Another design that was developed to address the problem of sub-reflector blockage is the twisted Cassegrain antenna.
This design work based on a simple principle of wave called polarization. An electromagnetic wave has 2 components (magnetic field and electrostatic field) that are always perpendicular to each other and perpendicular to the direction of travel.
The polarization of the wave is defined by the orientation of the electrostatic field, polarization can either linear (vertical/ horizontal) or circular ( left/ right hand circular/ elliptical).
The most interesting aspect of polarization is the polarizer or how to filter out a source so that the outcome polarised only in certain direction /plane.
Typically polarizer made from a piece of material with atoms arranged parallel or it could also be a parallel wire screen with the distance between the wires less than the wavelength of the radio wave that needed to be polarised, a rule of thumb for the allowed gap is usually about haft a wavelength).
A very common misconception is that electromagnetic wave and polarizer behave in similar fashion with a wiggling piece of string and a fence such as a horizontal polarised electromagnetic wave will be blocked by a screen with slit oriented vertically. This misconception is depicted in the photo below:
In reality, electromagnetic wave behaves very different from mechanical waves, the assumption that waves slip through the gaps between the wires is simply wrong.
A plate made of parallel wire mesh oriented horizontally wire will completely block and reflect horizontally polarised radio wave while letting the vertically polarised wave pass through with little obstruction and vice versa.
The reason for this is: when the electric field of a wave is parallel to the wire it will excite the electrons along the length of the wire ,since the length of the wire is many time its width electrons can move a lot and absorb most energy from the wave, the movement of electrons will induced a current, this current will create it’s own wave.
The secondary wave produced by induced currents will cancel the incident wave on the transmission side and behave as a reflected wave on the incident side of the surface.
On the other hand, when the electrical field of a wave is perpendicular to the wire, it will excite electrons along the width of the wire and because electrons cannot move very far across the width of each wire little energy will be reflected. EM wave polarization is depicted in the photo below:
It is important to note that, even though most imagines illustrate radio waves often shown only 1 magnetic field and 1 electric field, that does not mean that their electronic/ magnetic field only oscillate in that exact plane.
In fact, both electrostatic and magnetic field can be thought as made of sub-electrostatic/magnetic fields that are also perpendicular to each other, and they will add like vectors.
For example: for a vertically polarised wave, it mean that the resultant electronic field ( vectors ) of 2 sub electronic fields is vertical. When the 2 sub-electronic fields are in phase ( having the same peak and trough ) then the resultant electronic field will always be stationary in one plane.
However, if one of the sub-electrostatic fields is slower than the other then the resultant electronic field ( their sum vectors ) will start to spin around the direction of travel of the wave (this is often called elliptical polarization).
If one sub-electrostatic field is slower than the others by exactly a quarter wavelength ( also known as 90 degrees phase different ) then we will have circular polarization wave as depicted in the photo below:
To convert linear polarised to circularly polarised wave and vice versa, we need to slow down one sub-electronic field more than the other by exactly a quarter a wavelength.
To do that the most common option is to use a parallel arranged wire grid with the distance between each wire is about 1/4 wavelength and put it at an angle so that the wires making a 45 degrees angle with horizontal axis, this specific device is also called a quart-wave plate.
A linear polarised wave hit a quart wave plate will be transformed into circular polarised wave while a circular polarised wave hitting a quart wave plate will be transformed into linear polarised wave.
Based on polarization principal, a flat plate Cassegrain antenna consists of 2 reflectors of equal size. The sub-reflector reflects only horizontally polarised waves and let vertically polarised one pass through.
The primary reflector reflects all waves. The sub-reflector is a flat plate is placed in front of the primary reflector instead of a hyperbolic metal reflector.
The sub-reflector consists of 2 parts, the first part is a plate with slits at an angle of 45° and the second part is a plate with horizontal slit, the slits are less than a 1/4 wavelength apart.
The basics principal is simple, For example: let say a left-hand circular polarised wave was transmitted from the feed. The wave passes the quart wave plate first and is transformed into a horizontally polarised wave.
This wave will be reflected on the horizontal strained wires. The wave passes quart wave plate again but from the other side. The orientation of the fins is mirrored now and appears rotated by 90 degrees.
This has the effect, that the former change of polarization is rescinded. Therefore, a left-hand circular polarised wave travels back to the primary parabolic reflector.
The reflection on the primary parabolic reflector will change the left-hand circularly polarised wave to a right-hand one. After passing the quart wave plate a third time, this right-hand circular polarised wave will become a linear vertically polarised wave.
This one can cross the horizontal slit sub-reflector without interaction and is emitted therefore vertically polarised toward targets ( opposite thing happened in receive mode ).
While Parabolic, Gregorian and Cassegrain antennas all have very high gain ( small main beamwidth) relative to aperture size.
However, they all share the same disadvantages include: high sidelobes (make radars susceptible to low RCS targets and ground clutter) , reduced efficiency due to beam blockage ( beam blockage is a big problem for small radar such as the one on air to air missiles, offset design reduce this problem but they occupied bigger space thus not suitable for airborne application ).
Due to problems stated earlier, a new antenna design called slotted arrays was developed. A slot array consists of a metal surface, usually a flat plate, with a hole or slot cut out.
When the plate is driven as an antenna by a driving frequency, electromagnetic waves are emitted from each slot, one can understand each slot act as a small antenna and together they form an array.
Since the beam from each individual slot is weak their side lobes are also very small Main characteristics of slotted arrays are high gain, low side lobes and light weight.
Radiating pattern
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