How add a circuit or “black box” with known network parameters in series with radiating element.
Introduction
Monopole antennas are often loaded with inductors to enable operation of the monopole at a lower frequency. This how-to will show how to add an inductor somewhere along the monopole wire using a non-radiating network. The steps outlined in this how-to can be applied in a more general sense to place any circuit or so-called "black box" with known network parameters (S, Z, Y, etc.) in series with a wire or metallic strip.
Details of the circuit to be added to the model
An inductor was obtained from a manufacturer with measured two-port S-parameters. The inductor will be added in series somewhere along the monopole wire to enable loading of the monopole. A non-radiating network will be used for this purpose.
Steps
Consider a monopole on an infinite ground plane fed a voltage source on a wire port against the ground plane.
The monopole is matched quite well for operation at around 100 MHz as seen in the graph of the input reflection coefficient.
An S-parameter Touchstone file is available describing the inductor performance over a frequency range. Note that Feko will interpolate within the bandwidth of the file, but will not extrapolate outside the bandwidth. An additional wire port (with the label L_port) is created some distance away from the feed point.
In CADFEKO a non-radiating network is created using the Touchstone file provided.
NOTE: For a simple ideal inductor, an inductor (series circuit) load could have been added to the L_port. However the circuit options to use for such loads are limited.
In the schematic view of CADFEKO, connect L_port with Port1. Port 2 is left open in the schematic view.
In CADFEKO model tree, a 0 Ohm load is placed on network port 2.
The comparison below shows the monopole is now resonating at a lower frequency due to the inductive loading.
Conclusion
It may seem less intuitive when working with, for example, S-parameters or Z-parameters, to use single connections for non-radiating networks. Usually graphical representations of network parameter "black boxes” are displayed with a signal pin and a ground pin (assuming a two-port S-parameter network).
It could be expected to use an “input pin” and “output pin” for the inductor in the non-radiating network model. When non-radiating networks are connected to ports in the geometry, these networks are implicitly seen as being in series with the port. For this reason it was required to “short” pin 2 of the of the non-radiating network. The image below gives a more intuitive graphical representation of how the black box connects to the monopole segments.
