You can change the characteristic impedance of a port by selecting it from the Port List and clicking the '''Edit '''button of the dialog. This opens up the Edit Port dialog, where you can enter a new value in the box labeled '''Impedance'''.
Click here to learn more about [[Using Lumped Sources to Model ===Modeling Transmission Line Feeds]].===
===Modeling Microstrip Line Ports===Using simple lumped sources, you can simulate a variety of transmission line structures in [[EM.Tempo]] including filters, couplers or antenna feeds and you can calculate their scattering [[parameters]]. This approach may become less accurate at very high frequencies when the details of the feed structures become important and can no longer be modeled with highly localized lumped ports. In such cases, it is recommended to use âDistributed Sourcesâ, which utilize accurate modal field distributions at the ports for calculation of the incident and reflected waves.
Using simple lumped sources, you can simulate a variety of transmission line structures in [[EM.Cube]]âs [[FDTD Module]] including filters, couplers or antenna feeds and you can calculate their scattering [[parameters]]. This approach may become less accurate at very high frequencies when the details of the feed structures become important and can no longer be modeled with highly localized lumped ports. In such cases, it is recommended Click here to use âDistributed Sourcesâ, which utilize accurate modal field distributions at the ports for calculation of the incident and reflected waves. This and next two sections explain how you can use simple lumped sources to model some popular transmission line feeds. Microstrip ports can be modeled with lumped sources placed underneath the microstrip line stretching to the ground plane through the substrate. To build a microstrip port with a lumped source, follow these steps: * First, draw the PEC microstrip using the '''Rectangle Strip Tool''' and end it at the desired port location.* Then, draw the substrate box and make sure that its extends at least λ<sub>g</sub>/8 beyond than the microstrip line (λ<sub>g</sub> is the guide wavelength).* If the microstrip's ground plane is the bottom of your simulation structure, set a PEC boundary for the bottom face of the computational domain. Otherwise, you do need to draw a PEC rectangle strip to represent a finite ground.* Connect the center of the microstripâs end to the ground plane with a PEC line using the '''Line Tool'''.* Place a lumped source with 1V amplitude and zero phase on the line pointing towards the line.* In the '''Port Definition''' Dialog, set the number of ports equal to one. Associate the lumped source with Port 1. Set the value of '''Port Impedance''' properly equal to the characteristic impedance of the microstrip feed line. {{Note|If you want to model a microstrip structure with a laterally infinite substrate and a laterally infinite ground plane, make sure to set the domain offsets in the ±X, ±Y and -Z directions equal to zero.}} {{isoimg|FDTD50.png|A microstrip line port on a substrate terminated from the bottom by a PEC boundary plane.}} ===Modeling Coplanar Waveguide Ports=== Using lumped sources, you can define coplanar waveguide (CPW) ports either with or without a bottom ground plane. In order to build a CPW port with a lumped source, follow these steps: * First, draw the PEC center strip of the CPW with its two PEC coplanar ground planes, all using the '''Rectangle Strip Tool''', and end them at the desired port location.* Then, draw the substrate box and make sure that it is at least λ<sub>g</sub>/8 longer than the CPW line.* Connect the two end corners of the center strip to the two side ground planes with two PEC line objects using the '''Line Tool'''.* Place a lumped source with 1V amplitude and zero phase on each line pointing towards the center strip.* In the '''Port Definition''' Dialog, set the number of ports equal to one. Associate both of the lumped sources with Port 1. Set the value of the '''Port Impedance''' properly equal to the characteristic impedance of the CPW feed line. learn more about [[EM.Cube]] sets the internal resistance of each of the two coupled lumped sources equal to twice the specified port (or line) impedance since the CPW port is a parallel connection of the two individual lumped sources. If you want to model a CPW with laterally infinite substrate and ground planes, make sure to set the domain offsets in the ±X and ±Y directions equal to zero. In the -Z direction, you need a nonzero offset to push the bottom CPML boundary down away outside the dielectric layer of finite thickness. Alternatively, you can terminate the bottom boundary by PEC and set its domain offset equal to zero to represent a CPW with a bottom PEC ground. {{isoimg|FDTD51.png|A coplanar waveguide (CPW) port on a dielectric substrate.}} ===Modeling Coaxial Line Ports=== Coaxial line is often used to feed various RF and microwave structures. In order to excite the dominant TEM mode of a coaxial line, it has to be fed symmetrically between its inner and outer conductors. Using lumped sources, you can define symmetrical coaxial line ports. To do so, follow these steps: * First, use the '''Cylinder Tool''' Lumped Sources to draw a solid PEC inner conductor and end it at the desired port location.* Next, using the '''Cylinder Tool''' again, draw a hollow PEC outer conductor with the '''Cap Ends''' option unchecked, and end it at the desired port location.* Then, draw the dielectric core of the coaxial cable using the '''Cylinder Tool''' under an active dielectric material group and make sure it is at least λ<sub>g</sub>/8 longer than the inner and outer conductors. Note that you only need to draw a solid dielectric cylinder rather than a hollow, pipe-like one. According to [[FDTD Module]]'s material hierarchy, the PEC core takes precedence over its enclosing dielectric cylinder.* Using the '''Model Transmission Line Tool''', connect the ends of the inner and outer conductors with four PEC line objects along the principal coordinate axes pointing from the inner conductor towards the outer conductor.* Place a lumped source with 1V amplitude and zero phase on each line.* In the '''Port Definition''' Dialog, set the number of ports equal to one. Associate all the four lumped sources with Port 1. Set the value of the '''Port Impedance''' properly equal to the characteristic impedance of the coaxial feed line. {{Note|[[EM.CubeFeeds]] will set the internal impedance of each of the four coupled lumped sources equal to four times the specified port (or line) impedance since the coaxial port is a parallel connection of the four individual lumped sources.}} {{isoimg|FDTD52.png|A coaxial line port using four symmetric lumped sources.}}
===Lumped Load===