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i Notes Chapter 2 lAn older term for hertz is cycles per second. Frequencies are also expressed in kilohertz (kHz, formerly kilocycles per second or kc/s) or megahertz (MHz, formerly megacycles per second or Mc/s). One kilohertz is one thousand hertz. One megahertz is one million hertz or one thousand kilohertz. 2Radio signals have frequencies, like sounds have pitch. Flutes are higher in pitch than trombones, and cellular base stations have higher operating frequencies than FM broadcast stations. FM broad- cast frequencies are from 88 to 108 megahertz (MHz). Base stations transmit and receive in a band of frequencies roughly nine times higher (about 850 MHz), near TV channel 69. 3Of course, the other parry might also be using a cellular phone, in which case his or her half of the conversation might pass back out over the airwaves via a base station. 4The technical term for this is full duplex operation. 5This is why, except in very unusual circumstances,we cannot—by direct reception—pick up UHF television stations from more than 60 miles away. Rural homes often have television receiving antennas on very tall masts co increase the reception range. 6The smaller the frequency difference between the desired and the interfering signals, and the stronger the interfering signal, the harder it is to decode the desired signal. Special signal-process- ing techniques can suppress interference, but there are limits to their effectiveness. 7One measures electric field strength in volts per meter, millivolts per meter,or microvolts per meter. One thousand millivolts per meter is the same as one volt per meter,or one million micro- volts per meter. Depending on the effective length of a particular antenna, and depending on the • strength of the electric field impinging on that antenna, an RF signal will appear at the antenna's output terminals. One measures that output signal in volts, millivolts,or microvolts (as opposed to volts per meter, millivolts per meter, or microvolts per meter, the units of electric field strength). There are one thousand millivolts (or one million microvolts) in one volt, the standard unit of electrical pressure or"potential difference." The voltage and the resistance in an electrical circuit determine the current flowing in that circuit. Current is measured in amperes or amps; electrical pressure,as noted above, in volts;and resistance in ohms. A fresh"AA"penlight battery generates 1.5 volts.A charged car battery generates 12 volts. Utility power is usually at 115 to 120 volts, but high-current circuits (e.g., those for electric stoves and furnaces) often are at 230 volts. Electrical pressure is like water pressure. Open a faucet a bit, and a certain amount of water will flow out each second. The water pressure overcomes the resistance put up by the walls of the pipe and the partially open valve.The flow increases if the pressure increases or the resistance decreases, that is, the faucet opens further. Similarly, more current flows in a circuit if the voltage increases or the resistance decreases. Receiving antennas usually generate output signals as strong as one volt only when they are close to the transmitting antenna,where fields are one volt per meter or more. 8The 32-dBµV/m contour is where FCC propagation curves predict a base station's signal will have a field strength of 39.8 microvolts per meter. Because the prediction curves include a statisti- cal factor, not every point within the contour will enjoy the predicted signal strength. At some points, the signal will be weaker due to local terrain effects (e.g., loss of line of sight); at others, the signal will be stronger. i m