BALDOVINO, Alan Joseph G. WIRLCOMResearch PaperMICROWAVE ANTENNAMicrowave antennas are a major component that is used in any microwave system. An antenna is equipment used in propagating microwaves into space. A single antenna can perform both transmitting and receiving in many modern applications today (Wade, 1998). Before going into the different types of antenna, here are some terms commonly associated and used with microwave antennas.ApertureThe aperture of an antenna is defined as the area that captures energy from a passing radio wave. The aperture of each antenna differs depending on the shape of the antenna. Antennas like the dish antenna and the horn have an aperture depending on the size of the dish or the mouth of the horn. Wire antennas, on the other hand, despite being small has an aperture in the shape of an ellipse with an area of around 0.132. The Yagi-Uda antenna also has a large aperture with a relatively smaller size (Wade, 1998). GainIn measuring the gain of an antenna, a hypothetical isotropic antenna is used as a basis. Theoretically, an isotropic antenna is an antenna that radiates in all directions. In reality, this does not happen since an antenna will always radiate more energy in certain direction than others. The gain of an antenna may be defined as the energy of radiated by an antenna compared to the energy radiated by an isotropic antenna with the same input power. It is usually the maximum value that is taken to be the gain. A large aperture antenna would have a larger gain compared to a smaller one. If it can capture or receive more energy in a certain direction then it can also transmit more energy in the same direction. The gain is given by the formula:

The gain is calculated with respect to the reference point, the isotopic antenna. It also calculated in its decibel form (Wade, 1998). Efficiency The efficiency of an antenna is defined as the ratio of power received to the power arriving. This may be understood through an illustration. An antenna is directed towards an isotropic antenna that is radiating from a distance. Since an isotropic antenna radiates on all direction, it is simple to calculate the energy arriving at the antenna. On the other, the energy received is also being measured on the antennas receiving end. Efficiency takes into consideration all losses experienced by the signal (Wade, 1998). There is no standard efficiency used for all microwave system, it is really dependent on its use and application. Of course a the higher the efficiency, the better it is for those that will be using the antenna (Wade, 1998). ReciprocityThe reciprocity of an antenna is defined as the performance of the antenna is exactly the same when it transmitting and when it is receiving a signal. Transmitting and receiving gains and antenna patterns are the same. The relative noise experienced by different kinds of antenna may differ even with identical antenna gains. This means that the signal-to-noise ratio may differ from one antenna to another (Wade, 1998). Directivity and BeamwidthThe directivity of an antenna is defined as the maximum gain of a given antenna compared to the average gain in all direction. This may be calculated with the formula of the gain of an antenna using 100% efficiency. The beamwidth is defined as the energy radiated toward a certain area. The beamwidth is closely related to the directivity. The higher the gain, the smaller the beamwidth of the antenna (Wade, 1998). SidelobesNo antenna in the real world scenario is able to perfectly radiate all the power it receives to a preferred or specific direction. There will always be wasted power inevitably radiated towards other direction. In Figure 1, there are some spillovers of radiated energy around the antenna based on the given antenna pattern. The spillovers of radiated energy are compared with main lobe in decibel form. The sidelobes must also be considered in designing an antenna because they will affect the amount of energy that will be radiated by the main lobe. Different antenna designs will have different forms of sidelobes. The main lobe determines the directivity of the antenna (Wade, 1998).

Figure 1-1: Antenna Pattern with sidelobesE-plane and H-planeThe antenna is a transducer that functions as a converter of voltage and current passing through transmission lines into electromagnetic field travelling in free space. It is composed of electric and magnetic field which perpendicular to each other. The electric field travels at the E-plane while the magnetic field travels at the H-plane which is perpendicular to one another. The polarization of an antenna always refers to the E-plane. To maximize the gain of the antenna, for any type, it is important to consider both the E-plane and the H-plane (Wade, 1998). Phase CenterThe microwave energy that propagates free space are all AC or alternating current, its voltage and current vary sinusoidally over time. For most antenna application, it is only the amplitude or average power of the signal that is considered. But certain applications like optical system also consider the phase of the signal as a function of time (Wade, 1998).

