Radio

Everything described in this article is simulated in SAS.

A radio is a two-way communication device that uses frequency modulated (FM) electromagnetic waves to transfer signals to receivers that are listening on the same frequency. The radio waves emitted by the radio device are subject to interference by signals at the same or nearby frequencies. Obstacles between transmitter and receiver disturb the propagation of the signal, and consequently reduce the signal strength.

Repeaters

Signal strength decreases as the waves have to travel greater distances through the atmosphere and upon encountering obstacles in their path. In order to transmit signals over greater distances and to avoid obstacles, a repeater can be used to reproduce and thereby boost the signal. Repeaters are often placed on elevated surfaces to have a clear line-of-sight between the transmitter and receiver.

Repeaters have a separate input and output frequency to avoid interfering its own receiver (effectively creating a loop). In order to use a repeater it is therefore required that the transceiver (radio device) supports separate input and output frequencies. Support for certain features differs per radio device.

Trunking

To minimize the required amount of frequencies used by radio operators, a technology called trunking can be utilized. A trunking radio system is controlled by a trunking controller which assigns frequencies on-demand and releases them them when they are no longer in use.

A practical example: in a traditional radio system separate frequencies are needed to create separate communication channels. For instance, police departments may use different channels for specific areas or incidents. In order to facilitate this segmentation in communication, a unique frequency is needed per channel. These frequencies are reserved permanently regardless of their usage, potentially wasting frequency space that is rarely used.

Trunking systems have a relatively small frequency pool assigned to them. These frequencies are assigned only when needed. When a request for transmission is received by the trunking controller, the trunking controller will look up a free frequency pair and assign them to that specific talkgroup. The trunking controller will then emit a signal to notify receivers within that talkgroup about the frequency assignment. The receivers will then automatically tune to the assigned frequency pair. This (data) communication between the trunking controller and receivers also uses electromagnetic waves and is subject to the same effects as any other signal, meaning that a receiver that does not receive the signal will not tune into the assigned frequencies.

Most devices will let the operator know when the frequency assignment process is finished and ready to transmit with an audible signal (usually a specific pattern of beeping tones). When the transmission is finished, the frequency pair that was used is released after which it is ready to be reserved again for new transmissions.

Interface

• /r, /xmit
  
  Closes the squelch and opens the transmitter to start transmitting. Transmitter is automatically closed after transmission, i.e. after you have typed your message.
  
  While the transmitter is open, background sound (like background chatter, a starting vehicle, gunshots, etc.) is picked up and transmitted on frequency.
  
  Incoming signals are not received with a closed squelch.
   
• /rc
  
  Closes the squelch and opens the transmitter to start transmitting. Transmitter does not automatically close after transmission, i.e. after you have typed your message.
  
  While the transmitter is open, background sound (like background chatter, a starting vehicle, gunshots, etc.) is picked up and transmitted on frequency.
  
  Incoming signals are not received with a closed squelch.
   
• /fr [frequency] ([mode =b])
  
  Program the specified frequency (in MHz) into the currently selected channel.
  
  Available modes:
  
  b = both - sets the specified frequency as both input and output frequency (on supported devices)
  i = input - sets the specified frequency only as the input frequency
  o = output - sets the specified frequency only as the output frequency
   
• /analog
  
  Switches to analog mode for the currently selected channel. Only on devices that support analog mode.
   
• /digital
  
  Switches to digital mode for the currently selected channel. Only on devices that support digital mode.
   
• /rscan
  
  Puts the device in SCANNING mode. While in SCANNING mode, the device will quickly hop over all channels until an active transmission is discovered. When an active transmission is discovered, the device will automatically return to NORMAL mode.
   
• /rsearch
  
  Puts the device in SEARCHING mode. While in SEARCHING mode, the device will quickly hop over all supported frequency bands (HF, VHF, UHF, etc.) to search for active transmissions. When an active transmission is discovered, the device will automatically program the frequency into a free channel. When no more free channels are available, the device will return to NORMAL mode.
   
• /channel [channel]
  
  Switches to the specified channel.
   
• /squelch [squelch]
  
  Sets the squelch value. The squelch controls the minimum required signal strength for received transmissions. If the received transmission signal strength is below the squelch value, the transmission will not be heard.
   
• /ctcss [ctcss code]
  
  Sets the specified CTCSS code for the currently selected channel. Only in analog mode.
   
• /dcs [dcs code]
  
  Sets the specified DCS code for the currently selected channel. Only in digital mode.
   
• /rkey [encryption key]
  
  Sets the encryption key for the currently selected channel. Only on devices that support encryption.
   
• /dropradio
  
  Drops the currently active radio on the ground.
   

Glossary

• CTCSS: Continuous Tone-Coded Squelch System or Continuous Tone Coded Subaudio Squelch
  
  Used to share a single frequency with multiple users. It does this by adding a low frequency audio tone to the voice. Where more than one group of users is on the same radio frequency (called co-channel users), CTCSS circuitry mutes those users who are using a different CTCSS tone or no CTCSS.
   
• DCS: Digital Coded Squelch
  
  The digital variant of CTCSS (see above).