Electra transmitter aerial polar diagram

[Colin Taylor]

Electra transmitter polar diagram

I have been puzzled as to why Earhart's radio transmissions ended abruptly after her last call. Here is an analysis of the Electra's dorsal transmitter aerial radiation pattern. 

If the signal strength was the same in all directions the value recorded by the ground station would depend mainly on the inverse square of the distance to the ground station. The difference from this ideal value will then depend on the aircraft aerial radiation distribution.

Each radio transmission is plotted according to the time and the aircraft position. Each is assigned a signal strength from 1 to 5 based on what was heard and transcribed. The range to the ground station and the relative bearing from the aircraft is measured.  The signal strengths are plotted against range on a graph and compared with a notional inverse square datum. This datum is a simple y=1/x² fitted to the strongest signals. Five of the signals are on this datum while the rest are attenuated with no correlation to range. Each recorded signal is divided by the value on the inverse square line corresponding to the range. That gives the best signal an attenuation value of 1. These attenuation values are then plotted as strength and direction on a polar diagram and duplicated symmetrically to give the radiation pattern.

There are of course many different factors affecting the recorded signal strength, not least the properties of the ground receiver and the atmospheric conditions. None the less the diagram illustrates the effect of the engines, propellers and the tail empennage on the signal strength. The emissions are particularly weak between relative bearings 160 to 200.

The results give an indication as to why the Electra transmissions were not easily heard by Lae when the aircraft was pointing tail-on to the ground station and how the signals became readable when the aircraft turned relative to the ground station despite the increased range. I heard that a group of radio engineers tested a model of the Electra and got a similar result.

It does explain why Earhart's radio signals diminished once she turned away from Howland during the search for the Island. It also suggests that she may not have ever turned back again to point at the Itasca. Or maybe the radio blew a fuse again!   

[Martin X. Moleski, SJ]

I have been puzzled as to why Earhart's radio transmissions ended abruptly after her last call. Here is an analysis of the Electra's dorsal transmitter aerial radiation pattern.

They stopped hearing her after she changed from her nighttime to her daytime frequency.

"KHAQQ to Itasca we are on the line 157 337 wl rept msg we will rept this on 6210 KCS wait, [(3105/A3 S5 (?/KHAQQ xmission we are running on N ES S line)]"

See Ric Gillespie, "Last Words:"

At 08:43 Earhart said, "We are on the line 157 337. Will repeat this message on 6210."

At 08:55 she said, "We are running on line north and south."

There may have been a further transmission as late as 09:00.

That change of frequency may account for the breakdown of communications.

[Colin Taylor]

Am I right in thinking that her transmissions to Lae were on the daytime frequency 6210?

Which is more likely to change the received signal strength, a change of frequency or a change of aircraft orientation relative to the ground station?

Colin

[Martin X. Moleski, SJ]

Am I right in thinking that her transmissions to Lae were on the daytime frequency 6210?

I think you are correct.

And Lae heard nothing from her for the first four hours of the flight.

That is why it may not be entirely surprising that the Itasca lost contact with her when she switched to her daytime frequency.

Bob Brandenburg did a lot of work on radio propagation.

https://tighar.org/Projects/Earhart/Archives/Research/ResearchPapers/Brandenburg/radioriddle.html

Which is more likely to change the received signal strength, a change of frequency or a change of aircraft orientation relative to the ground station?

I don't know.

Although "post hoc" does not necessarily mean "propter hoc," neither does it rule it out.

She was heard loud and clear on the nighttime frequency.

As soon as she announced she was switching to her daytime frequency, they heard nothing from her on that frequency.

There seems to have been one last broadcast on 3210 that caught the radio room off-guard.

[Colin Taylor]

Brandenberg says that the maximum closest point of approach was 80nm; in other words, they got within 80 miles of Howland. He then says that the point of intercept of the 157/337 line of position was 80 nm from Howland at 1915GCT. This because the next transmission at 2015GCT must be 200nm away based on signal strength (1 hour at about 120 kts). But the 2015 signal was recorded as S5 in the radio log whereas the earlier '...200nm to go...' signal (possibly 250nm from Howland) was S3 (readable with difficulty). Therefore, the supposed 80nm intercept must be wrong.

Now, looking at the data in Table A1, the signal strength on 3105khz reduces with time (1900 to 2000GCT) but increases with reducing distance (150 to 100 to 50nm). If they flew away from Howland the radio signal at 2015GCT would have been weaker due to both increasing distance and the passage of time. Therefore, because the signal strength was maintained from 1915GCT to 2015GCT they must have been flying towards Howland during that time. So, the point of interception of the 157/337 line of position must have been at least 1 hour's flight time from Howland plus say 20nm since they did not see the island. Say 140nm.   That supports my exposition in Drift in the Dark.