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Date: Sun, 4 Jun 2006 21:01:18 From: Hue Miller Subject: Re: Howland Island DF

> Anyone seen other correspondence relating to this?. > By the way, the SF Chronicle reported that AE carried two 50 watt > transmitters, and listed her call letters KHAQQ. [ Ibid, p. 200]

Well, they got the callsign right, but the reporter managed to scramble the rest, which was 2 channel 50-watt output. ( And only a fraction of that is radiated; in the horizontal part of the antenna, the radiation is cancelled against the currents in the airframe, so the only radiating part is the vertical rise, which is not a whole lot.) A trailing antenna would have vastly improved her voice communications also, but would have been trickier to tune, because you either have to watch the current meter closely while you reel out the wire, or you have to have an accurately re settable counter on the reel. But that would have been a one time thing, once in the air, and it sure would beat fiddling with direction finder on the shortwave channels, in a race against the clock.

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Ric Gillespie, June 21, 2000, Forum.

1. When the airplane was delivered to AE in July 1936 it had a WE 13C transmitter and a WE 20B receiver, the same radios (or so it would seem) that it had when it disappeared. However, there was no dorsal antenna on the airplane at all. There was a belly antenna identical to the one that was apparently lost at Lae with a lead-in that entered the fuselage right under the copilot's seat where the 20B receiver was mounted. The only other antenna on the airplane was a trailing wire that deployed from the extreme tail of the airplane, just under the navigation light. At that time the airplane appears to have no DF capability at all. What, in your opinion, was the function of the belly antenna at that time?

2. Sometime around October 1936 the Hooven Radio Compass was installed. This involved a separate receiver mounted on a fuel tank in the cabin, a dome-shaped antenna on the cabin roof, and another belly antenna that ran parallel to the original belly antenna but on the opposite (port) side of the airplane. The trailing wire in the tail remained unchanged. What, in your opinion, was the function of the new belly antenna? What, in your opinion, was the function of the original (starboard) belly antenna at this time?

3. In mid-February 1937 Earhart flies the airplane to New York to announce her planned World Flight. While she's on the east coast Western Electric installs a new dorsal vee antenna. All the other antennas remain unchanged. What, in your opinion, was the purpose of this antenna?

4. Back in California in late February the trailing wire is moved from the extreme tail to deploy from a mast under the cabin. Right around March 1st comes the big change in DF equipment. Hooven's Radio Compass and it's domed-shaped antenna go away and are replaced by the Bendix loop over the cockpit. The belly antennas -- both port and starboard -- remain unchanged. What, in your opinion, is the function of each of these antennas at this time?

5. The airplane goes to Hawaii, gets wrecked, and comes back to Burbank for repairs. When it come out of the shop several changes are apparent in the antenna set-up.

• the dorsal vee has been lengthened by moving the mast forward.
• the trailing wire is gone.
• the port side belly antenna (that had been added when the Hooven DF was installed) is also gone.

Unchanged are the Bendix loop over the cockpit and the starboard side belly antenna. What, in your opinion, is the function of the belly antenna at this time?

There is no change to the airplane's antenna configuration while it is in Miami or later (until the belly antenna gets knocked off in Lae.)

Related articles

• NR16020 antennas.
• The Hooven Report.