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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
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