Is a Full Wave Antenna Better?

Background

• An antenna̵7;s wavelength is the length of the radio waves it receives or transmits. Radio waves have a size defined by their frequency and the speed of light: lower frequencies, such as for AM broadcasts, have wavelengths measuring hundreds of meters, while microwaves measure a centimeter or less. A full wave antenna is one whose length matches the wavelength of the radio waves for which the receiver or transmitter operates. Electronic designers also use antennas with fractional sizes, such as half and quarter wave, as well as those with no simple relation to the wavelength.

Efficiency

• The more efficient the antenna, the better it is at converting radio waves into electrical signals. Though an antenna of any length pulls in some signal, a full wave antenna receives the most; this is because it is tuned to the incoming radio waves. Half and quarter wave antennas are almost as efficient as full wave designs, which makes them effective substitutes in many applications.

Broadcast Versus Receive

• A broadcaster̵7;s concerns differ from those of a receiver. For the broadcaster, any signal energy that the antenna does not transmit is wasted power; if the waste is excessive, it stresses the equipment and runs up the electric bill. High-frequency devices such as cell phones and walkie-talkies use full-wave antennas to maximize efficiency. Lower-frequency transmitters such as AM radio stations use quarter-wave antennas; full wave antennas for low frequencies are too long to be practical. By contrast, a radio receiver handles weak signals -- thousandths or millionths of a watt. The radio̵7;s amplifier must boost the signal regardless of the antenna̵7;s efficiency. Though antenna efficiency has some bearing on the design of radio receivers, it is much less important than size and cost; full wave antennas usually merit consideration only for ham radio and other specialty receivers.

Mobile Applications

• At AM and FM frequencies, a radio signal̵7;s wavelength runs from tens to hundreds of meters, making full wave antennas unsuitable for use in vehicles. Handheld devices such as cell phones work at GHz frequencies. These short wavelengths make full wave antennas practical.