Antenna Theory Flashcards
Define the term “Antenna”?
An antenna is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver.
Explain why an Antenna can be considered as a Transducer?
An antenna is a transducer designed to transmit or receive radio waves which are a class of electromagnetic waves. In other words, antennas convert radio frequency electrical currents into electromagnetic waves and vice versa.
What range of frequencies within the Electromagnetic Spectrum are Radio Waves?
Radio waves have frequencies as high as 300 gigahertz (GHz) to as low as 30 hertz (Hz).
What is the speed of a Radio Wave?
Radio waves travel at the speed of light (approximately 300,000 km/s) in vacuum.
Explain the structure of a Radio Wave?
All Electromagnetic Waves consist of two separate fields.
Fields are known as the “E Field” and “H” Field;
• E Field = Electric Field
• H Field = Magnetic Field
Electric + Magnetic = Electromagnetic
Define how an Antenna produces Radio Waves?
Radio waves are produced when the electric field rapidly changes. There has to be an alternating current.
This field sends a wave outward from the wire.
When the current is removed, the field collapses which again sends a wave.
State how wavelengths are measured?
Wavelength is measured in metres
The measurement is the distance between adjacent points in a series of periodic waves
Denoted by the Greek letter λ (Lambda)
Define the term “Frequency”?
Frequency is the number of waves that pass a point in one second.
The unit of measure for frequency is Hertz (Hz).
1000 Cycles/Sec = Kilohertz – KHz
1,000,000 Cycles/Sec = Megahertz – MHz
1000,000,000 Cycles/Sec = Gigahertz - GHz
What is the relationship between Wavelength and Frequency?
The lower the frequency, the longer the wavelength.
The higher the frequency, the shorter the wavelength.
Frequency and wavelength are said to be;
Inversely Proportional
Calculate the wave speed (in m/s) for a wave with a frequency of 10,000,000Hz and a wavelength of 30m?
Velocity is a constant 300,000,000 meters per second
Velocity (V) = Frequency (f) x Wavelengh (λ). (m/s)
• Calculate the wavelength (in metres) of a wave with a frequency of 440Hz travelling at 340 m/s?
Velocity is a constant 300,000,000 meters per second.
Frequency (f) = Velocity (V) x Wavelengh (λ). (Hz)
Calculate the frequency (in Hz) of a wave travelling at 300,000,000 m/s with a wavelength of 0.15m?
Velocity is a constant 300,000,000 meters per second
Wavelengh (λ) = Velocity (V) Divided by Frequency (f). (m)
Define the term “Propagation”?
The movement or spreading of waves.
What are the three modes of Propagation?
- Ground Waves
- Sky Waves
- Space Waves
What are the four factors that affect the Angle of Propagation?
- Height.
- Frequency.
- Angle of Elements
- Terminating Resistance.
Define Ground Wave Propagation?
Ground Wave propagation is a method of radio wave propagation that uses the area between the surface of the earth and the ionosphere for transmission.
Explain how Sky Waves can be transmitted over large distances?
By using the Ionosphere,
The lower frequency waves reflect off the Ionosphere and essentially “skip” around the earth.
Explain Space Wave Propagation?
The space waves can travel through atmosphere from transmitter antenna to receiver antenna either directly or after reflection from ground in the earth’s troposphere region.
What are Polar Diagrams used for?
Polar diagrams are used for plotting the radiation patterns of antennas as well as other applications like measuring the sensitivity of microphones in different directions.
Explain Sky Wave Propagation?
The sky waves are the radio waves of frequency between 2 MHz to 30 MHz. These radio waves can propagate through atmosphere and are reflected back by the ionosphere of earth’s atmosphere.
Explain the term “Gain” with regards to an Antenna?
As an antenna, the gain describes how well the antenna converts input power into radio waves headed in a specified direction or radio waves arriving from a specified direction into electrical power.
How is Gain measured?
dBi (decibels relative to an isotropic antenna).
State an example where a high gain antenna would be beneficial?
TV Antennas. If you mount a TV antenna on your roof and know the tv broadcast antennas are to the south, then it is preferred to have a high gain antenna.
State two applications where a low gain antenna would be utilised?
Mobile Phone Antennas.
Vehicle Antennas.
State the two ways an Antenna can radiate?
- Omni-Directional
- Directional (Uni/Bi)
State the two categories of polarisation for Antenna Systems?
- Horizontal
- Vertical
What is the frequency band for High Frequency?
3MHz to 30MHz
What is the frequency band for Super High Frequency?
3GHz to 30GHz
What is the frequency band for Ultra High Frequency?
300MHz to 3GHz
What is the frequency band for Very High Frequency?
30MHz to 300MHz
State one advantage and disadvantage of a Broadband Antenna?
Advantages;
- Broad Spectrum of available Frequencies.
- Reduces the need for multiple antennas
- Potentially ensures less cost, equipment and servicing required.
Disadvantages:
- Complex in design.
- Some Antennae need large ground areas.
What type of propagation does High Frequency Antennas use?
Sky Wave Propagation.
What type of propagation does Ultra High Frequency use?
Space Wave/Line of Sight Propagation
What type of propagation does Super High Frequency use?
Space Wave/Line of Sight Propagation
State three considerations that should be factored in when installing antenna systems?
Antenna Frequency
Polarity
Water Ingress Protection
Climate
List four common faults with antenna systems?
- Incorrect Antenna for desired frequency
- Different polarities between Tx and Rx
- Broken/Missing elements
- Water Ingress.
State the advantages and disadvantages of Bayed and Stacked Antenna Systems?
Advantages:
i. Performance & Gain of a system Gain” as a parameter measures the directionality of a given antenna. An antenna with a low gain emits radiation in all directions equally, whereas a high-gain antenna will preferentially radiate in particular directions. Specifically, the Gain, Directive gain or Power gain of an antenna is defined as the ratio of the intensity (power per unit surface) radiated by the antenna in a given direction at an arbitrary distance divided by the intensity radiated at the same distance by a hypothetical isotropic antenna:
Disadvantages:
ii. More connectors I.e. more dB loss. Complex in design. Limitations of structure