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  • jalves61 4:26 pm on February 11, 2020 Permalink | Reply
    Tags: , Microwave/RF Engineering   

    Keysight ADS – Short Circuit Terminated Ideal Transmission Line 

    Using ADS, a parameter sweep can be used to confirm the results of a short circuited transmission line. The input impedance of a transmission line is given as

    input.png

    When the line is terminated by a short circuit, ZL = 0 and the equation reduces to only the imaginary part of the numerator. For integer multiples of the wavelength, the input impedance is equal to zero. At odd multiples of a quarter wavelength, the input impedance becomes infinite and looks like an open circuit.

    The following circuit is constructed to test the results. A parameter sweep with the variable theta (the electrical length) is used.

    schem.PNG

    The results are shown below. As expected, the reactance alternates between inductive and capacitive for different electrical length values. The reactance is infinite at quarter wavelength multiples and zero at integer multiples of the wavelength. The current is shown to be lagging the voltage by 90 degrees. The major conclusion to be made is that a transmission line does not behave like a lumped element circuit because voltages and currents are different at different lengths along the line.

    results

     
  • jalves61 2:55 pm on February 9, 2020 Permalink | Reply
    Tags: , Microwave/RF Engineering   

    Keysight ADS – Open Circuit Analysis 

    Expanding upon the previous project, open circuit analysis can be used to find equivalent per unit length capacitance and conductance values for the dielectric part of the transmission line.

    ocschem

    The same process is used for the open circuit analysis with new equations for capacitance and conductance. The calculated values from the simulation window are compared to the simulated values from the AC analysis.

    oc

     
  • jalves61 2:20 pm on February 8, 2020 Permalink | Reply
    Tags: , Microwave/RF Engineering   

    Keysight ADS – Extraction of Lumped Element Model from Coaxial Line using Short Circuit Analysis 

    The following ADS project is meant to analyze an RG58 coaxial cable and extract a “lumped element model” containing discrete components intended to represent distributed values. The lumped element values for R, L, G, and C for ideal coaxial lines can be obtained from the following equations.

    equations

    “a” is the radius of the inner conductor and “b” is the radius of the outer conductor. It is important to note that since these are ideal values, the actual simulation will differ from calculated values. R and G are nonreactive and therefore will be quite similar, however C and L which are frequency dependent will vary.

    Like with other ADS projects, creating variables is an easy way to change component values, especially when these need to be duplicated. In the ideal transmission line palette, the COAX_MDS component can be found.

    The Dielectric loss model can be changed to Frequency independent, as shown. This will prevent the frequency dependent parameters from changing from the calculated values.

    ads_1.PNG

    First, a short circuit analysis can be performed in order to determine resistance and inductance values (shorting out the dielectric parameters C and G). Using the “name” option at the top of the screen, the input wire can be named “Z_SC”.  An AC simulation can be performed with the schematic shown.

    schematic.PNG

    The results for per unit length resistance are shown from the simulation. Decibel scale is used for the x axis and the y limits are changed to get a better looking plot.

    res

    The following image demonstrates placing equations in the data display window and using a calculated value to compare with a simulated value.

    eqn

    As expected, the results of the calculated and simulated values agree (they are both close to zero). Although, the resistance differs a bit (not sure why). The inductance normally would vary, but because the frequency independent model was selected for the coax cable, they are exactly the same at 100MHz.

    measured

     

     

     

     
  • jalves61 6:07 pm on February 6, 2020 Permalink | Reply
    Tags: Microwave/RF Engineering   

    Ferrimagnetic Materials – Circulators and Isolators and Ferrite Phase Shifters 

    When designing microwave and RF components, a non reciprocal device can be obtained by using ferrimagnetic components. Sometimes, it is a good thing for a device to be reciprocal (when the ports of the S parameter matrix are reversible), but in the case of RF devices such as circulators or isolators, it is important for power flow to only move in one direction or to have directional dependence. When directional dependence is present, permeability and permittivity become a tensor rather than a constant and the material is said to be anisotropic.

    Ferrimagnets are different than ferromagnets such as iron or steel in the sense that ferrimagnets have high resistivity and directional dependence at mictrowave frequencies. Both are very strongly magnetic.

    A circulator is a three port device which can be matched at all ports and lossless at the same time. It can couple power in direction or the other, but not both directions. If the reverse direction is desired, the Scattering matrix can be transposed. For a ferrimagnetic circulator this is achieved by changing the polarity of the magnetic bias field. Most of the time a permanent magnet is used, but an electromagnetic can be used for the circulator to function as an SPDT switch.

    An isolator is a two port device which only functions in a single direction. The scattering matrix shown below, implies that the device is nonreciprocal (asymmetric matrix) and lossy due to disobedience to the unitary matrix properties.

    isolator.jpg

    An isolator can prevent damage to a high power source by forcing the power to flow only from the source to load. Any reflected power due to an impedance mismatch will be absorbed by the isolator.  The two main types of ferrite isolators are resonance and field displacement isolators.

