10.8.5 Why Is Coaxial Cable Used for Radio Frequency Systems?
- What Is a Coaxial Cable?
- Why Does Coaxial Cable Confine the Electromagnetic Fields?
- Why Isn't Ordinary Twin Cable Suitable for RF?
- Why Are 50 Ω and 75 Ω the Most Common Impedances?
- How Much Signal Is Lost in a Coaxial Cable?
- Why Does the Skin Effect Become Important?
- Where Is Coaxial Cable Used?
- Why Has Fibre Optic Cable Replaced Coaxial Cable in Many Networks?
- Does Coaxial Cable Still Have a Future?
- Why Is Coaxial Cable Still So Important?
- What Should You Remember?
Short Answer
Coaxial cable is widely used for radio-frequency (RF) systems because it efficiently guides electromagnetic energy while preventing it from radiating into the surrounding environment. Its unique concentric construction confines the electric and magnetic fields within the cable, providing excellent shielding, predictable characteristic impedance, low radiation, and good immunity to external interference. These properties make coaxial cable ideal for connecting transmitters, receivers, antennas, satellite equipment, television systems, laboratory instruments, and many other RF devices.
What Is a Coaxial Cable?
A coaxial cable consists of four principal components:
- a central conductor that carries the signal;
- a dielectric insulating material surrounding the centre conductor;
- an outer conductor, usually formed from braided wire, foil, or solid tubing; and
- an outer protective jacket that provides mechanical protection.
The name coaxial comes from the fact that the inner and outer conductors share the same central axis.
This apparently simple geometry gives coaxial cable several important electrical advantages that distinguish it from other transmission media.
Why Does Coaxial Cable Confine the Electromagnetic Fields?
One of the defining characteristics of coaxial cable is that almost all of the electromagnetic energy is confined within the dielectric separating the inner and outer conductors.
The electric field extends radially between the two conductors, while the magnetic field forms concentric circles around the inner conductor. Because the outer conductor completely surrounds the inner conductor, these fields remain almost entirely within the cable. As a result:
- very little energy is radiated from the cable;
- external electromagnetic interference is largely prevented from entering the cable; and
- nearby cables experience minimal interference.
This shielding property is one of the principal reasons coaxial cable became the standard transmission medium for RF systems.
Why Isn't Ordinary Twin Cable Suitable for RF?
At low frequencies, two parallel conductors often perform perfectly well.
At radio frequencies, however, they present several problems. The electromagnetic fields surrounding the conductors extend into the surrounding space, allowing energy to radiate away from the cable. Nearby electrical equipment may also induce unwanted signals into the transmission line. These effects increase transmission losses, produce electromagnetic interference (EMI), and make system performance much less predictable.
Coaxial cable largely eliminates these problems by confining the electromagnetic fields within its outer conductor.
Why Are 50 Ω and 75 Ω the Most Common Impedances?
One of the most recognisable characteristics of coaxial cable is its characteristic impedance.
Although many impedance values are possible, two have become dominant. 50 Ω cable is used primarily for radio transmitters, receivers, radar systems, satellite communications, instrumentation, and wireless communication equipment. 75 Ω cable is used mainly for television distribution, cable television networks, video systems, and satellite television receivers.
These values were not chosen arbitrarily. A characteristic impedance of approximately 77 Ω provides the lowest attenuation for air-filled coaxial cable, while an impedance of approximately 30 Ω provides the highest power-handling capability.
The widely adopted value of 50 Ω represents an engineering compromise between low attenuation and high power handling, making it particularly suitable for communication systems.
The 75 Ω standard is closer to the minimum-loss condition and is therefore preferred for receiving systems where transmitted power is not a consideration.
How Much Signal Is Lost in a Coaxial Cable?
Every transmission line introduces some attenuation.
In coaxial cable, these losses arise primarily from two mechanisms. The first is conductor loss, resulting from the finite resistance of the conductors. At higher frequencies this increases because of the skin effect, which forces current to flow only within a thin layer near the conductor surface. The second is dielectric loss, caused by small amounts of energy being absorbed within the insulating material separating the conductors.
