Estimates of signal strength

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Signal Strength Quantifiers

Mike Everette, October 22, 2001, Forum.

As for reported signal strengths in terms like "S-5" this really does not have any relationship to the meter reading. In fact a signal reading high on the S-meter scale can still be covered up in strong noise.

The operator in those days used a scale of 1 to 5.

  • S-1 means "signal very weak, in the noise, barely intelligible if readable at all."
  • S-2 means "weak signal, still in noise, but readable, maybe with difficulty."
  • S-3 means "signal stronger, noise, but readable."
  • S-4 means "fairly strong signal, some noise, very readable."
  • S-5 means "very strong, clear signal."

This is SUBJECTIVE. There is a LOT of room for interpretation. An operator's ears might tell him/her that a signal is S-1 if any appreciable amount of noise is heard. Or, that a signal is NOT S-5 if there is ANY NOISE AT ALL heard. It's a judgment call. Different operators hearing the same signal may disagree.

Estimates of signal strength on the final flight

Ric Gillespie, January 9, 1999, Forum.

In the report submitted by Air Corps Lt. Daniel Cooper on July 27, 1937, there is a summary of the strength of transmissions that the Itasca received from Earhart on the morning of July 2. All times are local.

Time Quality S-code Highlight of transmission
0345 very faint S1 "Will listen on hour and half on 3105..."
0445 faint S2 (not confirmed by log)
0600 fair S3 "Two hundred miles out..." (actually rec'd at 06:15)
0646 good S4 "About 100 miles out.."
0741 very loud S5 "We must be on you but can not see you..."
0750 very loud S5 "We are listening(?) but can not hear you..."
0758 very loud S5 "We rec'd your signals but unable to get a minimum."
0843 very loud S5 "We are on the line 157 337"

"The radio operator reported that from 0741 on, her signal strength was at a maximum and judging from her volume, she was practically over Howland. All this seems to indicate that she passed close to Howland, probably within 50 miles" (Cooper).

This sounds like a steadily strengthening signal over a period of five hours.