Intro Flashcards
A structure used to facilitate the transition between a guided wave and a free space wave.
Antenna
Define reciprocity
The directional pattern of an antenna during transmission is the same as during reception.
Extends to infinity; it is dominated by radiated fields, with electric and magnetic fields in phase and orthognal to each other in the direction of propagation.
Far Field region
E-field and H-field are 90 degrees out of phase with eachother. Because they are not in phase, the energy is not radiated away as EM waves.
Reactive Near Field
Here the EM fields are a combination of reactive and radiative. Some energy is stored in the near field and some is radiated away.
Radiative Near Field
The power transmitted per unit solid angle. Does not depend on distance.
Radiation Intensity
Ratio of power density in a specified direction to the power density averaged over all directions at the same distance from the antenna.
Directivity.
Define Antenna Gain
Directivity x radiation efficiency.
The increase in power density in the desired direction relative to the power density that would be acheived with an isotropic antenna.
The measure of the power from the antenna at a certain distance. Formula depends on directivity.
Power Density.
Effective Area
The area over which the antenna effectively captures EM radiation. It is the amount available at the reciever antenna.
Ewuates the power at the terminals of a receiveantenna as the product of power density of the incident wave and the effective aperture of the receiving antenna under idealized conditions given another antenna some distance away transmitting a known amount of power.
Used to find the ideal power received at an antenna from basic information about the transmission.
Friis transmission formula
What is a radiation pattern?
A plot of the gain.
Range of frequencies over which the antenna can effectively transmit and receive.
Antenna Bandwidth
An antenna whose length is 1/4 of a wavelength and therefore the maximum amount of current flows through the antenna.
Resonant antenna
Can a verticle polarization be recieved by a horizontally polarized antenna?
No.
Used for tracking
Common in surveillance radar
Common in air search radar
What is a Hertzian dipole?
Infiniteseimally small dipole with omnidirectional, doughnut-shaped radiation pattern. Ideal antenna.
Define Beamwidth
The angular range over which the antenna can radiate or receive. It is is the width of the main lobe of the radiation pattern.
How are beamwidth and frequency related?
The larger the physical size of the antena relative to the wavelength, the narrower the beamwidth.
The wavelength of an EM wave is inversely proportional to the frequency.
As the frequency increases, the wavelength decreases, the antenna’s physical size becomes a larger proportion of the wavelength, narrowing the beamwidth.
TLDR: Higher freq -> Narrow beamwidth
What is an Isotropic Antenna?
A theoretical antenna which radiates equally in all directions (dough-nut), same as a Hertzian dipole.
What is dBi?
Measures the power transmitted in a specified direction compared to an isotropic radiator, on a logarithmic scale.
Would you expect the beamwidth to be wider or narrower at the maximum frequency and why?
High Frequency -> Low Wavelength -> Large physical size compared to wavelength -> Narrow Beamwidth
Define Directivity
A measure of the spatial distribution of the radiation pattern and how focused it is in a specific direction. Does not account for ability to radiate power.
Define Gain
The ability to radiate more power in a particular direciton compared to a reference antenna.
What does this graph mean?
Below the Resonant Frequency, the antenna will appear capactivie. More electrical energy will be stored in an Electric Field in the Reactive Near Field.
Above the Resonant Frequency, the antenna will appear inductive. More electrical energy will be stored in a Magnetic Field in the Reactive Near Field.
At Resonant Frequency, the inductive and capacitive effects will cancel and there is peak antenna efficiency.