Both losses increase with frequency. Consequently, a cable that performs well at 10 MHz may exhibit much greater attenuation at 10 GHz.
This explains why microwave systems often minimise coaxial cable lengths or replace long coaxial runs with waveguide or optical fibre.
Why Does the Skin Effect Become Important?
At direct current (DC), electrical current flows throughout the entire cross-section of a conductor.
As frequency increases, changing magnetic fields induce currents that force most of the current towards the outer surface of the conductor. This phenomenon is known as the skin effect.
Because only a thin outer layer of the conductor carries current, the effective cross-sectional area decreases and the electrical resistance increases. Higher resistance leads directly to greater attenuation.
To minimise these losses, high-quality RF cables often employ silver-plated conductors, whose excellent surface conductivity reduces resistance where the current actually flows.
Where Is Coaxial Cable Used?
Coaxial cable has become one of the most widely used transmission media in radio-frequency engineering.
Typical applications include:
- connecting radio transmitters to antennas;
- satellite television installations;
- cable television distribution networks;
- radar systems;
- microwave communication equipment;
- radio astronomy;
- laboratory test and measurement equipment;
- spectrum analysers and signal generators;
- wireless base stations; and
- amateur radio systems.
Although many long-distance communication links now employ optical fibre, coaxial cable remains the preferred medium for many short and medium RF interconnections.
Why Has Fibre Optic Cable Replaced Coaxial Cable in Many Networks?
During the second half of the twentieth century, coaxial cable formed the backbone of many communication systems.
Long-distance telephone networks, early computer networks, and cable television systems all relied heavily upon coaxial transmission. Today, optical fibre has largely replaced coaxial cable for long-distance communication because it offers:
- dramatically lower attenuation;
- vastly greater bandwidth;
- complete immunity to electromagnetic interference;
- improved security; and
- lower operating costs over long distances.
Nevertheless, coaxial cable continues to play a critical role wherever radio-frequency signals must be transported over relatively short distances.
Does Coaxial Cable Still Have a Future?
Despite the rapid expansion of fibre-optic communication, coaxial cable remains indispensable in many applications.
Every radio transmitter requires an efficient means of connecting to its antenna. Satellite ground stations continue to employ coaxial interconnections. Laboratory instruments require accurately controlled impedance transmission paths. Cable television systems still rely extensively on coaxial distribution within hybrid fibre-coaxial (HFC) networks. Even where fibre carries information over long distances, coaxial cable often provides the final RF connection between equipment.
Rather than disappearing, coaxial cable has become increasingly specialised, serving applications where its unique combination of shielding, controlled impedance, and mechanical robustness provides clear engineering advantages.
Why Is Coaxial Cable Still So Important?
Coaxial cable represents one of the great successes of communications engineering.
Its simple concentric geometry provides predictable electrical characteristics, excellent shielding, efficient power transfer, and low electromagnetic interference. For almost a century it has enabled reliable radio communication, television broadcasting, satellite systems, radar, scientific instrumentation, and countless other technologies.
Although fibre-optic systems now dominate long-distance communications, coaxial cable remains one of the most important transmission media for radio-frequency engineering and will continue to play a vital role wherever RF signals must be transported efficiently and reliably.
What Should You Remember?
- Coaxial cable confines electromagnetic fields within the cable, greatly reducing radiation and interference.
- Its concentric geometry provides excellent shielding and predictable characteristic impedance.
- The most common characteristic impedances are 50 Ω for RF communication systems and 75 Ω for television and video distribution.
- Attenuation increases with frequency because of conductor losses, dielectric losses, and the skin effect.
- Coaxial cable is widely used for antennas, radar, satellite systems, cable television, laboratory instrumentation, and wireless communications.
- Optical fibre has replaced coaxial cable for many long-distance links, but coaxial cable remains the preferred medium for many RF interconnections.
- Coaxial cable continues to be an essential component of modern communication systems because of its excellent electrical performance and mechanical reliability.