Chapter 2 - Section 1: Basic principles Flashcards
When are HF communications equipment installed on aircraft?
When they are flying routes where, for some part of the flight, aeronautical radio stations are out of VHF range
What is the frequency bands for HF communications?
2 - 30 MHz
With regards to HF, what do modern equipment provide?
SSB and AM modes of operation on any one of 28,000 frequencies spaced 1 kHz apart
What are the advantages of SSB over DSB?
- Uses less power
- Uses less than half the bandwidth
- Better signal to noise ratio
- Less prone to selective fading
By how much is the signal to noise ratio better in SSB over DSB?
9 dB
What is interlock with regards to the HF system?
Only one transceiver can transmit at one time
What is the common method by which to interlock the HF system?
Only one aerial suitable for transmitting
Two aerials can be used for receiving
What will happen where an interlock is fitted?
- Inhibit Tx2 (by o/c on PTT2 line)
- Earth second aerial
- Earth audio output of Rx2
- Switch aerial 1 from Rx1 to Tx1
- Earth RF input to Rx2
- Earth aerial input to Rx1
What does the MHz control switch do?
Operates, via an autopositioner, to control the tuning elements of the HF oscillator
What does the kHz control switch do?
Control an autopositioner to select the tuning elements to tune a variable frequency oscillator (VFO)
Where is the antenna tuner unit located?
Where the associated aerial lead-in enters the aircraft
How does AGC differ in SSB and DSB?
DSB - Derived from detection of the carrier RF
SSB - Derived from detection of sideband RF
Where is real sidetone taken from in DSB and what advantage does this confer?
Taken from the PA output to aerial circuit
Proves the transmitter is operating correctly
How is sidetone developed in SSB?
PA output is sampled and rectified but the DC content is used to operate a relay
This will make available the output of the modulator as sidetone to the AIS