Figure 1-2: Phase and Phase CenterIn Figure 2, it illustrates the effects of the different phase when seen in its antenna pattern. Whenever two electromagnetic signals is received by an antenna, there three possible things that may occur: it can be in phase, out of phase and partial out of phase. When the two signal are in phase, their amplitudes add together to form a solid waveform. This also produces a smooth pattern. When a signal is out of phase, the signal are off by 180 degrees, they cancel out generating almost no signal at all at the receiving end of the signal. When a phase is partially out of phase, it generates a signal is the average of the two signals. When scene in its antenna pattern, it can be seen to have some noise patterns (Wade, 1998). Inverse Square LawThe inverse square law of the antenna states that the received power of the antenna is inversely proportional to the square of the distance. This happens because the area illuminated by the beamwidth angle begins to increase as the square of the distance from the source. Similarly, the power of the unit area must decrease by the same ratio. Since the are of the receive antenna has not changed, the received power must decrease proportionally (Wade, 1998). Path lossThe path loss between two antennas may be estimated using the Friis transmission loss equation. The formula is given by:

The path loss is a function of the distance between the two antennas, the gain of both the receiver and transmitter and also the wavelength (Wade, 1998). Types of Antenna:Microwave antenna has many use and purposes depending on its application. Different types of antenna have different purpose and application in the real world scenario. There are four main types of antenna: reflector antennas, lens antennas, array antennas and frequency sensitive antennas. Reflector antennasThe reflector antenna is antenna type that uses a reflector to focus the electromagnetic signal to make it directional either to the vertical plane, the horizontal plane or both. A spherical wavefront normally radiates in all direction. To make a an antenna highly direction, it must be able to make a normally spherical wavefront to a plane wavefront. The parabolic reflector is an antenna that is known for its high directivity (Microwave Antennas, 2014). Parabolic Reflector A basic property of microwave is that it travels in a straight line and it can be reflected. This properties of microwave has been considered when they designed the parabolic reflector. Figure 3 illustrates what a parabolic antenna looks likes and how it works (Microwave Antennas, 2014).

a. Parabolic Reflector Radiation

b. Actual Parabolic Reflector AntennaFigure 1-3: Parabolic Antenna The microwave source is given at point F (Figure 3a) and its radiates in all spherical wavefront towards the reflector. When the wave hit the reflectors surface it is shifted by 180 degrees. But when it is reflected, the wave travel outward in a parallel path. When they reach the line XY, they are all in the same length. The signal is now radiated with high directivity. The antenna will still experience some signal spillover causes side lobes to form with the antenna pattern. There are many variation of the parabolic antenna. Each variation has its own special application (Microwave Antennas, 2014). Truncated Paraboloid The truncated paraboloid is a reflector that is shorted vertically. It is used to generate a beam that spreads out vertically. The main application of this type of reflector is in radar detection to generate more accurate determination of bearing. Since the beam is spread vertically, it can detect objects at different altitude with changing the angle of the antenna. It can also be used in target height-finding systems if it is oriented horizontally.

a. Vertically Truncated Paraboloid

b. Horizontally Truncated ParaboloidFigure 1-4: Truncated ParaboloidOrange-Peel Paraboloid The orange-peel paraboloid, as its name states, is shaped like an orange peel. The orange-peel shape generates a beam that is wide in the horizontal plane and narrow in the vertical plane. This type of antenna is normally used in height finding equipment (Microwave Antennas, 2014).

Figure 1-5: Orange-peel ParaboloidCylindrical ParaboloidThe cylindrical paraboloid is an the cross section of a parabolic cylinder. This type of antenna causes the reflector to be directive in a single plane only. The signal is fed to the reflector using waveguides or dipoles. Changes in the width of the parabolic section causes changes in the beam shape it generates. This antenna is typically used in radar systems and ground control approach radar systems (Microwave Antennas, 2014).

Figure 1-6: Cylindrical ParaboloidCorner ReflectorThe corner reflector is made of two flat conducting sheets that meet to form an angle (the corner). It is driven by a half wave radiator located on a line which bisects the angle formed by the two sheets (Microwave Antennas, 2014).

Figure 1-7: Corner Reflector Horn RadiatorsSimilar to parabolic reflectors, the horn radiators is very directive in nature with microwave frequencies. The advantage of these antennas is that they are useful for wide frequency bands because they do not use resonant elements. These are typically used with waveguides because they both function as an impedance matching devices and a directional radiator. These may be constructed in varying shapes such as rectangular, pyramidal and conical as given in Figure 7 (Microwave Antennas, 2014).

