Ports

Ports are best understood as the virtual 3D counterpart of physical ports on RF/microwave components, such as the standard 50 Ω input or output ports on circuit boards, signal generators, oscilloscopes, and especially Vector Network Analyzers (VNA). A port is treated as a lumped circuit, located at a defined position. It acts as a voltage source or load with a resistive impedance. It can inject a signal to the Device-Under-Test (DUT) or measure the DUT’s response, either from its own signal or from another port.

Internally, a port is implemented by locating the Yee cells occupied by the port, and setting the numerical values of the electric fields in these cells based on the excitation waveform. Thus, a port can be understood as a source that injects electromagnetic energy into the simulation, helping establish initial conditions for the system. Simultaneously, a lumped resistor and a probe are also created at the same location as the port, allowing it to provide a matched load for the signal, or to measure the voltage or current at this region.

../_images/vna_ports.svg — The “port” in openEMS serves a purpose similar to the physical ports on Vector Network Analyzers and circuit boards. Both kinds of ports are used to inject an input signal at a particular point in the Device-Under-Test (DUT), and to measure what comes out at another point. The DUT is thus characterized as a black box, solely represented using its input-output relationships without an internal structure. Note that port implementations are fundamentally different in physical instruments (via circuits) and in openEMS simulations (by loading numerical values into Yee cells). Image by Julien Hillairet, from the `scikit-rf` project, licensed under BSD-3, modified for clarity.

Port Types

Port Type	Matlab/Octave	Python	Notes
Lumped	`AddLumpedPort()`	`AddLumpedPort()`	General Purpose
Curved	`AddCurvePort()`
Microstrip	`AddMSLPort()`	`MSLPort()`
Stripline	`AddStripLinePort()`
Coplanar Waveguide	`AddCPWPort()`
Generic Waveguide	`AddWaveGuidePort()`	`WaveguidePort()`
Rectangular Waveguide	`AddRectWaveGuidePort()`	`RectWGPort()`
Circular Waveguide	`AddCircWaveGuidePort()`

Note

Some port types are not ported to Python yet.

Usage

Matlab/Octave:

z0 = 50;

start = [-100 0 0];
stop  = [-100 0 50];
[CSX port{1}] = AddLumpedPort(CSX, 5, 1, z0, start, stop, [0 0 1], true);

start = [100 0 0];
stop  = [100 0 50];
[CSX port{2}] = AddLumpedPort(CSX, 5, 2, z0, start, stop, [0 0 1], false);

Python:

z0 = 50

port = [None, None]

start = [-100 0 0]
stop  = [-100 0 50]
port[0] = fdtd.AddLumpedPort(1, z0, start, stop, 'z', excite=1)

start = [-100 0 0]
stop  = [-100 0 50]
port[1] = fdtd.AddLumpedPort(2, z0, start, stop, 'z', excite=0)

Port Attributes

Matlab / Octave	Python	Domain	Definition
`ZL_ref`	`Z_ref`	Impedance	Reference Impedance
`uf_inc{n}`	`uf_inc[n]`	Frequency	Incident Voltage
`uf_ref{n}`	`uf_ref[n]`	Frequency	Reflected Voltage
`if_tot{n}`	`if_tot[n]`	Frequency	Total Voltage
`if_inc{n}`	`if_inc[n]`	Frequency	Incident Current
`if_ref{n}`	`if_ref[n]`	Frequency	Reflected Current
`if_tot{n}`	`if_tot[n]`	Frequency	Total Current
`P_inc{n}`	`P_inc[n]`	Frequency	Incident Power
`P_ref{n}`	`P_ref[n]`	Frequency	Reflected Power
`P_acc{n}`	`P_acc[n]`	Frequency	Accepted Power (Incident - Reflected)
N/A (see notes)	`ut_inc[n]`	Time	Incident Voltage
N/A (see notes)	`ut_ref[n]`	Time	Reflected Voltage
`ut_tot{n}`	`ut_tot[n]`	Time	Total Voltage
N/A (see notes)	`it_inc[n]`	Time	Incident Current
N/A (see notes)	`it_ref[n]`	Time	Reflected Current
`it_tot{n}`	`it_tot[n]`	Time	Total Current
`raw.U.TD{1}.val{n}`	`u_data.ui_val[0][n]`	Time	Raw Voltage (`ut_tot` Recommended)
`raw.U.TD{1}.t{n}`	`u_data.ui_time[0][n]`	Time	Raw Time of Voltage Samples
`raw.I.TD{1}.val{n}`	`i_data.ui_val[0][n]`	Time	Raw Current (`it_tot` Recommended)
`raw.I.TD{1}.t{n}`	`i_data.ui_time[0][n]`	Time	Raw Time of Current Samples

Note

Voltage symbol. u is the unambiguous symbol of voltage (\(U\)) in ISO/IEC convention, so frequency-domain variables have the prefix uf, time-domain variables have the prefix ut. In American literature, symbols such as \(V\), \(E\) and \(\mathcal{E}\) are used.

Incident and reflected signals. In Matlab/Octave, only total time-domain port voltage and current are given, while their incident, reflected components are not. They can be calculated using the following expressions:

ut_inc = 0.5 * (ut_tot + it_tot * ZL_ref)
ut_ref = ut_tot - ut_inc

it_inc = 0.5 * (it_tot + ut_tot ./ ZL_ref)
it_ref = it_inc - it_tot

Usage

Port Selection

In openEMS, ports are ideal sources of E&M fields, but they are not ideal launchers of E&M waves into structures due to a discontinuity at the boundary between the port and the structure. If port placement is not optimized, this region of discontinuity may introduce artifacts such as reflections or excitation of spurious modes. Optimizing the placement and implementation of a port reduces these artifacts. This can be done by using smooth transitions or by shaping the electric fields initially injected by the port.

In openEMS, the standard port is the lumped port that works with most structures. If an optimal transition is needed, openEMS also provides optimized implementations of curved, microstrip, stripline, coplanar waveguide, and coax cable ports.

Most specialized ports in openEMS are signal integrity optimizations rather than strict requirements. However, in enclosed waveguides, specialized ports are required to excite those structures properly. These waveguides only have one conductor, unlike the usual two-conductor transmission lines. An ordinary port can’t excite them correctly, as the waveguide is essentially a DC short circuit. Special waveguide ports must be used to excite the unique TE-mode waves. These include general waveguide ports, rectangular waveguides ports, and circular waveguides ports

Note

Like physical ports on real devices, the virtual ports in openEMS are not perfect. They’re ideal sources of E&M fields, but they are not ideal launchers of E&M waves into structures. A port creates a region of discontinuity, so they may introduce artifacts. Optimizing the placement and implementation of a port reduces artifacts. Alternatively, these artifacts can be removed through calibration or de-embedding algorithms, an advanced topic beyond the scope of this tutorial.

../_images/error-box.svg — The artifacts introduced by a two-port measurement can be viewed as two linear circuits (left error box, right error box) cascaded in series with the DUT. All three circuits are represented as three matrices, called their S-parameters. Measurement error can be reduced by making error boxes nearly transparent using optimized port transitions. Alternatively, by mathematically removing the port’s contributions from the measured response using linear algebra, a process known as calibration or de-embedding (image by Ziad Hatab et, al., licensed under CC BY-SA 4.0 [1]_)

Port Implementation

Ports are a high-level concept in openEMS. Internally, they’re implemented by first calling AddExcitation() to create a source of E&M field. Later, AddLumpedElement() and AddProbe() are used to add termination resistances and probes. One can create new port types based on these low-level primitives.