What Is Antenna Impedance?

The connection between the antenna and the feeder is called the input or feed point of the antenna. For a linear antenna, the ratio of the voltage to the current at the input of the antenna is called the antenna's input impedance. For the mouth-type antenna, the voltage standing wave ratio of the feeder is often used to represent the impedance characteristic of the antenna. Generally, the input impedance of an antenna is a complex number. The real part is called the input resistance and is represented by Ri; the imaginary part is called the input reactance and is represented by Xi.

The input impedance of the antenna is related to the antenna's geometry, size, feed point location, operating wavelength and surrounding environment. When the diameter of the wire antenna is relatively thick, the input impedance changes smoothly with frequency, and the impedance bandwidth of the antenna is wide.
A radiating system composed of many unit antennas close to each other is called an antenna array, and the elements in the antenna array interact in a complicated manner. This phenomenon is called mutual coupling. Not only related to its own excitation, but also to the current on adjacent antennas. In an N-element antenna array, the input impedance of any unit is V _n and I _n are the voltage and current of the input terminal of the n-th unit; Z is the self-impedance of the n-th unit when the other units are open; Is the mutual impedance between the nth unit and the mth unit. The definition of mutual impedance is that in the formula, i is the number of each unit except m. According to the reciprocity theorem, Z = Z _mn . ^[1]

The main purpose of studying antenna impedance is to achieve matching between antenna and feeder. To match the transmitting antenna with the feeder, the input impedance of the antenna should be equal to the characteristic impedance of the feeder. To match the receiving antenna with the receiver, the input impedance of the antenna should be equal to the complex conjugate number of the load impedance. The receiver usually has a real impedance. When the impedance of the antenna is complex, it is necessary to use a matching network to remove the reactive part of the antenna and make their resistance parts equal.
When the antenna and the feeder are matched, the power transmitted from the transmitter to the antenna or from the antenna to the receiver is the largest. At this time, no reflected wave will appear on the feeder, the reflection coefficient is equal to zero, and the standing wave coefficient is equal to 1. The degree of matching between the antenna and the feeder is measured by the reflection coefficient or standing wave ratio at the input end of the antenna. For a transmitting antenna, if the matching is not good, the radiated power of the antenna will be reduced, the loss on the feeder will increase, the power capacity of the feeder will also decrease, and the transmitter frequency will be "pulled" in severe cases. That is, the oscillation frequency changes.
For a mouth-type antenna, in order to achieve a matching state, a matching element capable of generating opposite reflection should be added near all discontinuities that generate reflections, so that they cancel each other out. The frequency band of the antenna is determined by the combined frequency band of these elements. ^[2]

Antenna impedance may include both reactance and resistance components. In most practical applications, we are looking for a purely resistive impedance (z = R), but this ideal situation is difficult to achieve. For example, a dipole antenna has a theoretical impedance of 73 when it reaches resonance in a vacuum. However, when the frequency of the signal sent to the antenna is not the resonance frequency, a reactance component (± jX) appears. Above the resonant frequency, the antenna has an inductive reactance and the impedance is Z = R + jX. Similarly, when it is lower than the resonance frequency, the antenna has a capacitive reactance and the impedance is z = R-jX. In addition, in the space close to the ground, the resistive part may not be 73, but may be some value of 30-130. Obviously, no matter how much the coaxial cable is selected, it is likely to be inappropriate.

In practical radio applications, in order to connect a complex load (such as an antenna) to a purely resistive source, the most common situation is to construct a matching network between the load and the source. The impedance of the matching network must be equal to the conjugate of the complex impedance of the load. For example, if the load impedance is R + jX, the impedance of the matching network must be R-jX; similarly, if the load impedance is R-jX, the impedance of the matching network must be R + jX. ^[2]