    Another two port nonreciprocal RF device is the ferrite phase shifters. Phase shifters are generally used in test and measurement systems or in phased array antennas where the antenna beam can be steered using the device. It is also possible to design a reciprocal phase shifter. In fact most phase shifters are reciprocal in the sense that they provide an equal phase shift in both directions.

     
  • jalves61 5:36 pm on February 5, 2020 Permalink | Reply
    Tags: Microwave/RF Engineering   

    Impedance Matching – Single Stub Tuning 

    One way to avoid impedance matching a transmission line to a load with lumped circuit elements is to implement Stub matching. Stubs are sections of transmission line that are terminated by either an open circuit or a short circuit. They can be connected in series or in parallel to the transmission line a certain distance from the load. For microstrip and stripline circuits, parallel configurations are preferred whereas series configurations are preferred for slotline or coplanar waveguides.

    Two variables when designing a stub are: the reactance of the stub and the distance between the stub and the load. The idea is that at a specific distance, the susceptance or (reciprocally) the reactance of the load should be cancelled out by the stub. This leads to the cancellation of reflection from the load. This reactance/susceptance value is determine by the length of the stub. The difference in length between a short and open stub is a quarter of the wavelength (which can be confirmed by the Smith Chart).

    The Smith Chart can be used to identify the length required for the stub admittance to equal 1+jb by traveling from the right hand side of the chart clockwise until the reactive parts are equal and opposite. You can also draw an SWR circle using the load impedance/admittance and find where the circle intersects the 1+jb circle. Then the same process for finding the stub length can be used. On a Smith Chart, lengths are always a function of the wavelength.

    stub.png

     

     
  • jalves61 4:55 pm on February 4, 2020 Permalink | Reply
    Tags: Microwave/RF Engineering   

    Limitations of Impedance matching Networks and the Bode-Fano Criterion 

    When designing an impedance matching network for an RF application, it is important to know the limitations of the design. For example, the maximum reflection coefficient is ideally quite small, but the bandwidth should be large. The Bode-Fano Criterion can specify the limitations of various load configurations to specify how exactly this tradeoff can occur. In addition, complexity of the circuit must be taken into account. The equations differ depending on the configuration of the load impedance.

    bodefano

    The lossless matching networks are passive and lossless. These equations lead to the conclusion that increasing the bandwidth of the matching network can be achieved by increasing the maximum reflection efficient in the passband. In addition, a reflection of zero cannot possibly be achieved unless the bandwidth is zero. This means that a perfect match can only be achieved at a finite number of discrete frequencies (you can’t have a straight line of zero reflection within the passband, only at specific points in the passband). It also shows that as R and C increase, match quality decreases. Circuits with higher Quality factors (store energy longer) are harder to match than low Q circuits.

     
  • jalves61 7:15 pm on January 24, 2020 Permalink | Reply
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    Fundamental Parameters of Antennas 

    To understand the details behind antennas, the vital interface between free space and a transmit/receive system, it is important to fully understand the basic properties of antennas in order to understand their performance.

    One of the main properties of an antenna is its radiation or antenna pattern. This is defined as a mathematical function of the radiation properties of the antenna as a function of space coordinates. It is important to note that this pattern is determined in the far field region (there are three main regions when studying antenna radiation: reactive near field, radiating near field, and far field). This can be a trace of the Electric or magnetic field (field pattern) or the spatial variation of the power density (power pattern). These are generally normalized with respect to the maximum value and typically are plotted in decibel scale to accentuate minor lobes. Minor lobes are any lobes that are not the major lobe. In split beam antennas, there can be multiple major lobes. The following image shows a directive antenna’s radiation pattern. Side lobes are generally undesirable and should be minimized if possible.

    antennapattern.jpg

    The Half Power Beamwidth (HPBW or sometimes just beamwidth) can be determined by drawing two lines from the origin point to the -3dB (half power) point and seeing the resultant angle.

    Antennas are generally compared to “isotropic” antennas. These are hypothetical antennas that radiate power equally in all directions. This is not to be confused with omnidirectional antennas, which radiate power equally in the azimuthal direction. The E and H planes are defined as the plane containing the electric field vector and direction of maximum radiation and the plane containing the H vector respectively.

    The three main regions around an antenna are the reactive near field, radiating near field and far field. In the reactive near field, the radiation is reactive (eg. the E and H fields are out of phase by 90 degrees. Because the waves are not in phase and transverse, they do not propagate. In the radiating near field, the waves are not purely reactive and propagate, however the shape varies with distance. In the far field (where the radiation pattern originates from), the radiation pattern does not change with distance and the waves are transverse.

    One of the major characterizing aspects of antennas is the directivity. This is equivalent to the ratio of the radiation intensity in a certain direction over the hypothetical isotropic radiator intensity.

    directibity.jpg

    The denominator represents the average power radiated in all directions. The function is the normalized radiation pattern as a function of both the elevation and azimuthal angles. It is also possible to calculate partial directivities in either the theta direction or the phi direction and total directivity is the sum of these two. For a highly directive antenna with a very narrow major lobe and negligible minor lobes, the solid angle can be approximated by the product of the half power beamwidths in two different planes.

