Antenna impedance is a measure of the resistance to an electrical signal in an antenna. Many factors have an impact on an antenna’s ability to transmit a signal, including the environment that the antenna is in and the design and composition of the antenna. Understanding antenna impedance is important when designing components that connect an antenna to a receiver or transmitter.
The ratio of voltage to current, which is equal to antenna impedance, is expressed in units called ohms. The antenna impedance represents the power that is absorbed by the antenna as well as the power that is dispersed by it as it comes into contact with an electromagnetic wave. Different wavelengths of electromagnetic radiation will give the same antenna different impedance values.
In electronics, ohms are a measure of the resistance within a wire, and a measurement of zero ohms means that there is no resistance, while a measurement of infinite ohms indicates that there is complete resistance. The antenna impedance would be zero ohms if the voltage and the current stayed the same from one point in the antenna to another. This is never the case in real-world antennas, however, which generally have an impedance of somewhere between 15 and 1,000 ohms.
Environmental factors can have an impact on antenna impedance. The objects around an antenna, the height of the antenna above the ground, and whether the antenna is enclosed indoors or exposed to the elements all change its resistance when exposed to an electromagnetic signal. Under optimal conditions, a simple half-wave dipole would have an antenna impedance of 75 ohms. Coaxial cable, which is used in conjunction with half-wave dipole antennas for transmitting television signals, is matched to this impedance and also has an impedance of 75 ohms.
Impedance matching is an important consideration when designing antennas of many different types. as it involves the transfer of electrical energy through the antenna. The antenna must transfer this information into a receiver of some sort, allowing human observers to collect the information picked up by the antenna. This information travels through the antenna and various other cables, such as the coaxial cable that feeds information into a television. Finding cables that match the same voltage to current ratio helps keep the signal strong and clean as it passes from one device to another.