Basic Radio Principles

Question 1

Describe the basic features of electromagnetic radiation waves

Answer 1

Electromagnetic waves are formed by an oscillating magnetic and electric wave acting perpendicular to each other.

Travel at speed of light (300,000,000 m/s)

Question 2

Describe where radio waves exist within the electromagnetic spectrum.

Answer 2

Radio waves are electromagnetic waves that are found at the lower frequency end of the electromagnetic spectrum.

Question 3

What is a cycle

Answer 3

the complete oscillation of the wave around a mean value (shown by the central black line in the diagram)

Question 4

What is amplitude

Answer 4

the maximum point of deviation away from the mean value, otherwise known as wave height.

Question 5

What is wavelength

Answer 5

wavelength is the physical distance required to complete one cycle.

Question 6

What is frequency and its relation to wave length

Answer 6

the amount of times a wave completes a cycle in a second, expressed in hertz (Hz).

They are inversely proportionally, increase of frequency is a decrease in wavelength and the reverse.

Question 7

Calculate frequency, given wavelength.

Calculate wavelength, given frequency.

Answer 7

Wavelength = 300,000,000 / frequency

(Lambda) Y = v/f

Question 8

What is phase and what’s meant by in phase and out of phase (phase difference )

Answer 8

refers to a given point in a wave’s cycle. (Basically its position) done by referring to a circles 360º

If a wave is in phase it’ll increase signal strength, but, if two waves are 180º out of phase, they’ll cancel each other out.

Question 9

What is attenuation

Answer 9

the reduction of signal strength over a given distance.

Question 10

Describe polarisation of a radio signal and its relationship to the orientation of transmission and receiving aerials.

Answer 10

Polarisation refers to the geometric orientation of the electrical wave oscillation when transmitted. (E.g vertical aerial transmits vertically polarised electrical wave)

using an antenna to receive a radio signal, the antenna orientation must match the radio wave polarisation: (vertically polarised wave, receiving aerial must also be orientated vertically and same for horizontal)

Question 11

Explain modulation of a carrier wave

Answer 11

The adding or superimposing of data on a wave.

• Audio signals (speech and music) and other data can be transmitted to a remote receiver.
• The basic wave signal transmitted from the aerial at a specific frequency: Carrier Wave.
• Added waveform for modulation: Modulating Wave

Question 12

distinguish between amplitude modulation (AM) and frequency modulation (FM) and what should be added to FM

Answer 12

FM - Frequency modulation uses varying frequency in the transmitted radio wave to deliver a modulated wave.

Due to the varying frequency there must be available sidebands (otherwise known as bandwidth) around the selected carrier frequency

e.g radio wave of 100Hz being transmitted there must be an available bandwidth of e.g 10Hz each side to transmits the modulated wave bandwidth of 90-110Hz used.

AM - Amplitude modulation uses varying amplitudes in a radio wave to deliver a modulated signal.

Question 13

State the relative advantages and disadvantages of AM and FM

Answer 13

FM Advantages:

• The efficiency of transmission is high, meaning a strong signal with less power required to produce it
• Less static and atmospheric interference (otherwise known as “noise”) due to the constant amplitude

FM Disadvantages:

• Extra frequencies are required
• The radio transmitter and receiver are generally more complex than AM

AM Advantages:

• Simplicity - transmitters and receivers are less complex
• AM waves can travel long distance

AM Disadvantages:

• More power is required to produce an AM signal
• Susceptible to atmospheric interference - static

Question 14

Describe single sideband (SSB) and state the advantages and disadvantages in its use.

Answer 14

single sideband essentially halves the required bandwidth. This means more power can be applied to the remaining “side” to give a clearer and stronger transmission.

Advantages

• for a given power output, SSB has a larger reception range.
• Because the required bandwidth is reduced, it allows more useable frequency channels to be allocated.

Disadvantages
- transmitter/receiver complexity, meaning a much higher cost is involved

Question 15

Describe surface waves and the effects of diffraction, scattering, and wave tilting

Answer 15

Radio waves typically travel in line of sight. Lower wave frequencies are able to bend and follow the surface of the earth though.

Diffraction - spreading of waves around an object. Much easier for lower frequencies. Wont always fully conform however (like on a mountain) leaving a zone without coverage.

Scattering - when waves scatter or bounce off an object or terrain. Occurs in higher frequency bands. This means its harder for them to bend around a mountain causing a greater zone of no reception.

Wave tilting - when a radio wave comes in contact with the earths surface it essentially bends over itself and conforms to the surface allowing it to travel beyond the horizon.

Question 16

Describe sky waves and the effect of frequency, critical angle, skip distance and dead spaces. And what’s the frequency range

Answer 16

Sky waves occur when a radio wave reflects off the ionosphere.

Critical angle - smallest angle at which a skywave can reflect off the ionosphere and return to earth. Any smaller it’ll penetrate

Skip distance - the distance the wave skips, measured from the transmitter to the point where the first sky wave returns.

Dead Space - the space where the aircraft will receive no radio signal between the end of the surface wave and the first returning skywave.

Frequency - If a frequency is too low, it will be absorbed into the ionosphere and fail to produce a return wave.

If the frequency is too high, it will penetrate through the ionosphere and could continue into space.

Frequency band for sky waves is medium-high frequency

Question 17

Describe direct waves and the rule-of-thumb formula for calculating maximum range of reception.

Answer 17

Very High Frequency (VHF) waves and above are known as direct waves or “line of sight” waves. As the name implies, these waves can only be received in line of sight transmissions, meaning they do not bend around obstacles

Range = 1.25√Tx height + 1.25 √Rx height

Question 18

Briefly describe the effects of static and atmospheric attenuation.

Answer 18

Static interference occur below VHF bands but can affect VHF bands

Common causes of static are like THUNDERSTORMS, TRAVELLING AT HIGH SPEEDS
GREATER IN SUMMER

Due to greater sky waves at night the static is greater during the day
SHF are most susceptible to attenuation

Question 19

Explain the relationship between frequency and refraction in the ionosphere.

Answer 19

VLF-Tend to get absorbed by the ionosphere
MF/HF- Are optimum frequencies of returning sky waves
HF< - Continues into space with no return

Question 20

Describe the changes to the height of the ionosphere at night and the effect of this change on the optimum useable HF frequencies.

Answer 20

At night the lower level of the ionosphere disappears which changes the optimum frequencies.

During the night the optimum useable HF frequencies are lower frequencies as attenuation has not ceased. Higher frequencies may actually penetrate the ionosphere

Question 21

Explain the optimum useable frequency of an HF signal.

Answer 21

3-30MHz

Question 22

What is square law of attentuation

Answer 22

3 times the distance = 9 x signal loss