DATA SHEET

BROADCASTING & RECEPTION

Licence type

5MBS has an FM licence of a type known as a 'foothills licence'. This class of licence was created by the Australian Broadcasting Authority [now the ACMA] as a result of pressure from the early FM broadcasters in Adelaide for an extra licence based in the Adelaide CBD with a signal beamed at the Adelaide Hills. It was their claim that signals broadcast from Mount Lofty or other high points in the Adelaide Hills were not being received in the gullies and foothills areas because of shading by the hills. FM signals are like light rays and travel in straight lines.

Where we broadcast

5MBS is licensed to serve the metropolitan area of Adelaide with the signal concentrated on the south-eastern suburbs [Glen Osmond, Burnside etc]. The transmitter and transmitter antenna are situated on top of a hotel/motel at 208 South Terrace Adelaide. A less strong signal is transmitted to other areas but with a modern FM receiver and a proper FM receiver antenna, the 5MBS signal should be received across all of the metropolitan area.

In 2005, 5MBS installed a new modern transmitter antenna to maximise the signal in its broadcast area. Tests carried out after the installation of the new antenna showed that the broadcast signal met the levels required by the ACMA in the 5MBS broadcast area. The number of people living in the area covered by the 5MBS signal is approximately 700,000.

FM broadcasting

5MBS broadcasts on the FM band. FM broadcasting provides a high-quality signal that is virtually free of interference. The frequency is 99.9 Megahertz [sometimes called 99.9FM]. Stereo broadcasting

5MBS broadcasts in stereo which gives the 'presence and realism' which is essential for Fine Music broadcasting. There are two channels - the right and left. You will need a stereo receiver to receive our signal in stereo. However, if your receiver is a mono receiver [such as a portable or a bedside radio] you can still hear 5MBS but the sound will be mono.

In areas where the signal strength is low, stereo sound can become distorted. If your receiver has a stereo/mono switch, you can often improve reception by switching your receiver to the mono mode. The sound will then be high quality mono.

Getting good reception

Good FM radio reception depends on the signal strength in your area and the antenna you are using. Electronics and hi-fi stores sell suitable antennas and they are quite inexpensive. They come with instructions for installing them and connecting them to your receiver. Often they come with an amplifier which will boost low level signals. Before you purchase an antenna, check that the store will take it back if it proves to be unsatisfactory.

Signal strength

Signal strength depends on several things. The most important are the distance you are from our transmitter and the nature of the surrounding area. In general, if you are living within line of sight of the city and are on a hill, you will receive an adequate signal.

Poor reception will often occur if you live near or surrounded by high buildings, in a gully or an area lower than the generally surrounding area, or out of line of sight of the transmitter, or in a hilly or heavily wooded area.

A suitable radio is essential for good reception. In general, the better the quality of your radio receiver, the better your reception will be.

If you have difficulty receiving our signal, the best place to start is with the radio and its antenna.

Common Radio Reception Problems

Hissing

If your radio hisses on stereo but not on mono it is probably because it is receiving a weak signal. FM radio receivers need a stronger signal to decode the stereo component of a signal than for the mono component. A weak signal can be caused if you are too far from our transmitter or if your signal path is blocked by large buildings or hills.

A good quality outdoor antenna, correctly positioned to pick up the best signal will improve reception. If necessary an amplifier can be fitted to the antenna to boost a weak signal.

If your radio is a portable with no provision for plugging in an external antenna, you should try adjusting the position of the radio's own antenna to improve the reception. Adjusting the length of the antenna by pulling it in or out is usually an option. Alternatively, move the radio to somewhere else in the room as reception can vary a great deal over short distances. Radio reception is often better near windows or upstairs rather than downstairs. Try to keep your FM radio away from other electrical equipment such as refrigerators, computers, TV sets and power tools.

Twittering or discordance

Overloading causes uneven and discordant reception and this in turn can cause background twittering sounds. Overloading occurs when a radio antenna receives a very strong signal. It is usually only a problem if you live close to the transmitter. This distortion is present both in stereo and in mono reception. To resolve overloading an attenuator, available from radio and TV shops, can be easily plugged in between the antenna and the antenna socket.

Distorted 's' sounds

Multi-path distortion is characterised by sibilance, which is the distortion of 's' and 'z' sounds to 'shhhhh'. It is caused by the transmitted signal travelling to the listener's radio receiver via more than one path. This is usually caused by the signal being reflected off hills or tall buildings. The reflected signal arrives at the antenna a moment later than the direct signal because it has travelled further. The reflected and direct signals then interfere with each other causing the distortion. It is most commonly heard on car radios and when stationary, can often be rectified by moving the car just a few centimetres.

At home, the best way to minimise multi-pathing is to use a directional rooftop antenna, which only picks up signals direct from the transmitter and rejects signals that arrive at the back or side of the antenna. It is also sometimes possible to mount the antenna so that the house screens it from the reflections but not from the wanted signal. If multi-pathing is affecting a portable radio try moving it to a different position in the room.

