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VHF radiocommunications

Typical recreational transceiver
– the Microair M760

Rev. 4 — page content was last changed November 28, 2009

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The Microair 760 is a very lightweight, low power consumption, panel-mounted, Australian-designed COMMS band transceiver and NAV band receiver; it is suitable for light recreational aircraft.* The unit is illustrative of the control and operating functions to be found in most modern electronic transceivers.

*The audio identification channels (in the NAV band between 108.0 and 117.975 MHz) of aerodrome navigational aids are often used for aerodrome weather reports so VHF voice reception in the NAV band is very useful for acquiring weather and other information in-flight.

The first two sections in the following material are edited extracts from the Microair M760 transceiver installation and user manual.

4.1 Transceiver control functions

Microair 760 front panel

1. Priority Switch
The priority switch is a push-down switch designed to activate an emergency frequency. When pushed down briefly, the radio will go into memory mode and select the frequency stored in the memory 99 position — usually the international aviation distress frequency 121.5 MHz.
2. Volume / On / Squelch knob
The M760 is turned on by rotating the volume knob. A positive 'click' is heard and felt at the start of the rotation to indicate the on/off position. The volume is increased by rotating the knob clockwise, and decreased by rotating counter-clockwise.

The squelch is adjusted by rotating the ring behind the volume knob. There is no automatic level set for the squelch; however, the ring has a large manual adjustment to suit all situations. Rotate the ring clockwise to increase the squelch threshold and counter-clockwise to lower the threshold. When the squelch is 'broken' (i.e. the static hiss can be heard) the annunciator light emitting diode [LED] lights green.

Note: This does NOT mean you are receiving a signal on the selected frequency!
3. Receive / Transmit annunciator
The LED operates red or green, and indicates the following states:
  • clear (off): radio is squelched, and is not receiving a signal

  • green: squelch is broken or a signal is received

  • red: radio is transmitting

  • flashing red: radio has transmitted for over 30 seconds (warning)
The flashing red signal may draw the user's attention to the fact that the aircraft may have a stuck PTT button. There is no likely scenario where a valid transmission exceeding 30 seconds would be required.
4. Mode switch
The mode switch is a push-down switch. When pushed down briefly, the radio will step to the next operating mode. The M760 has three operating modes:
  • active/standby mode
  • channel mode
  • intercom adjust mode.
5. Frequency adjust knob
The frequency adjust knob is used to change display values and characters. Rotate the knob to scroll values or characters up or down. Press the frequency adjust knob inwards briefly to move the cursor to the next display item to adjust. In the active/standby mode, only the standby frequency can be changed directly — the active frequency cannot be altered directly by the frequency adjust knob.

In channel mode the pilot can scroll alphabetically by turning the frequency adjust knob. Adjustment is restricted to the pre-programmed values stored in memory.
6. Toggle switch
The toggle switch is a push-down switch. When pushed down briefly in active/standby mode, the active and standby frequencies exchange places. Hold the toggle key down for three seconds to activate the scan function.
7. Liquid crystal display
The display has two lines, each of eight characters of information. In normal operations the active frequency appears in the top line and the standby frequency in the lower line, as shown.

4.2 Operating procedure

The M760 should always be turned off before starting the aircraft, to protect the radio from transient voltages.

There is a requirement (AIP GEN 3.6 para 8.2) that pilots should monitor 121.5 MHz before engine start and after engine shutdown, to check for transmissions and to ensure that your own distress beacon is not activated.

After starting, the radio can be turned on, and the squelch adjusted so the static hiss can be heard through the headphones. The LED annunciator will light green while the hiss is heard. Use the hiss tone to adjust the volume to an appropriate level. With the volume set, turn the squelch ring to break the squelch and eliminate the hiss. The LED annunciator light will go clear.

The M760 can now be adjusted to the correct active and standby frequencies, by scrolling and pressing the frequency adjust knob.

The M760 will transmit when the PTT button is held down unless the CD lockout function is enabled and the unit is currently receiving a signal. When transmitting the user will hear themselves speaking through their own headphones via the sidetone system. The LED annunciator will light red.

If the transmission lasts longer than 30 seconds, either because you have a lot to say, or because the PTT has stuck, the LED annunciator will flash red. When this happens, check the PTT immediately. If you find no obvious fault, turn the radio off and then on again. If the LED is still red, turn the radio off and leave it off.

4.3 Fault tracing

Pre-flight check
The following check list is applicable to all hand-held and fixed panel-mounted transceivers and associated system components:

1. No apparent power
  • All necessary main/avionics power switches ON?
  • Transceiver switch ON?
  • Aircraft battery/transceiver batteries drained?
  • Fuse blown/circuit breaker popped?
2. No background hash heard
  • Volume turned down?
  • Squelch set too high?
  • Headset correctly plugged?
  • Headset compatible with transceiver?
  • Earpiece volume controls turned down?
  • Antenna missing or damaged?
  • Antenna feedline coax correctly plugged in at both ends?
3. No signal reception or transmission response
  • Correct local frequency set as active?
  • Other stations (e.g. airfield Unicom) not listening out?
  • Squelch set too high?
  • Volume turned down?
  • Loudspeaker too far from ears?
  • Microphone compatible with transceiver?
  • Microphone not close to lips?
  • PTT or microphone switch inoperative?
  • Airframe structure blocking signal?
  • Airfield structures blocking signal?
Interference problems
When airborne, continuing excessive background crackle, hum or howling heard in the headset/speaker is usually associated with the components, active wires and ground wires within the electrical system. The engine ignition system may also be a source of RF interference. The identification of the cause, and fixing the problem(s), may well be a difficult and protracted process requiring expert help.

Groundschool – VHF Radiocommunications Guide

| Guide content | Abbreviations and acronyms |

| 1. Transmitter licensing | 2. R/T phrasing | 3. VHF characteristics and radio operation |

| [4. Microair 760 transceiver] | 5. R/T procedures | 6. Safety and emergency procedures |

| 7. Aviation Distress Beacons | 8. Understanding SAR services |

Next –  R/T procedures The next section of the VHF radiocommunications guide outlines radiotelephony procedures