Antennas: types, polarisation, patterns, gain and ERP

Your antenna matters more than any other part of your station. In this lesson: what the antenna does, polarisation, the common types, where they send their signal, and how gain turns into effective radiated power.

An antenna turns electrical signals into radio waves, and back again when you receive. The waves are polarised to match the antenna. A horizontal antenna radiates horizontally polarised waves, and a vertical antenna radiates vertically polarised waves. At V H F and U H F, signals are received best when both stations use the same polarisation. At H F it matters less, because the polarisation often changes when the signal is bent by the ionosphere.

Here are the antennas you'll need to recognise. A half wave dipole is a wire or rod fed in the middle, and it's about half a wavelength long from end to end. A quarter wave ground plane is a vertical element with radials spreading out below it. A five eighths wave vertical is a longer vertical, often used on vehicles. An end fed wire is fed at one end, usually through a matching unit. You'll also meet the Yagi, a beam antenna. All of them work the same way at any frequency, but H F antennas are much bigger than V H F antennas, because size depends on wavelength. A dipole for two metres is about one metre long; one for forty metres is about twenty metres.

A radiation pattern, or polar diagram, shows how strongly an antenna radiates in each direction. A horizontal dipole, seen from above, radiates most at right angles to the wire, and least off the ends, making a figure of eight. A Yagi concentrates its signal towards the front, with very little behind it. A dipole mounted vertically, a quarter wave ground plane and a five eighths wave vertical all radiate equally in every horizontal direction. They're called omnidirectional.

Gain comes from an antenna's ability to focus its signal in one direction, so a Yagi usually has more gain than a dipole. Gain is measured in decibels, compared with a half wave dipole. The exam gives you a table: three decibels doubles the power, six decibels multiplies it by four, and ten decibels by ten. Multiply the power fed to the antenna by its gain to get the effective radiated power, or E R P. So ten watts into a Yagi with six decibels of gain gives forty watts E R P. Gain can also be compared with an imaginary antenna that radiates equally in all directions. That gives E I R P, and ten watts E I R P is the same as six point one watts E R P.