Page 87 - Communication IFR_Neat

Frequency)

9.4 VHF FREQUENCY SPREAD

The frequencies in the part of the VHF band that may be of concern to the

pilot are as follows:

(AM stands for amplitude modulation and FM for frequency modulation)

9.5 VHF FREQUENCY SEPARATION

Sidebands and Bandwidth.
The spread of side frequencies above and below the carrier frequency are known

respectively as the upper and lower sidebands. The total spread of frequencies

in the modulated emission is known as the Bandwidth of the signal. A voice (or
music) transmission consists of many different audio frequencies, up to at least 5

kHz, impressed on the carrier wave. Consequently many side frequencies exist in

the modulated signal, which may have a bandwidth of at least 10 kHz. Such a
signal is classified as an A3E emission; an example is VHF R/T.

VHF Bandwidth
The bandwidth allocated to VHF frequencies is at present for the most part 25kHz

or 0.025mHz i.e. the spacing between one channel and another.

Wherever channels are separated by 25kHz, only the first five digits should be
used, not more than 2 significant digits after the decimal point. In the case of

these being 2 zeros, a single zero is considered significant:

118.0 transmitted as ONE ONE EIGHT DECIMAL ZERO
118.025 transmitted as ONE ONE EIGHT DECIMAL ZERO TWO

However, this is being reduced to 8.33kHz and is already mandatory for aircraft
using the upper airspace over Europe under Eurocontrol. Wherever VHF channels

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