Figure 1-8: Horn Radiators Lens AntennasLens antenna convert spherically radiated microwave energy into a plane wave through a collimating lens. The collimating lens causes all radial segment of a spherical wavefront into a parallel path. The source of the signal acts like a gun that shoots the signal towards the lens. Waveguide TypeThe waveguide type is composed of several concave metallic strips which are placed parallel to the electric field of the radiated energy. This can be seen in Figure 8a. The stripped are positioned at about half-wavelength apart.

a. Waveguide lens function

b. Three-dimensional Waveguide lens Figure 1-9: Waveguide LensAntenna ArrayThe antenna array is known for being sharply directive. It is constructed using simple half-wave dipole elements. The antenna works be positions these antennas together that will form elements that add to each other while other cancel each other out. A reflector helps by causing the radiation to go in one direction. Common example of an antenna array is the bedspring array. It is called as such because it highly resembles a bedspring. It is an example of a unidirectional antenna. It is normally used in two-dimensional search radar to obtain the range and bearing of a given target. Figure 9 gives an actual image of a bedspring antenna (Microwave Antennas, 2014). ]Figure 1-10: Bedspring Antenna (http://www.interfacebus.com/bedspring-antenna-array.jpg)

Frequency-Sensitive AntennaThe array antenna shown earlier are mostly transmitting devices. The frequency sensitive antenna is now the receiving end. Each array contains a slot that will receive a particular frequency. Bearing and elevation data is done by moving the antenna by rotation and elevations (Microwave Antennas, 2014).

Figure 1-11: Frequency Sensitive Antenna

WAVEGUIDESWaveguides is a transmission medium used to propagate microwaves. They are known to be the most efficient way to transfer electromagnetic energy. A waveguide is constructed similarly to coaxial cable lines without its center conductor. They are design using highly conductive materials such as metals and maybe rectangular, circular or elliptical in shape (Waveguide, 2014).

Figure 2-1: Shapes of Waveguides

Advantages and DisadvantagesWaveguides have its advantages and disadvatages over wired and coaxial transmission lines. An advantage of waveguides is that it greatly reduces the copper losses experienced by the signal passing through it. They are unlike wired transmission lines that have large copper losses because they have very small surface area. Coaxial cables have a large outer conducer and smaller inner conductor. When microwaves pass through it, the carrying area of the inner conductor becomes small due to a phenomenon called skin effect. The skin effect causes the effective resistance of a conductor to increase. The electromagnetic field motion causes the energy transfer in the coaxial cable. The current-carrying area of the inner-conductor limits the magnitude of the field. The small size of the center conductor is reduced even further with the effects of the skin effects. This causes coaxial cables to work less efficiently compared to waveguides. Waveguides also have a lower dielectric loss compared to the wired transmission and coaxial cables. The dielectric loss for both wired and coaxial lines are caused by the increase in heat of the insulating material between the conductors. The insulation, when heated, begins to behave like a dielectric of a capacitor. This causes heating thus resulting in loss of power. In real application, the problem is not due to dielectric loss but rather the breakdown of the materials. Since the waveguides are hollow, the dielectric material that passes through it is only air, and it has a much lower dielectric loss compared to other materials. Consequently, the waveguides may still experience loss due to standing waves. This causes the arcing which greatly decreases the efficiency of energy transfer and may cause damage to a waveguide. Radiation loses are kept at a minimum because the electromagnetic fields are completely contained within the waveguide material. Waveguides are also capable of handling more power compared to the two other transmission materials (Waveguide Theory and Application , 2014). Waveguides also has its own set of disadvantages. The biggest disadvantage of the waveguides is found in its design. The width of a waveguide is approximated at the value of half the wavelength of the frequency of the wave that is transported. It has become impractical to make waveguides for frequencies below 1 GHz because they are too large in dimension. The frequency range of any system that use waveguides is limited by the size of the waveguides. Unlike wired transmission and coaxial cables, waveguides are very difficult to install since they are rigid and hollow-pipe in shape. They require the use of special coupling materials to ensure proper operations. Waveguides are also very costly to use because they may have silver or gold plating inside its walls to reduce the skin effect (Waveguide Theory and Application , 2014). Energy PropagationEnergy is propagated through a waveguide through electromagnetic fields. An electromagnetic field has two components, the H field and E field. It is the interaction between these two fields that energy is propagated through the waveguide. Figure 2-2 shows an illustration of the E-field pattern of a voltage sine wave that is applied to a waveguide of one-wavelength. The electric fields are represented by the arrows that is seen on figure B and C. The E field is closely related to the sine-wave causing variation in it density, especially when voltage is applied. The figure illustrates the instant that the applied voltage wave is at its peak. They vary continuously from zero to peak value.

