Hi,
Current project is to move from a 1/4 wave whip antenna to a ceramic chip type (e.g. Johanson 1575AT43A0040). I have a pi matching network on the pcb and can look 'into' the antenna with my vector network analyser and get the conjugate impedance. I have tried to simply match that to 50 ohms (per the datasheet for the GPS receiver we are using) but it doesn't see to perform anything like as well as the 1/4 wave whip.
Question - Do I need to look 'into' the receiver chip with the VNA and match that to the antenna? If so is it still the S11 parameter I measure or something else?

TIA.

Bob

 

Answering your question, yes, you need at least to know the input impedance or S11 parameters of the receiver chip. Knowing so might give you only some information but it does not guarantees to succeed. There are too many open variables that can influence the design:
1- What do you mean by: "it doesn't see to perform anything like as well as the 1/4 wave whip"?
2- Is the input of the receiver chip balanced or unbalanced? Transmission line design from the RF chip to the antenna.
3- Which are the dimensions of your PCB/chip antenna ground plane compared to the suggested in the chip antenna datasheet? The impedances might be matched, but your antenna still doesn't work. The ground is part of the chip antennas.
4- Is the clearance area suggested for the chip antenna respected in your new PCB design? Also related to antenna performance.
5- Take care of measuring the S11 parameters of the antennas in a repeatable way according to positioning and proximity to hands, metals, casing, etc.

Understanding all this usually takes a semester in EE field.