Hello all,
I'm pretty new to CST Studio, and I'm trying to learn the ropes by mimicking the results of a patch antenna I found in a paper in CST Studio. I've constructed the antenna in CST Studio, but I'm a little confused on how the waveport system in CST Studio works. I have the basic layout of the antenna attached, as borrowed from the paper. My main question is: when constructing the waveport in CST Studio, would I put the waveport fully against the square patch (Image A in my attached images), or would I construct the waveport on some type of strip (Image B in my attached images)?
I have watched quite a few videos on CST Studio, but none have really talked about what a waveport really does or how it may interact with things like traces on a PCB, so I would love any information!
I would really appreciate any advice you may have!
(The paper I'm trying to learn from is "Design and Evaluation of Reconfigurable Intelligent Surfaces in Real-World Environment" if anyone wants to check it out!)
A CST wave port mimics a transmission line, so you put it somewhere where the signal would be entering the circuit in a transmission line, such as microstrip, stripline, coplanar waveguide, coax, slotline. waveguide ...........
The port is at right angles to the transmission line and the first thing CST does is calculate the modes that can propagate in the transmission line, so the metal and the dielectric at the plane of the port (and a few grid cells into the model) must be those of the transmission line, and the port must be big enough to include all the significant fields. For example in microstrip the port needs to be at least three times the width of the strip and three times the height of the substrate. There will be a better guide than that somewhere. For simulation it is usually enough that the impedance doesn't change significantly if you make the port bigger.
In the simulation, the port launches the mode it has calculated (pattern of fields) from the port, and detects any of that mode coming back out.
Thank you so much for your response, Malcolm White ! You've really cleared it up! I have a follow up question, if that's okay. When designing the transmission line for an antenna, I know width matters and that the transmission line should often be 50 Ohms. I was wondering if the length of the line matters, and if it does what a recommended length would be? I have attached an image of my antenna, where I have added 2 wide transmission lines (both of 50 Ohms) that taper into much thinner lines. My frequency response changes greatly if I change the length of the wide or thin lines. I was wondering if you knew of any relation of line length to frequency response? Thank you so much!
You need to learn microwave engineering. You have done several things wrong that are very basic mistakes.
The whole point of transmission limes is that if they are matched by their characteristic impedance at each end it doesn't matter how long they are, except for attenuation. All the signal goes in, travels along the line and goes out the other end, except for attenuation, and there is no reflection at either end.
A waveguide port launches a wave into a transmission line of the same impedance. If the impedance changes there will be a reflection. There are lots of ways to control these reflections and to make them zero over a desired bandwidth. This is a whole area of filter and transformer design.
Microwave components are nearly all designed so that they have 50 ohms input and or output impedance, so when joined together there is no reflection. It doesn't have to be 50 ohms. There are better choices for some applications, but it is handy to be able to join things together and know there is no reflection.
A short transformer can make the reflection from a change in impedance small again. One of these is a quarter wave transformer. Microstrip patch antennas often have these because the antenna itself is not matched to 50 ohms, and the transformer makes it look like 50 ohms over the operating band, so it can be put at the end of a 50 ohm transmission line of any length.
I can't tell what you have done with your circuit. Is there a ground? Are the tracks microstrip? If so then then you have a huge change of impedance from the wide bits to the thin bits, with a big parasitic capacitance too, and there will be a big reflection. If there is a reflection at both ends of the thin bits then the length will cause a time delay between the reflections which will result in constructive and destructive interference depending on the frequency.
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