Figure 2-2: E-Field and WaveguidesFigure 2-3 shows the magnetic field pattern in its cross-sectional view. The field is strongest at the edges of the waveguides where the current is at its peak. The field is also at weakest where there is no current. The figure shows only the pattern at only one instant in time, this will reverse when the field will continue to change with changes in the input (Waveguide Theory and Application , 2014).

SIDE VIEW

Figure 2-3: H Field and WaveguidesModes of Operation Waveguides have many modes of operation depending on the input placed into the device. The strength of the field is indicated by the spacing of lines, that means that the closer the lines constitutes to a stronger field. The region at maximum voltage in the field moves continuously down the sine-wave pattern. Ideally, it must meet the boundary condition, the field must be zero at the b wall. Waveguides are mostly operated at the dominant mode. They are always operated at the dominant mode because it is the most efficient mode of the waveguide. Most waveguides are design to accommodate only the dominant mode. A waveguide must ha a wide dimension of at least half of the wavelength of the frequency it propagates to operate in the dominant mode. The wide side is normally kept a minimum value that will still allow the dominant mode to work in it. However, the waveguides are not limited to operate in the mode other than the dominant mode. The waveguide is designed differently so that it can accommodate other modes to propagate in its guide. This may be done by varying the sizes of the wide and long side (Waveguide Theory and Application , 2014).

Figure 2-4: Waveguide operating other than the dominant modeThe dominant mode of a rectangular waveguide is given in Figure 2-5. The design of the rectangular waveguide is made to accommodate only the dominant mode. The numbering system of the different modes is done by adding subscripts to the mode it operates on. When there is a field pattern in a dimension, a value 1 is given while if there is no field pattern, a zero is given. For a rectangular waveguide operating at dominant mode, the a side is the only on with a field pattern. So the mode is denoted as TE10 (Waveguide Theory and Application , 2014).

Figure 2-5: Rectangular waveguide in TE10 mode. The dominant mode of a circular waveguide is given by the Figure 2-6. The circular waveguide also operates in the TE mode. The circular waveguide is operated at the TE11 mode because along the diameter, the lines go from a minimum to a maximum then back to minimum (Waveguide Theory and Application , 2014).

Figure 2-6 Circular Waveguide in TE11 mode. Waveguide Input/Output Methods The waveguide uses special devices to input the energy into the device. The three devices that are used to input/inject or remove energy from a waveguide are called probes, loops and slots. A probe is inserted to into a waveguide to provide and supply it with microwave energy. The current the flows sets up the E field.

FRONT VIEW

SIDE VIEW

ISOMETRIC VIEWFigure 2-7: Probe in a Rectangular Waveguide (in 3 different views)Figure 2-7 provides a clear illustration of a probe that is inserted in a rectangular waveguide. The different views provides a clear pictures of how the field functions inside the waveguide.Notably, the most efficient place to put the probe is at the center of the a wall which is parallel to the b wall. It is also about a quarter-wavelength away from the shorted end of the waveguide. That location is where the E field is maximum in the dominant mode. Conclusively, this is where the energy transfer or coupling is maximum. The quarter-wavelength spacing is given by the frequency required to propagate at the dominant mode. The probe size indicates the frequency, bandwidth and power-handling capabilities. The bigger the probe, the higher the bandwidth, the larger its capacity (Waveguide Theory and Application , 2014). Impedance MatchingImpedance matching is an important aspect used in waveguides. Since transmission systems are not always perfectly matched to their load devices, they produce standing waves causing losses power and reduction in capacity. Impedance changing devices are placed into the waveguides to change its impedance. These are normally placed at the source of standing waves.

Figure 2-8: Waveguide with irises. The figure above shows a type of impedance matching device, the iris. These are used to introduce a inductive or capacitive component into the waveguide. The iris is mde of metal plates that contain an opening to let the wave pass. The iris is normally located at the transverse plane. The equivalent impedance component is highly dependent on the size and orientation of the opening. A horizontal opening gives a capacitive value while a vertical opening provides an inductive value. A combination may also be achieved and it will have both capacitive and inductive values (Waveguide Theory and Application , 2014).

