How to make the simple DIY FM dipole antenna design

To make the simple DIY FM dipole antenna, only a few items are needed. Typically these will be:

1、Twin flex - twin mains flex is idea but we used some old speaker flex.
2、Tie wrap - to secure the centre of the dipole and prevent the flex opening out beyond what is needed.
3、String or twine to secure the ends of the dipole to relevant fixing points (if required).
4、Connectors - if it is to be connected to coaxial cable.

One advantage of using mains flex is that when used as a feeder for radio frequency signals this type of wire is a reasonably close approximation to 75 ohm twin or open wire feeder. This is convenient if a reasonable length is needed. For making our FM dipole antenna, we used some cheap speaker wire.

To make up the VHF FM dipole antenna, first the cable should have the two insulated wires split back away from one another and opened out. The centre should then be secured to prevent the cable opening out any further. One method of doing this is to use a tie wrap such as those available from most electronics components stockists. The length of wire which has not been split can then be used as the feeder for the antenna.

The overall length for the antenna should be about 150 cms, i.e. each leg should be 75 cms. This length should make the resonant frequency fall slightly in the lower half of the FM broadcast band, but often the more popular stations may be found in this region. If the resonant frequency is required to be higher then the antenna can be shortened slightly.
It is quite easy to calculate the length from one of the following equations:

length (inches)=5905 * A / f

Sometimes the equations vary a little in the constants used, as this depends on a variety of factors including the wire used, environment, frequency and the like. However it is a very good starting point and certainly good enough for making the FM dipole antenna.

The ends of the wire can be knotted as shown to enable it to be attached to some twine or string to set it up on a loft space, etc. If this is done, the length should be taken to the extremity of the wire and any wire that is part of the knot or doubled back should not be included in the length. The knotting of the wire will add some inductance to the end of the wire, possibly making it a little long, but it should be fine for reception.

As we already had some coax cable installed around the house, our antenna was connected to a coax connector, and a mating version was attached to the coax cable. It is noting that twin feeder, such as that formed by the flex does not perform well when routed for long lengths through a house and signal losses will rise - better to use coax as this is not affected in the same way.

When installing the antenna, as far as is possible in a roof space, the antenna should away from metal objects as this will reduce the signal levels. In particular the ends of the antenna are more sensitive to nearby metal objects.

We strung our antenna up in the loft or roof space. As many VHF FM stations use vertical polarisation these days, we mounted the dipole in a vertical fashion: one end attached to a convenient nail in the wooden constriction of the roof, and the other end held down by a weight. The coax was lead away at right angles - as much as you can in these circumstances!

If you need any fm/tv equipment, please feel free to contact us: [email protected]

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