Wider Is Better?

A Discussion On Bandwidths

By Jonathan Imberi

Bandwidth is the amount of space in the radio frequency spectrum that a signal occupies. The information rate determines the bandwidth of a transmission. Therefore, a pure, continuous unmodulated carrier has a very small bandwidth with no sidebands. Because a television transmission contains a great deal of information it is several megahertz wide.

The determining factor in how well you can receive one signal in the presence of another signal that is very close in frequency is called receiver bandwidth. The enjoyment you'll experience will depend greatly on how well you can isolate the signal you are receiving from all others nearby.

Bandwidth is merely a measure of selectivity, or, how wide a range of frequencies is received with the receiver tuned to a particular frequency. For example, if a receiver has a bandwidth of 6 KHz it means that you can receive signals 3 KHz above and below the tuned frequency. The narrower the bandwidth the greater the selectivity, making it easier to copy one signal when there is another one close by in frequency.

Special intermediate frequency (IF) filters built into each receiver determine its selectivity. Some receivers have several filters so you can choose different bandwidths. They are necessary because some emission types occupy a wider frequency range than others. A 250 Hz bandwidth filter is excellent for separating CW signals on a crowded band, but it is useless for listening to SSB, AM, or FM transmissions. A wider filter is needed to allow all the transmitted information on SSB, AM, or FM transmissions.

2.8 kHz is the standard filter selectivity of receivers designed for SSB voice operation. SSB usually has a bandwidth between 2 and 3 kHz. A human voice contains frequencies higher than 3 kHz, but all of the sounds necessary to understand speech are between 300 Hz and 3,000 Hz. Most amateur voice transmitters limit the bandwidth of a transmitted audio signal between 300 Hz and 3,000 Hz. The bandwidth, 2700 Hz, is the difference between these limits. If you use a filter selectivity of 2.8 kHz, you can see that your receiver will reproduce the full range of transmitted audio.

Although 2.8 kHz is also usable on CW, a narrower bandwidth is needed to prevent adjacent CW signals from seeping through at the same time. Amateurs prefer a filter bandwidth of 500 Hz or even 250 Hz for CW operation. A radioteletype signal has a bandwidth that is wider than a CW signal, but a 500 Hz or 250 Hz will work for this mode too.

CW signals have the narrowest bandwidth of any amateur emissions. Radioteletype emissions are wider than CW, and SSB signals are even wider than that.


The frequency of the transmitted RF signal varies an amount that depends on the strength of your voice when you transmit FM voice emissions. When you speak louder, the frequency changes more than when you speak softly. The frequency, the pitch of your voice, or another signal used to modulate the transmitter controls how fast the frequency will change. Higher frequency tones vary the frequency at a faster rate than low frequency tones. Frequency deviation is the instantaneous change in frequency for a given signal. The frequency swings just as far in both directions, so the total frequency swing is equal to twice the deviation. There are also sidebands that increase the bandwidth even further. A good estimate of the bandwidth is twice the maximum frequency deviation plus the maximum modulating audio frequency:

Bw = 2 x (D+M)

where:

Bw = bandwidth

D = maximum frequency deviation

M = maximum modulating audio frequency

An FM transmitter using 5 kHz deviation and a maximum audio frequency of 3 kHz uses a total bandwidth of about 16 kHz. A typical FM signal may have an actual bandwidth somewhat greater than this. A good approximation is that the bandwidth of an FM voice signal is between 10 and 20 kHz.