Antenna Types Part 2 Flashcards
Horn Antennas
What is the advantage of Horn Antennas?
No resonant elements -> They can operate over a wide bandwidth
How does a parabollic reflector work?
The waves all bounce off the reflector and form a planar beam.
On reception, waves will reflect off the parabola and arrive in phase at the focal point.
Does a Parabolic Reflector surface need to be solid?
No. A mesh of less than 1/10 of a wavelength is required.
“This is a measure of the uniformity of the electric field across the antenna’s aperature. Typically an antenna will have maximum gains if the electric field is unfirom in amplitude and phase across teh aperature. Increasing the focal length will improve this factor.”
Aperature Taper Efficiency
“Energy that travels away from the feed horn but does not get reflected by the dish. This can be improved by decreasing the focal length or increasing the reflector size but that is a trade-off with the Aperature Taper Efficiency.”
Spillover Efficiency
“Small deviations in the shape of the reflector degrade performance, especialyl for high frequencies that have a small wavelength and become scattered by small surface anomolaies.”
Surface Error
“Polarization of the transmitted and recieved signals must match otherwise there is loss”
Cross Polarization
“The physical structure of the feed and other elements of the antenna often mask part of the reflector.” (Feed blocks the reflector)
Aperature Blockage
“Some energy from the reflector will also reflect off the horn support strecture and increase side lobes. Some energy wil travel back into the horn and increase the Standing Wave Ratio in the waveguide.”
Non-Single Point Feed
What are causes of side lobes?
Reflector Imperfections
Diffraction at the edges of the dish or from the feeder support structure.
Uneven radiation of the main dish.
Cassegrain Antenna
“The angular range within which an antenna radiates or receives electromagnetic energy effectively.”
Beamwidth
What are advantages of a Cassegrain Antenna?
Compact - easier to train and elevate
Receiver or transmitter can be mounted directly near the horn - minimizes losses
What is the disadvantage of the Cassegrain Antennaio
Sub-reflector obstructs the main reflector.
This can be mitigated by using a large main reflector and placing the feed radiator as lcose to the sub-reflector as possible.
How does the CEROS overcome the obstruction of the sub reflector?
The main reflector is designed to twist the polarization by 90 degrees.
The sub-reflector reflects horizontally polarized waves but allows vertically polarized waves to pass through unaffected.
This also protects against cross-pol jamming.
Why would we use different shapes of paraboloids?
Reduced size but loses directionality in one of the planes of the paraboloid.
Slotted Waveguide
Advantage and disadvantage of slotted waveguide.
More durable and easy to construct.
Slotted Waveguide
