Practical Matching Circuit

[Neil_Preston]

Hi,
I've build a microstrip antenna with a probe feed. I've manufactured the antenna and decided to build a matching circuit. I've taken the antenna s-parameters from the network analyzer and put it into ADS. Then I've build a lumped matching circuit using a matching tool. After that I've replaced the calculated values with the closest values based on manufacturer's datasheets. After that I've manufactured the matching PCB circuit. However the results didn't match and now I'm trying to figure what's wrong. Maybe it's because I used a barrel interconnector to connect two female sma's. However, I don't know how can I get rid of it. Maybe there is a way I can integrate the probe into the circuit, like cutting the connector and soldering the pin to the circuit.
Hope you can help.
Thanks,
Neil

 

[vfone]

Whatever results you get from the simulator, due to a lot of unknown parasitics, you have to do a manual fine tuning of the matching components to find the final values.
One barrel connector is not introducing high mismatch between components.

 

[biff44]

There are three possible reasons.

1) your antenna input impedance depends on both the antenna itself AND the surrounding ground plane. When you added a matching network to the antenna, you changed the ground plane shape and therefore the antenna impedance.

2) your initial measurement was faulty. It is pretty easy to screw up WHERE the antenna is electrically located (ie the reference plane extension). I often will measure something like an antenna, and then use a knife to cut the trace to the antenna I just measured, and remeasure with an open circuit, and then drill a hole thru the PCB and short it out and remeasure a short circuit. That way I can be somewhat sure that the reference plane I chose is a correct one.

3) you are measuring something that is hard to do, such as a narrowband antenna that has a very large reactance slope, or the impedance you measured is near the edge of the smith chart where a very small measurement error ends up being a very large error once you try to match it to the center of the smith chart

 

[Mazz]

Maybe a simpler reason...

Neil
did you take into account (designing the matching network) the electrical lenght between the section in which you have measured the antenna impedance (the feed point) and the matching network itself?

If not, the barrel, sma connectors and lines may affect the result. Maybe some more detail will help us to understand better.

I hope it can help.

Mazz