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Two Way Radio for the 21st Century


Aerial Options

for MOTOROLA Mobiles

The physical size of an aerial is directly proportional to the wavelength of its operating frequency. However at the VHF and UHF frequencies licenseable in the UK a standard aerial would be rather too large to use on a car, so the favourite compromise is to use a quarter-wave aerial. This gives a reasonable balance between efficiency in radiation and size, with the advantage of a fairly flat response which allows multi channel operation. It also has an omni-directional radiation pattern, just what is wanted to optimise the range of a moving vehicle.

To calculate the size of the aerial we will need some physics and since we normally refer to a system's operating frequency, rather than it's wavelength, we use the imperial approximation:

     Length in feet = 234 / frequency in cycles per second. 

This gives us quarter-wave whip lengths at the typical band mobile transmits of 38", 16" and 6".

Since the roof of a typical car is approximately 6ft wide and long, it begs a centrally mounted aerial to take advantage of the metal ground plane all around it. Since the ground plane radius should be at least as long as the aerial radiator for best operation we find that the aerial whip should be 3ft or less in length.

At VHF Low Band the quarter wave aerial is the instant answer, at VHF High Band it works well, but there is space for a larger aerial, and by cheating a bit in fitting a matching or loading coil we can use a 5/8 wave or 1/2 wave aerial with whip lengths of around 40 inches. The 5/8 wave provides a more horizontal radiation, less signal is broadcast vertically up or down, meaning more goes to the horizon, and is therefore frequently named as a gain aerial. However it has the disadvantage of having a fairly narrow bandwidth, which means any gain in signal is soon outweighed by mistuning, and if the licenced operating channels are widely spread, as is common for organisations using both speech and data channels, the tuning has to be compromised heavily. The 1/2wave aerial needs no ground plane, which suits it to smaller vehicles such as motor cycles, but requires a relatively bulky and expensive loading coil. Consequently it is less popular except for specialist applications.

At UHF frequencies many more exotic aerials are popular, such as 1/2 over 1/4 wave - two radiators matched by a central coil to provide extra gain.

Quarter wave aerial

A VHF LB quarter wave aerial - about 36" long

Quarter wave aerial

A VHF HB quarter wave aerial - about 16" long

Five eighths wave aerial

A VHF HB 5/8 wave gain aerial - about 40" long

Five eighths wave aerial

A VHF HB 5/8 wave open coil gain aerial - about 40" long

Five eighths over quarter wave aerial

A UHF HB half over quarter wave gain aerial - about 30" long