8. "Would you explain how CB (citizens band) radios and VHF radios work? What kind of waves are they?"
CB radio and VHF radio waves are members of the electromagnetic wave "family" and so they travel at the speed of light. CB radio waves have frequencies of about 27MHz (27 x 106 Hz), i.e., with wavelengths of about 11m, and VHF radio waves have frequencies around 156-157MHz (156-157 x 106 Hz), i.e., with wavelengths around 1.9m.
CB radio is a two-way voice communication service for private individuals in automobiles, trucks, homes and offices. A typical CB radio consists of a transmitter-receiver, called a transceiver, and an antenna for transmission and reception. The communication range is normally up to about 15 miles although sometimes "skip" occurs, see below, that can extend the range very considerably. CB radio was initiated in the US in 1947 and was allocated the 27MHz band in 1958. Although open to the public there are some rules, for example, over the position and height of antennas and there is a limit on the power output of transmitters (4W). CB radio covers a designated range of frequencies, from 26.965MHz to 27.405MHz and there are 40 channels - equivalent to the "stations" in the AM and FM bands - each separated by 0.010MHz (i.e., 10kHz), the same as the separation of stations in the AM radio band. None of these channels is assigned to a specific individual or organization although channel 9 (27.065MHz) is the emergency channel that is monitored by highway patrol and police departments and citizen volunteer groups.
The production of CB radio waves is very similar to AM waves, i.e., it involves amplitude modulation. The signal, i.e., the voice, is used to modulate the amplitude of a carrier wave, which in this case is one of the 40 channels in the CB radio band. So, in order to receive the signal, the receiver will have to be tuned to the same carrier wave frequency (channel) as the sender (see question 7). There is some interesting physics in the problem that becomes clear if we take a simple case ... imagine a signal of a single frequency ws that modulates a carrier wave of frequency wc, as shown below.
Using a trignometric conversion we can write the combined AM wave as a sum of two sine waves! So, our AM signal can be represented as two waves with frequencies (wc-ws) and (wc+ws), as shown below
These two waves are called "side bands" and when the receiver is tuned to correct carrier wave frequency, or channel, wc, then de-modulation allows us to recover the signal (with frequency ws). It is not necessary to use both side bands to recover the signal, the use of a single side-band (SSB) is sufficient, and this is the basis of SSB radio. There is a limit on the separation of the two side-bands; it is called the bandwidth and in the case of CB radio (and AM radio) because the signals from two channels (or stations) must have unique frequencies, the bandwidth is limited to 10kHz, i.e., ±5kHz above and below the carrier wave frequency. So, signals with frequencies greater than 5kHz cannot be transmitted by CB radio; fine for voice ... lousy for music!
Normally, the range of CB radio is up to about 15 miles but CB waves can be reflected by the ionosphere around the Earth. The ionosphere is a region in the atmosphere starting about 50km above the Earth's surface where there is a large concentration of free electrons and positively charged ions. It's existence was proposed independently by Arthur Kennelly (1861-1939) and Oliver Heaviside (1850-1925) to explain how Guglielmo Marconi (1874-1937) was successful in sending radio waves over great distances around the Earth. Since radio waves travel in straight lines it was thought that they would be "lost in space" rather than follow the curvature of the Earth. Kennelly and Heaviside suggested that radio waves are reflected back to Earth and so can travel much farther than simple "line-of-sight". At night, the base of the ionosphere is higher than during the day and is much more sharply defined; that's why reception at CB and AM wavelengths is much clearer and exists over greater distances at night (and also why the FCC requires most stations to reduce their power or go "off the air" completely at night). Waves in the FM radio range, microwaves and radar and not reflected by the ionosphere, they go straight through; that's good because how else would we communicate with satellites?
VHF radio is used mainly for marine communication, between ships or with marine installations. It is a two-way voice communication service that consists of a transmitter-receiver, called a transceiver, and an antenna for transmission and reception. The VHF band is from 156.050MHz to 157.425MHz with a channel separation of 0.050MHz (50kHz). A number of the channels are set aside for specific purposes, for example, port operations, navigational information, distress (channel 16 at 156.800MHZ) and the US Coast Guard. However, there are many channels available for public use. VHF radio differs from CB radio in that the transmitted signal is produced by frequency modulation (FM), see question 7. However, like CB radio, no ship nor individual is allocated a personal channel and so no conversation, within the range of operation, is private! For that reason, experienced boaters usually prefer VHF radio to cellular phones. Although their ranges are similar, if a distress is made on channel 16 there may be many "listeners" whereas only one party will be able to hear you on a cellular phone (and they will probably be on-shore!).
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