## S-parameters, evanescent modes?

## S-parameters, evanescent modes?

(OP)

Hi,

could someone help me with this.

Usually S-parameters are written for propagating modes.

Is there something wrong with including evanescent modes in the s-parameter formulation? In my situation I need to analyze microwave circuit components which are placed close to each other, so that I would need to consider evanescent modes on the artificial port boundaries?

Is the generalization straight forward?

thanks,

Svea

could someone help me with this.

Usually S-parameters are written for propagating modes.

Is there something wrong with including evanescent modes in the s-parameter formulation? In my situation I need to analyze microwave circuit components which are placed close to each other, so that I would need to consider evanescent modes on the artificial port boundaries?

Is the generalization straight forward?

thanks,

Svea

## RE: S-parameters, evanescent modes?

of Evanescent Waves from

Perfectly Matched Layers” , IEEE

Antennas and P ropagation Symposium

Diges t, pp. 1888-1891, July

1997.

## RE: S-parameters, evanescent modes?

Haim Haskal, "Matrix Description of Waveguide Disdcontinuities in the Presence of Evanescent Modes,"

IEEE Transactions on Microwave Theory and Techniques,

pp. 184-188, March 1964

## RE: S-parameters, evanescent modes?

## RE: S-parameters, evanescent modes?

## RE: S-parameters, evanescent modes?

## RE: S-parameters, evanescent modes?

## RE: S-parameters, evanescent modes?

## RE: S-parameters, evanescent modes?

## RE: S-parameters, evanescent modes?

thanks for showing your interest!

I found the Haim Haskal paper valuable I remember.

I used to work on a problem where we simulated pieces of waveguide junctions using the Finite Element Method (HFSS etc...) in the Frequency Domain (interesting comment there on the atomic clock, I was mostly looking at the fields in the frequency domain). This was indeed a problem of practical 'industrial' interest. The problem was that the excitation ports, i.e. truncation boundaries of the FEM model, were located close to geometric discontinuities in the model such that evanescent modes were necessarily present at the ports.

The analysis of a system of many connected junctions using circuit parameters (S-parameters for example) would then (perhaps) need to take into account also the evanescent modes. This is somewhat of a mode matching method, where the fields are to be made continuous at the intersecting ports by matching the modes on both sides of the intersection.

The simulators could model ports in a variety of ways (Perfectly Matched Layers, Impedance Boundary Condition, etc...), of which some would be appropriate for situations with multiple (incl. evanescent) modes.

Best luck,

Svea

## RE: S-parameters, evanescent modes?

## RE: S-parameters, evanescent modes?

So, if you now design something using that set of measured S parameters, your design will work wonderfully IF the next closest discontinuity is physically far enough so that 90% of the evanescent energy has died out by then. If, as one usually finds, you are trying to use multiple discontinuities close together (such as a T junction, as stub, and the gate of a FET all withing 200 mils of each other), then the originally measured S parameters for each of those microstrip elements are now wrong, and you will find measured performance deviating from the predicted.

In those cases, you should do an Electromagnetic Field simulator (HFSS, etc) with all of the discontinuities shown all at once.

## RE: S-parameters, evanescent modes?

and wouldn't it be great in this case not having to redo the field calculations (e.g. in hfss), but to keep on doing algebra using generalized scattering parameters that include information about the evanescent modes? Also in real world applications.

## RE: S-parameters, evanescent modes?

## RE: S-parameters, evanescent modes?

*******

The problem was that the excitation ports, i.e. truncation boundaries of the FEM model, were located close to geometric discontinuities in the model such that evanescent modes were necessarily present at the ports.

*******

At this point, it might be feasible to switch from FEM to FDTD. Computation of S-parameters with the present evanescent modes is, e.g., a routine procedure in

QuickWave-3D(www.qwed.com.pl).