I would like to know how to define the probe feed for a microstrip antenna which is directly connected by probe wire to coax connector through ground plane behind the patch
If you want to have a model for the coax feed through a ground plane, you have to construct something vaguely similar using microstrip feeds and vias. That's because Momentum in ADS does not have any concept of a "coax".
You can define a substrate stack like this:
Air ------- patch Sub1 Via ------- hole botSub Via ------- Feed
The "Via" layer can be mapped through both substrates at the same time. You have to choose the bottom substrate "botSub" thickness such that the microstrip you end up drawing gives you 50 Ohms or 75 Ohms or whatever the coax impedance is. You draw the feedline on the layer "Feed". The Via needs a clearance through the ground plane which is why a "slot" layer called "hole" is mapped to it. You should make the hole size such that the coax impedance of the combintation of hole diameter and via diameter in that "epsr" gives you the same impedance as the feed.
Thankyou, that seems like a logical approach, although I believe there would be a discontinuity at the microstrip to via transition that would not occur with the coax feed. Am I correct in assuming that the microstrip feed would be taken to the edge of the board and connected to single edge feed port referenced to the ground on the slot layer?
Another way I have thought of doing the feed is by defining two substrates between the patch and the ground plane and connecting an internal port to a via that goes from the patch to an internal conductor layer. The Sub1+Sub2 would give the height of the patch above the ground with Sub2 being much smaller than Sub1.
Air ------- patch Sub1 Via ------- cond2 (internal Port connection layer) Sub2 ------- GND
Unless your patch is electrically large (most of the time they are a half-wavelength), the microstrip feed would introduce a negligible discontinuity. The benefit is that you introduce no extra coupling underneath the radiating element. The aperture/via combo looks very similar to a coax.
Your approach would introduce extra coupling between the port and metal element (the port can't be hooked directly to a via - not allowed).
You can define a substrate stack like this:
The "Via" layer can be mapped through both substrates at the same time. You have to choose the bottom substrate "botSub" thickness such that the microstrip you end up drawing gives you 50 Ohms or 75 Ohms or whatever the coax impedance is. You draw the feedline on the layer "Feed". The Via needs a clearance through the ground plane which is why a "slot" layer called "hole" is mapped to it. You should make the hole size such that the coax impedance of the combintation of hole diameter and via diameter in that "epsr" gives you the same impedance as the feed.