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How to make a Simple Dipole Antenna It is very easy to create a simple 1/2 wave dipole, all you need is some lengths of wire such as the core of some mains flex or even a straightened out metal coat hanger, some co-ax cable and a connector for your scanners antenna input (usually BNC or SMA).
Connect one end of each cut wire to one side of a piece of choc block and to the other sides connect the core and shield of a length of 75 ohm co-ax. Put a suitable connector for your scanner onto the other end of the co-ax and hang the antenna in a suitable position with the element that is connected to the center core of the co-ax pointing upwards. The
balanced 75 ohm impedance of a dipole should really be matched
to the unbalanced impedance of the co-ax using a balun but this
is not as important for receiving only as it would be for transmitting.
The
mismatch will degrade the performance a little as it will reduce
the overall sensitivity and possibly allow the co-ax feeder
to act as an antenna and pick up some noise generated from within
the building (eg. computers).
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The
formula to calculate the length of the antenna is 147/frequency
in MHz, this give the total length of the dipole in centimetres.
For example, to make a 150MHz dipole: 147/150 = 98cm so each
element of the dipole should be 49cm