PENETRATING

EXTENDED THROUGH Figure 2-9: Post and ScrewsAnother way to introduce impedance matching to a waveguide is by adding posts or screws made of conductive materials. Above shows an illustration of how post and screws function as a impedance matching device. A penetrating screw inserted to a waveguide is equivalent to a capacitor while screw extending through is equivalent to an inductor. Waveguides that are not properly match will have an effect in the overall transmission of any system. This will cause reduced power and be less efficient if not properly matched in the system. Iris and Screws may be used to change the impedance of a given waveguide. These are made of conductive materials to help introduce an equivalent impedance to the system. This will help the system increase or decrease the impedance to match it with any given load connected to it (Waveguide Theory and Application , 2014).

Waveguide TerminationStanding waves have a affect the overall efficiency of a waveguide when not properly match. Mismatch in impedance leads to standing waves. Other causes of standing waves are the abrupt changes in the impedance. A solution to decrease standing waves is to gradually decrease the impedance. This may be done using a horn to terminate a waveguide. Horns are simply antenna which have an advantage compared to other impedance matching devices. The horn function this time as an impedance matching tool that is done gradually.

Figure 2-10: Different types of waveguide horns

Waveguide DevicesThere are many waveguide devices that have been developed to provide a certain type of energy transmission. These are some of the common waveguide devices that have been used. These are the directional couplers, cavity resonators and hybrid junction . Directional CouplerThe directional coupler is waveguide device that is mainly concerned about the sampling energy within the waveguide that will be used by another device. The coupler normally sample energy that is unidirectional. It is possible to design coupler that can sample for both directions. Bidirectional couplers are the product of making a couple for both direction and is used in radar and communications system.

Figure 2-11: Directional CouplerThe design of directional couplers may be done in many ways. Based on Figure 2-11, it is made using an enclosed waveguide. The directional coupler has a wedge in the upper section that has an energy absorbing material on one side with a probe on the other side.

Figure 2-12: Incident wave in a directional couplerThe figure above shows the incidents wavefront in a waveguide. The illustration shows the incident wave passing through the coupler section through the holes. Since the waves that travel are in phase, they add up when they get to the coupler. The sample taken is only a small part of the energy that travels through the waveguide.

Figure 2-15: Reflected wave in a directional couplerThe figure illustrates the effect of a reflected energy on the directional coupler. Reflected waves that travel the coupler are not in the same phase as the signal. The signals are out of phase by 180 degrees. The two signals cancel out because of this. That is why in the coupler there is a material that absorbs.

Figure 2-14: Bidirectional-CouplerThe figure about is the bidirectional-coupler. The design is made to measure energy passing in both directions. The design has two pairs of holes in the waveguide. This helps in the sampling of the signal in the waveguide (Waveguide Theory and Application , 2014).

Hybrid JunctionHybrid junctions are waveguide devices that are designed with multiple junctions. There are many designs of hybrid junctions. The most common design of the junction is the T junction given in the figure below.

Figure 2-15: T JunctionsA waveguide with junctions does not necessary mean that the signal is equally divided among the junctions. Each type of junction has its own way of affecting the energy that propagates through it. As technology advances, the designs become more complex requiring more complex designs. The figure below shows a hybrid junction (Waveguide Theory and Application , 2014).

Figure 2-16: Hybrid JunctionCAVITY RESONATORA cavity resonator is defined as space complete enclosed using conductive walls that are capable of holding electromagnetic fields and possess resonant properties. The cavity resonator is known for its many uses in microwave technology and has many advantages. An important parameter that is known in cavity resonators is its high quality factor or q factor. Through this, the resonant cavity has a high power capacity (Cavity Resonator, 2014).

Figure 3-1: Sample Cavity ResonatorThe Q factor or quality factor is an important parameter in microwave technology of the cavity resonators. It is defined as the measure of the damping of resonator modes. It measures the strength of damping of the oscillating device or the relative line width. This parameter was initially used for LC circuits but now is also used for microwave cavities. The Q factor may understood in two ways: in terms of energy storage or in terms of resonance bandwidth. The q factor as an energy storage is 2 times the ratio of the stored energy to the energy dissipated per oscillation cycle. For cavity resonators, an oscillation cycle means the field oscillation period. In resonance bandwidth, the Q factor is defined as ratio of the resonance frequency 0 and the full width at half-maximum (FWHM) bandwidth of the resonance (Paschotta, 2014). It is denoted by the formula:

