phy1020.DVI

• Citizen’s Band radio. This is a radio service available for use by anyone, with no license required.
  Citizen’s Band (or “CB”) radio enjoyed a huge (but brief) boom of popularity during the 1970s, when
  it was often used as a mobile radio service by drivers of cars and trucks. Today CB radio is much less
  popular, but is still used by truck drivers.


• Other uses.Various other radio services are available for use by police, fire, military, taxicabs, maritime
  and aircraft communications, etc.

Table 43-2. Some 50 kW clear-channel AM radio stations that can be heard from the east coast of the U.S.

f(kHz) Call Sign City

640 KFI Los Angeles

650 WSM Nashville

700 WLW Cincinnati

710 WOR New York

720 WGN Chicago

750 WSB Atlanta

760 WJR Detroit

770 WABC New York

780 WBBM Chicago

830 WCCO Minneapolis

840 WHAS Louisville

850 KOA Denver

870 WWL New Orleans

880 WCBS New York

890 WLS Chicago

1020 KDKA Pittsburgh

1030 WBZ Boston

1040 WHO Des Moines

1060 KYW Philadelphia

1090 WBAL Baltimore

1110 WBT Charlotte

1120 KMOX St. Louis

1160 KSL Salt Lake City

43.1 The Ionosphere

The distance that radio waves can travel depends strongly on their frequency. At some frequencies, radio
waves are able to reflect off of a layer of ionized gas in the Earth’s atmosphere called theionosphere. The
ionosphere actually consists of three layers, calledD,E, andF.^2 The lowest layer is theDlayer, above
that is theElayer, and the highest layer isF.^3 During the day, sunlight ionizes these three layers, turning
them into a plasma. At night, when the sunlight is gone, the ions in theDandElayers re-combine with
the free electrons, and these layers become neutral. TheFlayer is high enough that the gas is at a very low
density—low enough that the gas particles do not have time to collide and re-combine with the electrons, and
theFlayer remains ionized all night.
Radio waves interact with the ionosphere in different ways depending on their frequency. Shortwave
radio frequencies, for example, are able to travel through theDandElayers to reach theFlayer of the

(^2) Layers of the ionosphere were lettered starting withDto allowA,B, andCto be used for possible other layers that might be
discovered below theDlayer. No such layers exist, though.
(^3) TheFlayer splits into two layers (F 1 andF 2 ) during the day, and merges back into a single layer at night.