Dipole aerial for FM broadcast

A half wave dipole aerial for the band 2 fm broadcast spectrum (87-108MHz)

A dipole is a basic aerial consisting of two opposing radiating elements which need to be cut to size to work with the frequency being used. Dipoles are known to be very reliable (even in harsh weather conditions) whilst also performing well in built up aeras. A dipole aerial can be constructed easily with Aluminium or copper tubing (I prefer alu) for the boom arm and elements (see below):

The half wave dipole formula
The following formula can be used to work out the total length of both radiating elements given the frequency.
300 (actually 299.782458) / f x 0 .935 x 0.5
The 0. 935 is the velocity factor of the material used for the elements and will vary depending on the material and the thickness. 0.935 is usually fine for alu and copper.
Example for 98MHz:

300 / 98 x .935 x 0.5 = 1.431m or 143.1cm for the total length (including the center cap)

BALUN – Balanced Unblalanced

A balun is needed to convert the unbalanced coax to a balanced output by isolating the the feeder cable (coax) using a small length of coax (rg58 is fine) to connect the out and inner of the feeder coax. Failing to fit a balun may cause the aerial feeder coax to radiate because of the bad matching causing interfernce, TVI (television interferance) for example. Your dipole will work without a BALUN but dont expect a good SWR. Click the pictures below for a better idea of where the balun is fitted:

The formula to work out the length of the coax for the balun is shown below:
300 / f x .68 x .25
Example for 98MHz:

300 / 98 x .68 x .25 = 0.520m or 52cm

The final product

1.balun to ensure coax doesnt radiate and preventing TVI
2.4MHz bandwidth
3.very low SWR typically 1.2:1 (better than 1.5:1 in all weather conditions)
4.rugged design built to last even in harsh weather conditions
5.SO239 connector for coax
6.folds up for easy storage and transport

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