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Radio Theory

The Radio Wave

A radio wave can be defined using two properties - frequency and amplitude, therefore, data can be transmitted by varying each of these two properties

Amplitude Modulation (AM)

  • Generally longer range

Frequency Modulation (FM)

  • Provides a less noisy signal
  • More impacted by physical barriers

AM vs. FM

(Military Aviation) Radio Bands

VHF FM Band

  • 30-88 MHz
  • 25KHz Channel Spacing
  • Calling Frequency: 30.0 MHz
  • Typically used for A/G communication - e.g. with a JTAC

VHF AM Band

  • 118-137 MHz (non-voice part: 108-118 MHz)
  • 8.33 kHz Channel Spacing
  • If possible, channels are spaced by 25kHz
  • Calling Frequency - 121.5 MHz
  • Used by Civilian Aviation

UHF AM Band

  • 225-400 MHz
  • 100 kHz Channel Spacing
  • Calling Frequency - 243.0 MHz (GUARD)
  • Used for Military Aviation communications

Restrictions on Radio Communications

Unfortunately, radio is not like a discord call, there are certain restrictions which we must be mindful of when operating a radio.

How do we tell if we're getting a transmission or just noise?

  • We set a 'squelch', this means that the radio only outputs when the receiving power is above a certain threshold.

What happens when we want to transmit and receive at the same time?

  • All aviation radios use simplex (sometimes called semi-duplex) channels - the same frequency to transmit and receive - this means that when you're transmitting, you cannot receive others' transmissions.
  • A traditional duplex channel will use one frequency to receive, and one frequency to transmit

What happens when multiple radios transmit at once?

  • AM - The radio waves interfere, if one station is transmitting at a much higher power, it will be heard over the other, if not, neither will be heard and the output is known as a 'blocking tone'
  • FM - The output rapidly switches between stations - known as 'picket fencing'

Guard Receive/Multi-Watch

Another feature of modern radios is the ability to operate on one frequency but receive on multiple - this functionality is known as multi-watch. Often it is used to monitor the band's calling frequency (hence the name GUARD receive).

Multi-Watch works by flicking between watched frequencies and listening for a transmission. If the radio detects a transmission on a watched frequency, it 'locks onto' that transmission, once the transmission is over it goes back to watching.