Frequency, wavelength, and antenna tuning

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Low frequencies, long wavelengths. High frequencies, short wavelengths.
Much of the discussion of the technical difficulties that Earhart and Noonan faced on the final flight involves consideration of frequencies and wavelengths used and the corresponding antennas and equipment used to generate or receive the various frequencies/wavelengths. The technical evaluation of the likelihood of whether a radio in St. Petersburg, Florida could have received transmissions from an aircraft stranded in the Western Pacific also uses this fundamental terminology. If you want to follow those arguments, you must familiarize yourself with these terms.

Contents

Frequency and wavelength

There is an inverse relationship between frequency and wavelength: the lower the frequency, the longer the wavelength; the higher the frequency, the shorter the wavelength.

The frequency of a wave is measured in cycles per second. Many of the historical records use "kcs" as an abbreviation for "kilocycles per second"--where "kilocycles" is "1000 cycle." At present, we use "Hertz" to stand for "cycles per second."

Abbreviations

cycles per second older system contemporary abbreviation
1 1 cycle 1 Hertz 1 Hz
1,000 1 kilocycle (1 kc or kcs) 1 kiloHertz 1 kHz
1,000,000 1 megacycle 1 MegaHertz 1 MHz
1,000,000,000 1 gigacycle 1 GigaHertz 1 GHz

Examples

7500 kcs = 7500 kHz = 7.5 MHz

2.4 GHz = 2.4 GigaHertz = 2.4 billion cycles per second (some cell phones and similar appliances)

Optimizing an antenna

Radio waves are transmitted from the movement of an electromagnetic wave along a conductor--the antenna wire. If we had unlimited resources, it would be best to have an antenna that was at least one wavelength long. This is not too difficult with high frequences (short wavelengths--or "short waves"). Popular HAM radio equipment operates on wavelengths of 2 meters, 6 meters, and 40 meters; it is not too hard to rig full-wave antennas for these wavelengths. A full-wave 2.4 GHz antenna is only 4.92" long.

If one cannot afford the space for a full-wave antenna, the next best thing is to use an antenna that is a half-, quarter-, eighth-, or sixteenth-wavelength.

Tuning coils can compensate to some extent for antennas that are not an even fraction or multiple of a wavelength.

Mike Everette, 23 October 2000 Forum.

If an antenna is a non-resonant length (as Earhart’s was) it must be tuned to compensate for its shortcomings if it is going to radiate power. Earhart’s radio had a limited capability to properly match, or tune, the antenna. The design of the unit was one which definitely encouraged harmonic radiation and a mistuned antenna further aggravated the potential for doing so.

Earhart's frequencies and antennas

As a general rule, TIGHAR prefers to use the metric system. In this table, the units are given in feet to match the units used in the sources. The old abbreviation "kcs" is used because that is what appears most often in the sources (where it is not taken for granted).

Wavelengths and antennas

The frequencies most often referenced in discussions of the final flight.
Frequency Full wavelength (meters) Full wavelength (feet) 1/4 wavelength (feet) Optimum antenna Actual antenna
500 kcs ~600 m (599.5849) 1968' 492' 246' 250' trailing antenna
"   "  "    "   61.5 46'/54' dorsal antenna
3105 kcs 96.5 m 316.9' 79.2' 39.6' 46'/54'
6210 kcs 48.28 m 158.4' 39.6' 39.6' 46'/54'
7500 kcs ~40 m (39.972) 131.2' 32.8' n/a n/a

7500 kcs was not a transmission frequency for Earhart. The radio equipment on NR16020 transmitted on 500 kcs, 3105 kcs, and 6210 kcs. The columns for optimum and actual antenna lengths are concerned with the Electra's transmission setup.

500 kcs frequency

500 kcs (or 500 kHz) was the universal emergency and calling frequency for ships at sea (once contact was made between two stations, they would switch to another frequency so as to clear the band for other traffic).[1] This meant that it was monitored continuously. Its traffic was all CW/Morse code. There would have probably have been no point in making a voice transmission on that frequency: "No one monitoring 500 would have been listening for/expecting a voice signal on 500."[1]

"The frequency of 500 KHz was essential for the round-the-world flight. Until the middle of World War II, this was the only universally monitored (guarded) distress frequency. Ships at sea, and maritime shore stations, were required by international regulations to maintain watch on 500 KHz. During specified parts of every hour, radio silence would be observed in order to listen for stations in distress. At all other times, 500 KHz was a calling frequency. Ships, shore stations and aircraft made initial contact on 500, then moved to other working frequencies like 410 or 425 KHz."[2]

"The 500 KHz frequency was not only a distress channel, but also a 'calling frequency.' Steamship companies maintained communications with certain coastal stations and these actually were conducted on 'working' frequencies like 418 or 425 KHz. However, the ships all were required to guard 500. So, to establish initial communication, a common 'calling' freq was essential ('I'll call you on 500, then we'll move off to 425' etc) and 500 was the logical choice.

"There was LOTS of traffic on 500, at all times.

"'Silent periods' were, however, required to be observed... about every 15 minutes, for at least 3 to 5 minutes. During those periods, operators were required to listen for transmissions from ships (or aircraft) in distress.

"Of course, a distress message could come at any time. If one was heard, everybody on the freq was required to QRT (cease transmitting) at once! Except for the station in distress, and whoever was in direct QSO (contact) with him."[3]

Daytime/nighttime frequencies

Notice that Earhart's daytime frequency (6210 kHz) is double her nighttime frequency (3105 kHz). The wavelength is halved as the frequency is doubled. This means that one antenna could serve both frequencies fairly well. A quarter-wave antenna for 6110 kHz would act as a one-eighth-wave antenna for 3105 kHz.

The reason for having a daytime frequency different from the nighttime frequency is that the ionosphere affects radio wave propagation. An old rule of thumb is "The higher the sun, the higher the frequency."

References

  1. Mike Everette, 7 Sep 2000 Forum.
  2. Mike Everette, A Technical Analysis of the Western Electric Radio Communications Equipment Installed on Board Lockheed Electra NR16020.
  3. Mike Everette, 20 October 2000 Forum.

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