RF Communication and Transmission Lines

Radio-frequency (RF) communication is the transfer of information using electromagnetic waves in the radio part of the spectrum. A message such as speech, music or data is impressed on a high-frequency carrier wave by a process called modulation, transmitted, and then recovered at the receiver by demodulation. Common modulation schemes include amplitude modulation, in which the carrier's strength varies, and frequency modulation, in which its frequency varies, along with more efficient methods such as single-sideband and double-sideband that conserve power and bandwidth.

The radio spectrum is divided into bands, from low frequencies through high frequency, very high frequency and up to microwaves, with each band suited to particular uses because of how the waves travel. Lower frequencies follow the ground and bend around obstacles, while higher frequencies travel in straighter lines and carry more information. At the transmitter and receiver, mixers shift signals between frequencies, and amplifiers raise their level, while filters and tuned circuits select the wanted band.

At radio frequencies the cable connecting equipment to an antenna cannot be treated as a simple wire, because the signal behaves as a wave travelling along it. Such a cable is a transmission line, and coaxial cable is a common example. A transmission line has a characteristic impedance, typically fifty or seventy-five ohms, set by its physical construction. When the line, the source and the load all share the same impedance the line is said to be matched, and energy passes through efficiently. A mismatch causes part of the signal to reflect back toward the source, wasting power and creating standing waves along the line, so impedance matching is a central concern in RF design.

Frequently asked questions

What is modulation in RF communication?

It is the process of impressing information such as audio or data onto a high-frequency carrier wave so it can be transmitted, then recovered at the receiver by demodulation.

Why is coaxial cable called a transmission line?

At radio frequencies the signal travels along the cable as a wave with a defined characteristic impedance, so the cable must be analysed as a transmission line rather than a plain conductor.

What happens if a transmission line is not impedance matched?

Part of the signal reflects back toward the source, wasting power and producing standing waves, which is why matching the line, source and load impedance matters.