If you are live under the flight-path of Adelaide Airport, you may hear interference when large aircraft are passing overhead. This is represented by a 'chuffing' sound. Similar sounds are sometimes heard when you are listening in a moving vehicle. It is almost impossible to overcome this type of interference which is generally very brief in nature.

Adjacent Channels

Adjacent channel interference is caused by a channel which is close in frequency to the station being listened to. It can sound like a twittering noise in the background and is sometimes known as 'birdies'. This problem is usually only apparent on FM stereo but if the interfering channel is very close in frequency, i.e. only 50 or 100 kHz away, the effect may also be heard in mono. If you are listening outside the geographical coverage area of the 5MBS transmitter, or if there are rare atmospheric conditions, you may suffer this interference problem. A good directional rooftop mounted antenna may solve the problem as with most radio reception problems providing that the interfering transmission is not coming from the same direction as the transmission you want. Some stereo tuners incorporate filters which can block out adjacent channels. If you are located within the main coverage area for 5MBS, see 'Interference Issues' below.

Interference Issues

For further information on interference issues, particularly those caused by other broadcasters or power utility companies, contact the ACMA. The ACMA produces a wide variety of useful fact sheets and brochures including 'Interference from CB and Amateur Transmitters' and offers an interference investigation service.

Background sizzling

Abnormal weather conditions can cause adjacent channel interference on television, while FM radio reception can be effected by sizzling noises in the background. This kind of reception problem is usually only apparent during periods of extremely high pressure, when temperatures increase dramatically to produce a much warmer drier atmosphere, usually with few clouds. Reception difficulties can occur due to interference from other UHF or VHF [FM radio and TV] signals, which are able to travel much further than the planned coverage area at times of high pressure. This is due to the changing properties of the lower layer of the earth's atmosphere where weather occurs. In normal conditions VHF and UHF signals travel through the troposphere in a curved path which allows good reception beyond the geometrical horizon. Any excess signals are usually lost in the earth's atmosphere, however, on rare occasions the excess or 'sky bound' signals are reflected back to earth outside the normal coverage area.

FM Antennas

FM Dipole Antenna

The FM dipole antenna is an economical and simple to install antenna which offers a considerable increase in reception quality. An FM dipole antenna is built from TV ribbon cable, which can be purchased from an electronics store.

How to make a folded dipole antenna

Use television ribbon cable. To receive 5MBS cut a length to 1.45 meters. Fold the cable in half to find the halfway point and mark this spot. Next, measure exactly 71.5 cm from this spot in both directions.

Now, at these two ends, cut the plastic shields around the copper wire and strip them to the ends of the cable. Be careful not to cut through the copper wire. Twist the ends of the copper wire together and solder the twisted wires together.

This makes a folded dipole where 'L' = 1.43 metres.

At the centre point that you originally marked cut away the plastic shield on one [only one] of the copper wires. Cut this wire in the centre of the region where the plastic has been cut away. Take a suitable length of ribbon cable [long enough to reach your radio receiver] and twist one wire to each of the centre wires of the dipole and solder the joint. [see diagram below]. You may need to solder a plug to the end of this cable if the only antenna connection to your FM radio is by a special socket. [see below].

Now, you can hide the dipole by running it along the top of bookshelves or you can tack it to a light wooden batten, e.g., 20-25 mm wide by 5-9 mm thick by 1.5 meters long. Stand or hang the dipole vertically inside the house or, if it improves reception, hang it outside. You can hang it from the guttering with a brass cup hook screwed into the end of the batten.

Some hi-fi tuners come with a ribbon-type antenna. Before you make one, check that there was not one in the box in which your tuner was delivered or one of your friends may have one which they are not using.

External FM Antenna

The best quality FM radio receiver system consists of an FM antenna, permanently erected externally. As FM radio signals occupy the same frequency band as some VHF television signals, an FM antenna is very much like a VHF television antenna. A VHF television antenna designed to receive TV channels 3, 4 or 5 may be used to improve FM radio reception.

A professional antenna installer can split the cable from your TV antenna and then run separate cables to the TV and FM receivers. If a splitter device is used, the signal strength is reduced. In some cases, particularly in low signal strength areas, an amplifier may also be needed. An amplifier boosts the signal and ensures that adequate signal strengths are supplied to both the radio and the television set. The amplifier will need to be connected to the electricity mains to provide it with the necessary power.

Connection to your tuner or FM radio

If your FM tuner or radio has a coaxial socket connection, it is simple and easy to make a connection from a ribbon antenna by using a 'balun'. These are available from electrical/electronic stores very cheaply and the TV ribbon can be screwed to the balun which is then plugged into the socket on the back of your tuner/radio.