Compare AM with FM

AM

FM

Bandwidth

Small (9kHz)

• Narrow band FM has a small bandwidth but the quality is not so good.
• Broadband FM has a much greater bandwidth and better quality too.

Cost

Low cost (Wind-up radios for use in "third world nations")

More expensive

Complexity

Simple

More complex

Noise Immunity

Susceptible to noise caused by lightning strikes, sparks in switches (including older car ignition systems) and some types of motor.

Better noise rejection because the FM can be amplified until limiting takes place. Any amplitude noise is removed by this process.

Distortion

Potentially serious distortion because diode demodulators don't work well with weak signals.

FM demodulators work very well. Although restricting the FM bandwidth causes some distortion, the problem is minimal compared with AM.

Fading

Signals traveling by different paths can arrive in or out of phase causing louder or fainter reception.

Multi-path reception occurs when the signal is reflected off buildings, terrain, vehicles or the ionosphere. The ionosphere changes greatly between day and night and it reflects medium and short wave transmissions (0.153 MHz to 30 MHz).

Because the FM can be amplified until limiting takes place, fading is not noticeable unless the signal becomes very weak.

FM also suffers from multi-path reception. Most of the reflections are from buildings, terrain or vehicles. This causes rapid fading (or flutter) and is most noticeable in moving vehicles.

Also listen out for FM radio signals bounced off aircraft. This causes a distinctive fading effect which sounds like an old steam railway engine slowing, stopping and then setting off again. It is most noticeable on weaker stations. Its called aircraft flutter or a chuff chuff.

Selective Fading

This effect can cause very severe distortion if the carrier has faded out but there are still sidebands. The programme content can be unintelligible. It is is caused by multi-path reception. The carrier fades out if the two signals arrive out of phase. The same effect can cause parts of the sidebands to fade. This causes a distinctive phasing effect on the received signal.

FM is mostly transmitted above 30MHz and multi-path reception is less common.

Range

Potentially global depending on the frequency used and the ability of the ionosphere to reflect the signals around the world.

Line of sight because VHF frequencies are normally used and the ionosphere does not reflect these. Freak atmospheric conditions can increase the range of VHF FM. These conditions usually occur in the evening after a hot still sunny summer day. The effect is similar to a mirage in the desert. Terrestrial TV reception can be poor during these conditions.

Transmitters

One AM transmitter on Medium Wave can cover most of the UK (a few hundreds of miles).

Many transmitters are needed to cover a country because of the line of sight limitation.

Local Radio

Not ideal because the range is too big.

Good for local radio because of the smaller range.