Oscillation and amplification are done using transistor networks for lower frequencies. LC circuits are used to make oscillator while transistors are used to design amplifiers. However, these types of design for amplifiers and oscillators only work for frequencies below 3 MHz because of the skin effect and stray capacitance and inductances. Cavity resonators were introduced to help efficiently oscillate and amplify frequencies above 3 MHz (Paschotta, 2014). The principles that work in the resonant cavity are based on the principles of the resonant circuits below. Each resonant is composed of its inductive and conductive parts. These work together to provide resonance to the circuit. The resonant cavity acts similarly to these. The metallic plates are the inductive component while the dielectric formed by these plates are capacitive in nature. Together they perform the same way as these resonant circuits (Ravindra.S.Kashyap, 2007).

Figure 3-2: Resonating CircuitThe design of the resonating cavity is made of conductors with a hollow interior. The ports are designed to hold RF signals. The metallic cavity works as the inductor while the spaces in its mouth functions as a capacitor. The cavity resonator works like an amplifier when a high frequency is injected to it, the output has a higher frequency. The illustration below shows how the cavity resonator works (Dall, 2006).

Figure 3-3: Illustration of design of cavity resonator

Figure 3-4: Rectangular Cavity ResonatorThe rectangular cavity resonator is one of the commonly used cavity resonator because of its simple and rigid design. This type of resonator is also known for its high frequency capacity.

Figure 3-5: Magnetic and Electric Field PatternThe figure above shows the magnetic and electric field pattern within a rectangular cavity resonator. Types of Cavity ResonatorThere are different types and designs of cavity resonators. Each types as its own structure and capabilities (Cavity Resonator, 2014). These are the 7 common types:1. Regulated Cavity Resonator2. Un Regulated Cavity Resonator3. Co-axial Cavity Resonator4. Capacitive Cavity Resonator5. Inductive Cavity Resonator6. Waveguide Cavity Resonator7. Reentrant Cavity ResonatorRegulated Cavity Resonator

Figure 3-6: Regulated Cavity ResonatorThe design of a regulated cavity resonator uses circular waveguides. A circular waveguide is plated with conducting material on its open ends. Based on the figure, the input and output ports are designed with cavity resonator itself. This is where the input and output devices are connected to provide its need purpose. Oscillate happens in the cavity when and input of higher frequencies are injected to it. The E- and H- Fields are made and the output may be used for further uses (Cavity Resonator, 2014).

Un-Regulated Cavity Resonator

Figure 3-7: Unregulated Cavity ResonatorThe design of the unregulated cavity resonator is based on the regulated cavity resonator. It is made using two regulated cavity resonators that is connected using a circular waveguide. Based on the given figure, the input is located in one cavity while the other is placed on the other cavity. Oscillation happens in the first cavity where the input is injected to the resonator. The oscillation is then transfer to the output through waveguide into the second resonator. The advantage of this type of resonator is that is has a greater bandwidth compared to the regulated cavity resonator. However, it has a disadvantage of a lesser gain (Cavity Resonator, 2014). Co-axial Cavity Resonator

Figure 3-6: Coaxial Cavity ResonatorThis type of cavity resonator is designed with a rectangular shape. It has an internal compartment that is hollow. The covering that surrounds the this resonator is a conduction mesh used in coaxial cables. The broad dimension is measured at the operating frequency and is /2. The input and output ports are also placed in the longer dimension. The advantage of this type of cavity resonator is that it has a relatively high bandwidth and has a wide range of input frequencies that can be amplified by the resonator (Cavity Resonator, 2014).

Capacitive Cavity Resonator

Figure 3-7: Capacitive Cavity ResonatorThis type of resonating cavity is capacitive in nature. The main design of the cavity is similar to the design of a regular waveguide cavity resonator with some minor changes. A rectangular waveguide is used for this resonator. The conductive plates used in the broad side of the waveguide are made to be flexible. This is made so that pressure may be applied to it to help vary some of its parameters. The illustration shows that the E field generated is perpendicular to the broad side. This increases the pressure and causes a disturbance to the electric field generating variation in the frequency of the cavity. This occurs because the pressure applied to the broad dimension increases. The operating frequency of this kind of resonator changes with applied pressure to the resonator. The advantage of this is the different input frequencies may be handled by the device and the gain remains the same (Cavity Resonator, 2014).

