Media UK

Antennas

WARNING!

Antenna technology is 5% Science and 95% Black Magic - and I don't mean the chocolates.

Any radio service which is transmitted to air needs an aerial. It's conventional to call a relatively compact rigid metallic structure an "antenna" - because it looks rather like the antenna of a moth. The term "aerial" is nowadays reserved for much larger non-rigid components such as wire systems strung between inactive mounting structures.

It's important to understand what an antenna does. It's simply a device for converting an electric current (in or on a conductor) into electromagnetic waves (usually launched into the atmosphere) - and vice versa. There is an equivalent theory that particles rather than waves are the method by which radio signals travel, but wave theory is much easier to understand and work with in a practical environment.

Most antennas work equally well as a transmission or reception device.

Electromagnetic waves or radiation (EMR), generated by a simple point source like the earliest spark gap generators, radiate like the surface of an inflating spherical ball, and are referred to as spherically polarised.

It's dificult to produce an efficient antenna which will produce this type of radiation, and so we use resonant metal surfaces which produce plane-polarised radiation - usually vertical or horizontal dipoles or developments of these two basic formats.

Spherical polarsiation mentioned above should not be confused with circular polarity, which means linear waves which are made to rotate by being launched from corkscrew-style antennas. Circular radiation must be specified as clock or anticlock as viewed from the base-point of the antenna.

Mixed polarity - often again mistakenly referred to as "circular" is the term for a system which launches two separate polarised waves at the same time and in phase.

These can be a vertical and a horizontal component, but more often for simplicity there are two radiators in the form of a multiplication cross, launching two plane-polarised waves, each inclined by 45 degrees from the vertical.

You'll often see this type of antenna in use at medium-power multi-user sites looking like a fat cross made out of tubes protruding from a large square metal grid fixed to the side of a tower. There is usually one unit on each of the four tower sides to achieve all-round coverage.

Don't confuse "tower" with "mast". A tower is a self-supporting structure like the Eiffel Tower or an electricity transmission tower (pylon). A mast is a slim structure held in place by a series of guy wires. Most so-called "mobile phone masts" are in fact towers.

The original BBC VHF/FM transmitters used a slot type of antenna which was simply a letter-box shaped slot cut into a tube, which although appearing to be arranged vertically, radiated a horizontally-polarised signal. The "skeleton slot" antenna used the same principle and is still seen on some older aerials and specialised UHF TV antennas.

Incidentally - the reason why FM transmissions were originally horizontally polarised when the system was introduced in the UK in 1955 was just so that the receiving antenna could be mounted horizontally on the same pole as the (most often vertical) TV antenna and not get in its way!

The simplest form of antenna is a half-wave dipole - not named as is rumoured after the old Welsh aerial wizard Dai Powell - but meaning a device with two levels of potential.

Rather than having to energise a conductor which is a whole wavelength long as logic might have it, a half wavelength functions perfectly well to launch electromagnetic waves.

A half-wave dipole is the most common of antenna types and is seen on just about every transmission facility where vertically-polarised signals are required to be launched omni-directionally.

Similarly, even a quarter wave radiator will work correctly if it is based on a "ground plane".

The type of vertical "flagpole" antennas widely used for low-power AM stations require a mat of copper strips laid radially in the ground for efficient operation. You'll often see the equivalent VHF antenna known as a "groundplane unipole" on the roof of a Fire or Ambulance station, looking like a skinny chimney sweep's brush.

The four radial ground plane elements produce an electronic mirror so that the quarter-wave element thinks it is operating above a metallic surface and that it "sees" a mirror image of itself. Strange, but true.

Also there are antennas typically used at airports and military installations which have the same feature using lots of downward elements forming a 'skirt' around the mounting pole.

Some antennas are enclosed in a complex housing, or are attached to some form of reflector or director device, but they all rely on a simple dipole or an equivalent component to function.

We're all familar with the satellite dish. The active part is buried inside the LNB (Low Noise Block - no, don't ask!) - which also houses a switchover system and a frequency translator. The dish is purely a passive reflector - like that in a car headlight - to concentrate the multiple bits of refelected energy very carefully into one place - where the actual antenna adds it all together.

The effect is just the same as using a magnifying glass and the sun to burn a hole in a piece of paper.