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FM dipole antenna

- A description of the dipole antenna used for the reception of FM broadcasts and the constructional details for an FM dipole antenna design..

One area in which dipole antennas are often used is for the reception of VHF FM broadcasts. Many Hi-Fi tuners and other radios have input sockets that will accept the input from a coaxial feeder, and where no external antenna is used, a dipole antenna can provide an excellent solution. The FM dipole antenna will provide greatly improved reception over many other improvised solutions that may be used.

It is quite easy to make a simple FM dipole antenna. They can be made in a variety of ways, and for minimal cost. They may prove to be the ideal solution for an internal FM antenna, possibly in the attic or roof space, or they may be used when a temporary antenna is needed. If they are to be used for internal use, then there is no need to use expensive materials to ensure they are not affected by the weather. Instead for internal use the VHF FM dipole can be made from commonly available materials and therefore the cost is likely to be minimal.


Dipole antenna basics

As described in other pages on the website (see the related articles link on the left hand side below the main menu), the dipole antenna consists of two poles or parts. For a half wave dipole each leg of the dipole will be an electrical quarter wavelength long.

The length of the dipole is determined by the frequency of operation. The FM broadcast band extends from 87.5 MHz up to 108 MHz. This is quite a wide bandwidth to be covered by a resonant antenna such as the dipole antenna, but as it is only used for reception the performance at the band edges is not as important as if it were to be used for transmitting.

The basic design for the FM dipole antenna is shown below.

Basic half wave dipole antenna

The basic half wave dipole antenna

In the diagram each leg of the dipole antenna is joined to the feeder. This can either be open wire / twin feeder, or coax can be used. Strictly speaking a balun should be used when coax or coaxial cable is used. This is because coax is what is termed an unbalanced feeder, i.e. the outer shield is connected to earth and the antenna is balanced. However for this application, no noticeable degradation should be seen and the VHF FM dipole antenna should operate quite satisfactorily without one. In this case the inner conductor of the coax is connected to one leg of the dipole and the outer conductor (braid) of the coax is connected to the other leg of the dipole antenna.


Simple FM dipole antenna design

One very simple way of making up a dipole with its feeder for experimental or temporary use is to use some of the white low current mains flex. 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. Another alternative to this is speaker wire. This can be used if the white mains flex is not available.

FM dipole antenna made from twin flex

FM dipole made from twin flex

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.

When installing the antenna, care should be taken to ensure that the antenna is kept away from metal objects as this will reduce the signal levels. In particular the ends of the antenna are particularly sensitive to nearby metal objects.

The FM dipole antenna can be mounted quite easily using string attached to the ends. The string that is used must be insulating, and as a result wire should not be used for this purpose.

This method of mounting the FM dipole antenna can enable the ends of the antenna to be kept clear of metal objects. If the end of the wire used in the dipole antenna is knotted to enable the string to be attached, then the physical length from the center to the outermost point of the wire is the required length, i.e. the wire included in the knot itself should not be included in the length.


VHF FM folded dipole antenna

Many VHF FM hi-fi tuners have a 300 ohm input as well as the standard 75 ohm one. This input will normally have screw terminals although they will sometimes have a special 300 ohm connector. This input is ideal for use with a VHF FM folded dipole antenna which can be made up very simply. It requires only the use of a length of 300 ohm ribbon cable (not the computer multi-stranded ribbon cable) which can be bought from most electronic component stockists.

The first stage is to cut a length slightly longer than that required for the dipole element. At either end the centre plastic should be cut back and the remaining wire on either side stripped and joined together. This should be done making sure that the overall length of the element is correct.

The next stage is to cut the bottom wire in the centre. The wires should be stripped back so that a second length of ribbon can be attached. This can be made any suitable length, bearing in mind that it is likely to introduce a reasonable amount of loss if it is run within the house close to other objects. This enables the 300 ribbon to be used as feeder to be connected. This may be any suitable length.

This cheap and easy VHF FM dipole antenna is suitable for areas with high signal strengths, or it may be used as a temporary measure. The 300 ribbon cable is generally clear and can be hidden quite easily. Often this type of aerial can be fixed behind a curtain rail or a large piece of furniture.


Summary

A dipole antenna is often an ideal solution for an antenna for receiving VHF FM broadcasts. The FM dipole antenna can be a cheap and effective solution, and they can be made in a variety of forms - only two ideas are given above, but it is possible to make a VHF FM dipole antenna in many more ways according to what may be available and what the requirements are.

 


Practical Radio Frequency Test and Measurement

Joseph J Carr


Practical Radio Frequency Test and Measurement

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