Radio propagation

Wikipedia article: "Radio Propagation."

• Radio waves can travel short distances in a straight line (line-of-sight propagation).

• Under certain conditions (time of day, frequency of the transmission), they also can bounce off various layers of the atmosphere and travel surprisingly long distances, even with a weak signal (ionospheric propagation, a.k.a skywave propagation).
  • Betty's notebook can only be authentic if the conditions were right for the signal to bounce off the ionosphere and reach her home in Florida. See Brandenburg's article, Harmony and Power: Could Betty Have Heard Earhart on a Harmonic?
  • daytime/nighttime frequencies The bands of the atmosphere that can and do reflect radio waves are higher in the daytime and lower at nighttime. The old maxim for best long-range transmission was, "The higher the sun, the higher the frequency." This is why the transmitter on the Electra had two voice frequencies, 3105 kHz and 6210 kHz.
• "You can see a graph of past solar cycles here. Look for the "Monthly Sunspot Numbers 1900-1999" graph. The higher the sunspot number, the better for radio wave propagation" (Brookner, Forum, 2010-12-22).

Excerpts from "The Chater Report":

Local interference prevented signals from the plane being intelligible until 2.18 p.m. The Lae Operator heard the following on 6210 KC –“HEIGHT 7000 FEET SPEED 140 KNOTS” and some remark concerning “LAE” then “EVERYTHING OKAY”. The plane was called and asked to repeat position but we still could not get it. The next report was received at 3.19 pm on 6210 KC – “HEIGHT 10000 FEET POSITION 150.7 east 7.3 south CUMULUS CLOUDS EVERYTHING OKAY”. The next report received at 5.18 p.m. “POSITION 4.33 SOUTH 159.7 EAST HEIGHT 8000 FEET OVER CUMULUS CLOUDS WIND 23 KNOTS”.

Miss Earhart had arranged to change to 3104 KC wave length at dusk, but signals were very strong and the plane was then called and asked not to change to 3104 KC yet as her signals were getting stronger and we should have no trouble holding signals for a long time to come. We received no reply to this call although the Operator listened for three hours after that on an 8-valve super-heterodyne Short Wave Receiver and both wave lengths were searched.

It was presumed the plane had changed the wave to 3104, the reason for that being that Miss Earhart claimed it to be a better night wave than 6210 and had used it on her flight from United States to Hawaii previously.

• Radio waves can also bounce off the ground, interfere with the line-of-sight waves, and affect reception (groundwave propagation).

"I will foolishly tread forward here. Perhaps the phenomenon of 'skip zone' was responsible [for no further transmissions being heard after 8:43 AM, when Earhart switched to her daytime frequency of 6210 kHz]: too far for good reception of ground wave, or direct wave, and not far enough away to receive her signal via skip propagation (sky wave.) I think i have seen old propagation charts which actually give distances for the skip zone, a dead zone of no reception, around the transmitting station."

Hue Miller, 23 February 2009 Forum

3105 Donut

TIGHAR Tracks 24 (2008) 3

Recent software advances have made it possible to computer model the propagation properties of the Electra’s transmitting antenna to an unprecedented degree of accuracy.

As a result, the long-held assumption that the closer the plane was to Howland Island the the stronger the signal heard by the Coast Guard would be, has been shown to be incorrect. A peculiarity in the antenna’s transmission pattern meant that if the plane was closer than about 80 nautical miles there was less than a 10% chance that Itasca would hear Earhart on 3105 kilocycles at maximum strength as recorded in the cutter’s radio log. Chances are the Electra was at least 80 and perhaps as much as 210 nautical miles from the ship at the time of the last transmission.

At 08:43-55 local time Itasca heard Earhart say, “We are on the line 157 337. Will repeat message. We will repeat this on 6210 kcs. Wait. We are running on line north and south.” The message came in at maximum strength. Given a newly discovered anomaly in the propagation pattern of the aircraft’s transmitting antenna, to have even a 10% chance of being heard at maximum strength, the Electra had to be somewhere within the “donut” shown. If on the line southeast of Howland, the plane was much closer to Gardner Island (Nikumaroro) than previously assumed.

6210 Problems

No transmissions were heard from Earhart on her "daytime frequency" of 6210 kcs until four hours after takeoff. Chater attributed this to thunderstorms, but it also may be that this is evidence of a "donut" in the transmissions on that frequency. The cause of such a big donut hole in the transmission pattern may lie in the history of modifications made to the antennas on NR16020. Each antenna has its own characteristic radiation pattern.

Skywave vs. Groundwave propagation

John Rayfield, Jr., Forum, 15 February 2015

On the frequencies that were being used by Earhart, the idea of the signal being stronger as the transmitter moved closer to the receiver might only apply to ground wave propagation and not skywave propagation. If the Itasca was receiving Earhart via skywave propagation, then Earhart could have been moving either closer to, or further away, from the Itasca as the signal increased in strength.

Several factors would influence the range achieved with skywave propagation, including

• the radio frequency being used,
• the time of day,
• the time of year,
• the 11-year sunspot cycle (the amount of sunspot activity on the sun),
• the presence (or lack of) solar 'storms',
• the altitude of the airplane,
• the design and location of the antenna on the airplane,
• and effective radiated power (ERP) of the transmitted signal from the airplane.

I've never thought that the Itasca was receiving Earhart, at the strongest point, via ground wave propagation. Of course, that was the assumption by everyone at that time, and even still today by many people. But, just because the received signal was very strong was not an 'absolute' indication that she was close to the Itasca. Based on the fact that nothing was found in searching within the distance from Howland Island, that ground wave propagation would have existed, could indicate that she was much further away from Howland Island than was 'assumed' at the point of strongest received signals.

Dave Ross Wilkinson, Forum, 2015-02-16

That's exactly my experience listening on the 20 meter ham band (except I was operating in Indiana). I had figured the polarization of the transmitted signal a lot to do with it, as well. Vertical polarization for ground wave, and horizontal polarization (e.g. a 20m beam) for skywave.

William G. Torgerson, Forum, 2015-02-18; corrected.

Another factor affecting radio propagation is the Solar Cycle (because it affects the ionosphere). Solar Cycle 8, which started in September of 1933, and ended in January of 1944, reached a peak in April of 1937 (not so far from July). Although the effect of the solar cycle is generally considered 'negative', these cycles can cause unpredictable events, especially in the HF bands.