Basic Dipole Design

All to frequently the receiving antenna is given little consideration. The simplest form of antenna is the Half-wave dipole. Element length is inversely proportional to its operating frequency. A Band 1 dipole will be almost twice the size as one cut for the f.m band. Do not be tempted to use a dipole far removed from its normal operating frequency. An f.m dipole may work in Band 1, but not very efficiently.

Typical dipole measurements are as follows: 1/4 wave = 71250 / frequency in MHz

Band 1: 40Mhz - 70Mhz

Centre Frequency = 56.1Mhz

1.270m 1/4 length

f.m Radio: 87.5Mhz - 108Mhz

Centre Frequency = 97.7 MHz

0.729m 1/4 length

Band 3: 173.2Mhz - 177.0Mhz

Centre Frequency = 175.1Mhz

0.407m 1/4 length

Band 3: 191.5Mhz - 201.2Mhz

Centre Frequency = 196.35Mhz

0.363m 1/4 length

DAB Radio:225.5Mhz - 227.0Mhz

Centre Frequency = 226.25Mhz

0.315m 1/4 length

Band 5 : 854.50mhz - 860.750Mhz

Centre Frequency = 857.625Mhz

0.083m 1/4 length

Dipole sizes for:

48.45mhz =  1.469m 1/4 length 

52.9mhz = 1.346m 1/4 length

199.700mhz = 0.357m 1/4 length

The dipole rods can be made from 12.5mm diameter alloy tubing. Weatherproof dipole connector boxes should be used if the antenna is to be mounted outdoors. Dipoles should be mounted vertically.

Outdoor antennas

It pays to apply a small amount of grease to the cable entry grommet of the dipole connector box to prevent ingress of moisture.

Apply grease to all bolts and wing nuts on both antennas and clamps to enable easy dismantling and cleaning at a later date.