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.

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