     

    Another important property is antenna efficiency, which is the product of reflection efficiency, conduction efficiency, and dielectric efficiency. This takes into account all possible loss: either from a VSWR greater than 1 due to an impedance mismatch between the feedline and the antenna and conductive losses due to Joule heating from both the dielectric and the conductive parts. The antenna gain can be defined as the product of the antenna efficiency and directivity.

     
  • jalves61 6:07 pm on January 17, 2020 Permalink | Reply
    Tags: , Microwave/RF Engineering   

    ARRL Examination Study (Part I) 

    The ARRL (American Radio Relay League) is an organization for amateur radio enthusiasts. In order to communicate using HAM radio, at least a technician license must be obtained. The following post is meant as a useful information guide for those wishing to obtain a license.

    The ARRL provides a complete manual as a study reference for HAMs. The book is divided into nine chapters: Basic info about ARRL, Radio and Signals, Circuit components, propagation and antennas, Amateur radio equipment, HAM communication, License regulation, operating regulation and safety. The questions come directly from each chapter (35 total, 26 to pass).

     

    For Radio and Signal fundamentals, it is important to know basic properties of waves including wavelength, speed of propagation, the relation between wavelength and frequency, identifying frequency bands, the frequency ranges of various bands used by HAMs and so forth. The fundamental equation for propagation of waves is c = fλ. Because radio waves are being transmitted by antennas through air, the speed of propagation is 300 million meters/sec. This is a constant value and therefore if frequency is increased, the wavelength decreases proportionally. This speed value is roughly equivalent to the speed of light in a vacuum. The property of radio waves used to identify different frequency bands is wavelength. HAMs tend to use the frequencies occupied by bands MF through UHF. It is important to know the frequency ranges of these bands.

    bands.PNG

    In this section, it is important to know prefixes for the SI unit system, so conversions between various values can be made. The following table should be committed to memory.

    SI.PNG

    The next section deals with modulation, which is a necessary function to transmit the correct signal to receiver. It is important not to set a transmit frequency to be at the edge of any band to allow for transmitter frequency drift, allow for calibration error, and so that modulation sidebands do not extend beyond the band edge. It is important to know about FM deviation (which is dependent on amplitude of the modulating signal) and that if the deviation is increased, the signal occupies more bandwidth. Setting a microphone gain too high could cause the FM signal to interfere with nearby stations. It is important to know the types of AM modulation (Double Sideband, Single Sideband, etc) and which modulation technique is best for various frequency bands. “Continuous wave” (Morse code-esque) modulation occupies the lowest bandwidth, followed by SSB modulation. The various advantages to certain modulation techniques should be understood. For example, SSB is preferential to FM because it occupies less bandwidth and has longer range. The bandwidth for each modulation technique is shown below.

    bandwidths.PNG

    The final section of Chapter two deals with radio equipment basics. A repeater should be understood to be a station that retransmits a signal onto another channel. The following is an image of a transceiver, which transmits and receives RF signals using a TR switch to switch between each function. A repeater uses a duplexer in place of this switch to transmit and receive simultaneously.

    transciever

     
  • jalves61 7:36 pm on January 10, 2020 Permalink | Reply
    Tags: Microwave/RF Engineering   

    Quarter Wave Transformer Matching – Using Theory of Multiple Reflections 

    There are two ways to derive an impedance value for a quarter wave transformer line. The transformer is an excellent tool to match a characteristic impedance to a purely resistive load where a large bandwidth is not required. It is much easier to find this relationship by examining it from an impedance viewpoint, however the theory of multiple reflections is an excellent topic because it illustrates the contribution of multiple impedance lines to the overall reflection coefficient.

    The following circuit with the matching transformer is shown below.

    reflection

    The addition of the matching transformer introduces discontinuity at the first port. Ideally, the addition of the transformer will match the load resistance to Zo, minimizing all reflection, as will be shown. the bottom figure provides a “step by step” analysis of each trip of the wave as it travels. When the wave first hits the Zo and Z1 junction, it sees Z1 as a “load” and does not yet see the actual load resistance. Depending on the impedance match, some of the wave will be reflected and some will be transmitted. The transmitted part of the wave then travels to the load and a portion is again reflected with reflection coefficient 3. As that portion of the wave travels back to the Z1 and Zo junction, the process repeats. This process continues infinitely and results in the following equation.

    gamme

    Using the definition of a geometric series and writing the reflection coefficient in terms of impedance, the equation reduces to

    reduce.PNG

    The reflection is seen to reduce to zero when Z1 (the impedance of the quarter wave section of transmission line) is set to q.PNG

     
  • mbenkerumass 2:51 pm on December 16, 2019 Permalink | Reply
    Tags: , , , Microwave/RF Engineering   

    Half-Wave Dipole Antenna Transmitter 

     

    Full PDF of project:

    ece435final_michaelbenker

    final

    121314151617

     
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