This chapter introduces the fundamental parameters of wireless communication, aiming to provide a better understanding of the role of antennas in communication systems. Wireless communication is carried out in the form of electromagnetic waves, making it essential to understand the propagation characteristics of waves.
In this chapter, we will discuss the following parameters:
•Frequency
•Wavelength
•Impedance Matching
•VSWR & Reflected Power
•Bandwidth
•Percentage Bandwidth
•Radiation Intensity
Now, let's take a detailed look at them.
Frequency：
According to the standard definition, frequency is the number of repetitions of a wave per unit time.In simple terms, frequency describes how often an event occurs. A periodic wave repeats every T seconds (one period), and its frequency is the reciprocal of the time period T.
Mathematically, it appears as follows：
$$f = \frac{1}{T}$$
•F represents the frequency of a periodic wave, while
•T is the time required to complete one full cycle.
Frequency is measured in hertz, abbreviated as Hz.
The figure above illustrates a sine wave, plotting voltage (in mV) as a function of time (in ms). This waveform repeats every 2t milliseconds; therefore, its period T = 2t ms, and its frequency f = 1/(2t) kHz.
Wavelength：
According to the standard definition, the distance between two consecutive peaks or two consecutive troughs is called the wavelength.
Simply put, the wavelength is the distance between two adjacent positive peaks or two adjacent negative peaks. The figure below shows a periodic waveform, with the wavelength (λ) and amplitude marked. The higher the frequency, the shorter the wavelength, and vice versa.
The formula for wavelength is:
$$\lambda = \frac{c}{f}$$
•λ represents the wavelength
•C is the speed of light ($3 \times 10^8$ meters per second)
•F is the frequency
The wavelength λ is expressed in units of length, such as meters, feet, or inches. The commonly used unit is the meter.
Impedance Matching：
According to the standard definition, impedance matching occurs when the impedance of the transmitter is approximately equal to the impedance of the receiver.
Impedance matching is required between the antenna and the circuit. The impedances of the antenna, transmission line, and circuit should be matched to achieve maximum power transfer between the antenna and the receiver or transmitter.
The Necessity of Matching
Resonant devices are capable of delivering optimal output within certain narrowband frequencies. As a resonant device, an antenna can achieve better output performance when its impedance is properly matched.
•When the antenna impedance matches the impedance of free space, the power radiated by the antenna will be effectively transmitted
•For a receiving antenna, its output impedance should match the input impedance of the receiving amplifier circuit
•For a transmitting antenna, its input impedance should match the output impedance of the transmitting amplifier as well as the characteristic impedance of the transmission line
Impedance is measured in ohms, denoted by the symbol Z.
VSWR & Reflected Power：
According to the standard definition, the ratio of the maximum voltage to the minimum voltage in a standing wave is called the voltage standing wave ratio (VSWR).
When the impedances of the antenna, transmission line, and circuit are mismatched, power cannot be effectively radiated; instead, a portion of the power is reflected back.
The main characteristics are —
•The parameter that indicates the degree of impedance mismatch is called the Voltage Standing Wave Ratio (VSWR)
•VSWR stands for Voltage Standing Wave Ratio and is also commonly referred to as SWR
•The greater the impedance mismatch, the higher the VSWR value
•To achieve effective radiation, the ideal VSWR value is 1:1
•Reflected power refers to the portion of forward power that is wasted. Reflected power and VSWR essentially describe the same physical phenomenon from different perspectives
Bandwidth:
According to the standard definition, the frequency band within a specified wavelength range allocated for a particular communication is called bandwidth.
When a signal is transmitted or received, it operates within a certain frequency range. This specific frequency range is assigned to a particular signal to prevent interference from other signals during transmission.
•Bandwidth refers to the frequency range between the high-frequency and low-frequency limits of a signal transmission
•Once bandwidth is allocated, it cannot be used by others
•The entire spectrum is divided into bandwidth segments, each assigned to different transmitters
The bandwidth we have just discussed can also be referred to as absolute bandwidth.
Percentage Bandwidth：
According to the standard definition, the ratio of the absolute bandwidth to its center frequency is called the percentage bandwidth.
The frequency within a band at which the signal strength reaches its maximum is called the resonant frequency, also known as the center frequency of the band, denoted as fC.
•The higher and lower frequencies of the band are denoted as fH and fL, respectively
•The absolute bandwidth is given by fH − fL
•To evaluate the width of a frequency band, it is necessary to calculate its fractional bandwidth or percentage bandwidth
The percentage bandwidth is calculated to understand the range of frequency variations that a component or system can handle.
•fH denotes the higher frequency
•fL denotes the lower frequency
•fc denotes the center frequency
The larger the percentage bandwidth, the wider the channel bandwidth.
Radiation Intensity：
Radiation intensity is defined as the power radiated per unit solid angle.
An antenna radiates more intensely in certain directions, which correspond to its maximum radiation intensity. The maximum possible range of radiation is characterized by the radiation intensity.
Mathematical Expression
Radiation intensity is obtained by multiplying the radiated power density by the square of the radial distance:
Where U is the radiation intensity, r is the radial distance, and （Wrad） is the radiated power density.
•U represents the radiation intensity
•r represents the radial distance
•Wrad represents the radiated power density
The above equation expresses the radiation intensity of the antenna. The radial distance is sometimes denoted by the symbol Φ.
The unit of radiation intensity is watts per steradian (W/sr), or watts per square radian (W/rad²).
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Post time: Mar-26-2026
