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Community AM radio: how do you listen to it? Do you wish to listen to your local community AM radio station but find that its frequency is “off the dial”? You can buy a cheap radio to cover the extended band or you can tweak your existing radio. We show you how. By RICK WALTERS The frequency range of the Australian AM band is from 531kHz to 1602kHz but recently a range above the top of the band (from 1611kHz to 1705kHz) has been allocated to “narrow band area services”. Community radio stations have been allocated frequen­cies in this new band. They operate on low power and are only meant, as the name implies, to cover a small area within a radius of 20-25km. They operate on a restricted bandwidth (narrow band) of ±3kHz and because of this, they do not transmit This Digitor AM/FM radio costs $12.95 from Dick Smith Electronics. It already covers all the new community stations without modification. Alternatively, you can tweak an old AM receiver if you have one lying around. wideband hifi sound. (Other AM stations do broadcast hifi sound but you’d never know it because most AM radios produce poor quality sound). For their intended use, community news and information, the narrow audio bandwidth is not a major drawback. The problem is that normal AM receivers, and especially AM/FM tuners with digital displays, will not cover this addition­al range. The same comment applies to car radios. So why not design a down-converter which would translate these stations into frequencies which can be tuned by a standard receiver? When we looked at the cost of a small AM/FM radio, we realised that the “converter” would cost many times more than the radio. We therefore decided that, in view of the cost of a down-converter and the relative difficulty in connecting it to an AM/FM tuner or car radio, the project was not really viable. OK then, could we convert an “offthe-shelf” AM radio to cover this extended band? Being the last of the big spenders (and having to get the money back from the boss), we bought a $12.95 Digitor AM/FM pocket radio from Dick Smith Electronics. When we got it back to the lab and opened the box, one of the interesting things about the radio was its dial coverage. It was scaled from 520kHz to 1710kHz, which meant it should already cover all the new stations. Checking this range using a signal generator showed that it actually tuned from 537kHz to 1720kHz. So who needs a converter? Just buy this radio and the problem is solved. Tweaking your own radio Maybe you have an old AM radio lying around and would like to try to 86  Silicon Chip extend its high frequency coverage. We can only give you a broad outline of the procedure involved as the location of the components you have to adjust will vary from one unit to another. The simplest way to move the tuning range higher is to reduce the inductance of the local oscillator coil (we are assuming that any set you adapt will be a superheterodyne). Now before we go any further we should state the drawback of this sort of modifi­cation. It will upset the tracking of the gang over the tuning range of the dial and so if you rely on stations appearing at particular parts of the dial, they will inevitably be shifted. However, the aim of the modification is to receive community radio stations. Identifying the oscillator coil Which one’s the oscillator coil? If the receiver uses small metal can coils, the oscillator will probably have a red spot on its slug. To decrease the inductance you need to rotate the slug anticlockwise. Once this is done, the aerial input coil should be peaked for maximum volume on the new frequency. If there is no red dot then you face a dilemma. If you are not the adventurous type, put the back on the radio and put it back on the shelf. After all, if you don’t twiddle anything, the radio will still work as it did. On the other hand . . . The oscillator coil is the only one that will move the received frequency dramatically when the slug is moved. Tune the radio to a local station around 1500-1550kHz and rotate the slugs 1/4 turn anticlockwise, one by one. If the volume changes but the audio stays clear, return the slug to the original position and try the next coil. The effect should be similar to tuning slight­ly off frequency, where the sound becomes slightly distorted or sibilants become pronounced. One of our photos shows the position of the oscillator coil in the radio we purchased. Once you have identified the oscillator coil, set the tuning pointer right against the high frequency end stop, then move it back a fraction. This allows you to easily find your local community station. Now adjust the oscillator slug until the station is heard at maximum level. Keep backing off the volume control The Digitor AM radio covers the range from 520kHz to 1710kHz and it uses a single surface mount IC which is soldered on the copper side of the PC board. OSCILLATOR COIL This photo shows the position of the oscillator coil in the Digitor AM radio that we purchased. Often, the oscillator coil will be identified by a red dot on its slug. If there’s no red dot, try rotating the coil slugs 1/4 turn anticlockwise, one by one, and observe the effect (see text). as you turn the slug as it is easier to distinguish this peak at low volume levels. The final step is to peak the aerial trimmer on the gang. This tunes the aerial coil on the ferrite rod to the new frequen­cy. There may be three or four trimmers, so again follow the same procedure. Move each in turn and if it appears to have no effect, move it back to the original position and try the next one. While these adjustments may upset the tracking and sen­sitivity a little at other frequencies, we feel that the main aim is to get the best possible reception on the new channel. If you want a list of the broadcasting frequencies of com­munity radio stations in your area, contact­the Australian Communications Authority by phoning (02) 6256 5555. Alternatively, you can get this information via their SC website at www.aca.gov.au January 1999  87