FM radio (sometimes known
as VHF) offers high quality listening and stereo sound but it can
sometimes be spoiled by reception problems and interference. Most of
these problems are well known and can be dealt with easily to enable
you to get the most out of FM broadcasts. The information below
describes the most common problems and tells you what you can do about
Weak Signal - Hiss
If your radio hisses on stereo but not on mono it could be because it
is receiving too weak a signal. This happens because radio receivers
need a stronger signal to successfully decode the stereo component of
the signal than for just the mono component. To solve this problem you
need a good aerial, correctly positioned to pick up the best possible
signal. Ideally it should be mounted outdoors, like a TV aerial. If
really necessary an amplifier can be fitted to the aerial in order to
boost a signal which is weak, for example, because it is attenuated by
a very long downlead. If your radio receiver is portable with no
provision for plugging in an external aerial, try adjusting the
position of the radio's own aerial by tilting and swivelling it to get
the best signal, (see Fig 1). If this fails, try moving the radio to
somewhere else in the room because FM reception can vary a great deal
over very short distances. It is often better near windows and may also
be better upstairs than downstairs.
Fig. 1: Adjust the position of the aerial to get the best
and try moving the position of the radio - perhaps near a window.
Multipath distortion is characterised by the specific distortion
of 's' and 'z' sounds (sibilants) such that 's' sounds like
'shhh'. In its more serious forms it effects all the sounds to make
them rough sounding or 'brittle'. It is caused, as the name implies, by
the transmitted signal travelling to the receiver via more than one
path: a common cause of this is reflection of the signal from hills or
tall buildings, (see Fig 2). The reflected signal arrives at the aerial
a moment later than the direct one because It has travelled further.
The reflected and direct signals then interfere with each other causing
The best way to minimise multipath distortion is to use a directional
rooftop aerial, i.e. one which will only pick up signals coming from
the direction of the transmitter, and will reject reflections which
arrive at its sides or its back. It is also sometimes possible to mount
the aerial so that the house screens it from the reflections but not
from the wanted signal. If you have a multipath distortion problem with
a portable radio try moving it to a different position in the room.
Fig. 2: Multipath reception - the radio signal arrives at the
via two or more paths. The time difference between these signal
paths causes 'multipath distortion' to the audio.
Adjacent channel interference is interference from a channel which is
close in frequency to the one on which you are listening. It sounds
like a twittering noise in the background and is consequently sometimes
known as 'birdies'. The problem is usually only apparent on 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.
The BBC's FM transmitter network has been designed to avoid such
problems but if you are listening outside the published service area of
the transmitter, or if there are rare atmospheric conditions, you
could still suffer from this problem.
As with most reception problems, a good directional rooftop mounted
aerial will probably solve the problem, providing the interfering
transmission is not coming from the same direction as the transmission
you want. Some stereo tuners incorporate 'birdie' filters which use
electronic circuitry to block out adjacent channels.
Overloading happens when a FM receiver receives too strong a signal. It
should only be a problem if you live close to the FM transmitting
station. Overloading causes intermodulation distortion which very rough
and discordant and this in turn will cause twittering 'birdies' in the
background. The distortion will be present both in stereo and in mono.
The cure for overloading is to fit an attenuator between the aerial and
the receiver. Attenuators are available from radio and TV shops and
they are easily plugged in between the aerial and the aerial socket.
They come in various values, quoted in decibels (dB), and your, dealer
should be able to advise you on the correct amount of attenuation you