KC8 - Basic Radio Communications

Question 1

Describe the characteristic and frequency ranges of HF, VHF and UHF communications system

Answer 1

HF
- 3 to 30MHz
- Long distance, uses atmosphere to bend transmitted signal to desired location

VHF
- 30 to 300MHz
- Short distance - line of sight (signal can bend around objects)

UHF
- 300 to 3000MHz
- Shorter than VHF - line of sight (wavelengths are the same size as buildings and terrain)

Question 2

Describe the relationship between Time and Frequency Domain

Answer 2

The purpose of Fourier Transform is to convert the time to frequency domain.

Question 3

Define modulation

Answer 3

A process where some parameter of a wave is varied in accordance with some parameter of another wave

Question 4

Describe Multiplexing Techniques

Answer 4

Transmitting multiple signals at the same frequencies generally will interfere with each other

Multiplexing and De-multiplexing will prevent that interference and ensure successful communication.

Question 5

List the differences between Analogue and Digital signals

Answer 5

Analogue Signals

• Infinite number of “values”
• Common signals
• Audio (narrow BW)​​​​​​
• Video (wide BW)
• Analogue systems tend to be design for particular type of signal

Cons
- Digitised analogue signal is an imperfect approximation of an - - – analogue signal due to noise
- repeater stations,
- static, multi-path, interference.

Digital Signals
- Finite number of “values”
- Greater noise immunity
- ability to use error-correction
- Can process different types of digitised signals

Cons
- Generally requires much larger bandwidth due to being only able to approximate the number of infinite values of a typical analogue signal.

Question 6

List the advantages and disadvantages of Data and Voice Communications

Answer 6

Voice Communications
Advantages
- Basic
- Simple & well understood protocols (e.g. AM/FM)

Disadvantages
- Time intensive
- Prone to errors
- High workload

Data Communications
Advantages
- Fast
- Accurate
- Flexibility
- Organised Information
- Less confusion on network (because of techniques such as multiplexing).

Disadvantage
- Bandwidth intensive
- Protocol specific

Question 7

State the reasons for using secure communications

Answer 7

The main purpose of secure communications is to limit the enemy’s detection and intelligence gathering capability?

Question 8

Describe the following concepts: Frequency Hopping

Answer 8

Frequency Hopping - the transmitter changes the carrier frequency during transmission in a pre-defined order

Question 9

Describe the following concepts: Spread Spectrum

Answer 9

Spread Spectrum - Transmitting smaller equal chunks of signal over different frequency channels