Mobile Service voice communications — Page 139, Lesson 124

I want to walk you through mobile service voice communications, starting with the two main modulation types you'll encounter. AM stands for amplitude modulation, and FM stands for frequency modulation — these are the two ways we impress information onto a radio wave. Let's begin with VHF frequency separation, sidebands, and bandwidth. When you modulate a carrier wave, you create side frequencies that spread both above and below the carrier frequency. The ones above are called the upper sideband, and the ones below are called the lower sideband. The total spread of all these frequencies in the modulated emission is what we call the bandwidth of the signal. Now, a voice transmission — or a music transmission — consists of many different audio frequencies. These go up to at least 5 kHz, and they're impressed onto the carrier wave. Because of that, many side frequencies exist in the modulated signal, and the bandwidth can be at least 10 kHz. This type of signal is classified as an A3E emission. A practical example of an A3E emission is VHF R/T — that's VHF radiotelephony, the voice communication you'll use in the cockpit. Let's move to VHF bandwidth allocation. Currently, for the most part, the bandwidth allocated to VHF frequencies is 25 kHz, which is 0.025 MHz. That 25 kHz is also the spacing between one channel and the next. Wherever channels are separated by 25 kHz, you should only use the first five digits of the frequency when transmitting it, and no more than two significant digits after the decimal point. However, when you're reading back a frequency — that is, repeating it back to confirm — you must say all six digits. If those six digits end with two zeros, a single zero is still considered significant. Let me give you two examples. The frequency 118.0 is transmitted as "ONE ONE EIGHT DECIMAL ZERO." The frequency 118.025 is transmitted as "ONE ONE EIGHT DECIMAL ZERO TWO FIVE." Now, this 25 kHz spacing is being reduced to 8.33 kHz — that's exactly one third of 25 kHz. This 8.33 kHz spacing is already mandatory for aircraft using the upper airspace over Europe under Eurocontrol. Wherever VHF channels are separated by 8.33 kHz, you must use all six digits of the numerical designator to identify the transmitting channel. And for all channels, you use three digits after the decimal. Here's an example for 8.33 kHz spacing: 118.005 is transmitted as "ONE ONE EIGHT DECIMAL ZERO ZERO FIVE." Notice that all six digits are spoken, and there are three digits after the decimal. So to summarize: 25 kHz spacing uses five digits transmitted but six digits on readback, with two digits after the decimal. 8.33 kHz spacing uses all six digits transmitted, with three digits after the decimal. This is critical for proper frequency identification in VHF communications.