For beginners stepping into the field of radio, it's common to focus too much on the power of radio equipment and inadvertently overlook the crucial role of the antenna system. For example, when signing in to a repeater network, you might encounter a situation where, despite turning the radio power up to 50W, the signal report you receive may not be as strong as that from a small handheld radio with just 5W of power. Besides factors such as distance and environment, the core issue is that the power generated by the radio cannot be effectively transmitted with low loss and is instead wasted in the antenna system. Therefore, before addressing this issue, the first step is to master the following 40 basic principles about antennas.

What is an Antenna?

An antenna is essentially a specialized transducer. Its primary function is to convert guided waves transmitted on a transmission line into electromagnetic waves that can propagate freely in an unbounded medium (usually space). It can also perform the reverse function. In radio systems, the antenna plays a critical role and is key to transmitting or receiving electromagnetic waves.

Classic Half-Wave Dipole Antenna

The maximum gain coefficient of an antenna refers to the ratio of the total input power and indicates how efficiently the antenna uses the radio frequency power. This is often expressed in decibels. Mathematically, it can be shown that the maximum gain of an antenna equals the product of the antenna's directivity coefficient and its efficiency.

The Invention of the Antenna

The antenna was invented by Russian scientist Popov. In 1888, after hearing about Hertz's discovery of electromagnetic waves, the 29-year-old Popov, who had initially planned to promote electric lighting, decided instead to explore electromagnetic waves. Popov believed that by controlling magnetic waves, he could reach the entire world, not just a small corner.

In 1894, Popov created a radio receiver using an improved metal filings detector. The receiver triggered an electric bell when it detected electromagnetic waves. One day, Popov noticed that the receiver's range increased unexpectedly. Upon investigation, he discovered that a wire connected to the metal filings detector improved the signal. He connected the wire to the detector, and the range increased dramatically, thus creating the radio antenna.

Antenna Classification

Antennas can be classified based on their characteristics and usage:

1. By Function:

    ○ Transmitting and receiving antennas.

    ○ Communication, broadcast, television, and radar antennas.

2. By Directivity:

    ○ Omnidirectional and directional antennas.

3. By Wavelength:

    ○ Extremely long, long, medium, short, ultrashort, and microwave antennas.

4. By Structure and Working Principle:

    ○ Wire antennas and surface antennas.

5. By Usage:

    ○ Handheld, vehicle-mounted, and base station antennas.

Key Antenna Parameters

    ● Antenna Efficiency: The ratio of power radiated by the antenna to the input power. It is always less than 1.

    ● Antenna Polarization: Describes the orientation of the electric field of the electromagnetic wave. It can be linear (horizontal or vertical), circular, or elliptical.

Common Antenna Types

1. Long and Medium Wave Antennas: Used for transmitting or receiving in long and medium wave bands. Common types include vertical, inverted L-shaped, T-shaped, and umbrella-shaped antennas.

2. Shortwave Antennas: These antennas are used for shortwave bands and are widely used in long-distance radio communication. Examples include symmetric, in-phase horizontal, folded dipole, and V-shaped antennas.

3. Ultrashort Wave Antennas: These antennas operate in the ultrashort wave bands and include Yagi, discone, and biconical antennas.

4. Microwave Antennas: Commonly used for microwave frequencies (meter-wave, decimeter-wave, centimeter-wave, millimeter-wave). Examples include parabolic, horn, and lens antennas.

4. Directional Antennas: These antennas emit or receive electromagnetic waves strongly in specific directions. Examples include parabolic antennas.

5. Omnidirectional Antennas: These antennas radiate or receive waves uniformly in all directions. A common example is the whip antenna.

Main Antenna Types and Their Applications

    ● Whip Antennas: Flexible rod-shaped antennas, typically 1/4 or 1/2 wavelength long, used for small communication devices and vehicles.

    ● Dipole Antennas: Symmetric antennas with two equal parts. The half-wave dipole is one of the most basic and widely used antennas.

    ● Yagi Antennas: A directional antenna commonly used for television reception and amateur radio.

    ● Rhombic Antennas: High-gain, wideband antennas commonly used in shortwave communication.