Inductive Cavity Resonator

Figure 3-8: Inductive Cavity ResonatorThis type of resonating cavity is inductive in nature. Similar to the capacitive cavity resonator, it uses a rectangular waveguide. The flexible conductive plates are now placed on the narrow dimension. The input and output ports are placed on the broader side, opposite to each other. This time the H field is perpendicular to the narrow side causing an increase in the pressure applied to the length of the dimension. The length is proportional to the wavelength of the given resonators. This results in the operating frequency of the resonator to decrease. This type of resonator operates in the TM mode of operation. It can also withstand different microwave length of frequencies and has a constant gain (Cavity Resonator, 2014). Waveguide Cavity Resonator

Figure 3-9: Waveguide Cavity ResonatorThis type of resonator is design using the common waveguide. The open ends of a waveguide are covered with conductive plates to make a hollow block. The input and output are place in one side of the block. The operation of this wavelength takes place in such a way that the E Field is perpendicular to the broad side while the H field is perpendicular to the narrow side. The frequency of operation is based on the broad side at half the wavelength (Cavity Resonator, 2014). Reentrant Cavity Resonator

Figure 3-10 Reentrant Cavity ResonatorThis type of resonator is design using two cavity resonator that is joined together using a rectangular waveguide. The input is in one resonator while the output is placed on the other resonator. They are connected perpendicularly as illustrated in the figure above. It has a wide bandwidth. It can also amplify and oscillate 3 MHz to 300 MHz (Cavity Resonator, 2014). Tuning Cavity resonators have been very popular because of its capacity to oscillate and amplify higher frequencies. Unfortunately, a cavity resonator is made to perform on a specific resonant frequency and it would be impractical to use multiple resonators just to accommodate a system that uses multiple frequencies. Tuning is introduced to the resonator. Tuning is the process of manually changing the resonant frequency of the cavity. This may be done by varying three parameters: cavity volume, capacitance and inductance (Waveguide Theory and Application , 2014). Volume TuningVolume tuning is a is literally changing the volume of the cavity resonator. This is illustrated in the figure in the following page. By varying the size of the cavity resonator, the resonant frequency is varied with it inversely. Increase the volume of the resonator translates to a decrease in the resonant frequency (Waveguide Theory and Application , 2014).

Figure 3-11: Volume Tuning. Capacitive TuningCapacitive tuning is done by placing an adjustable screw in the cavity resonator. This must be placed in the area where there is maximum E lines. By decreasing the distance of the plate in the cavity resonator, the capacitance is increased. This leads to decreases the resonant frequency of the resonator. When the capacitance is decrease, the resonant frequency increase. The figure below illustrates how the capacitive tuning is done (Waveguide Theory and Application , 2014).

Figure 3-12: Capacitive TuningInductive TuningInductive tuning is done by placing an adjustable nonmagnetic screw in the cavity resonator. This must be placed where the maximum H lines. By introducing a slug into the resonator, it generates an opposing H field in the resonator. This reduces the overall H field in the resonator causes the inductance to decrease proportionally. This increases the resonant frequency. When the inductance is increase this causes the resonant frequency to decrease. The figure below illustrates this process (Waveguide Theory and Application , 2014).

Figure 3-13: Inductive TuningBibliographyCavity Resonator. (2014). Retrieved May 14, 2014, from Daenotes.com: http://www.daenotes.com/electronics/microwave-radar/cavity-resonatorMicrowave Antennas. (2014). Retrieved May 12, 2014, from Navy-Marine Corps : MICROWAVE ANTENNASWaveguide. (2014). Retrieved May 13, 2014, from daenotes.com: http://www.daenotes.com/electronics/microwave-radar/wave-guide-construction-working-types-usesWaveguide Theory and Application . (2014). Retrieved May 13, 2014, from Navy-Marine Corps: http://www.navymars.org/national/training/nmo_courses/nmo1/module11/14183_ch1.pdfDall, R. (2006). Klystron Theory. Retrieved May 14, 2014, from Electronic Theories: http://www.electronicstheory.com/html/klytheo1.htmPaschotta, R. (2014). Q Factor. Retrieved May 14, 2014, from RP Photonics Encyclopedia: http://www.rp-photonics.com/q_factor.htmlRavindra.S.Kashyap. (2007). Waveguide Resonators. Bombay : EE 614: Solid State Microwave Devices & Applications.Wade, P. (1998). Antenna Fundamentals. Retrieved May 12, 2014, from WIGHZ: http://www.qsl.net/n1bwt/chap1.pdf