I got stuck with impedance matching for an antenna. Any added components in the matching network gave a result totally different from what a Smith chart predicts. Let me explain this clearer.
First, by shortening L2,C8,L5,C23, I measured my antenna "raw" impedance (without matching) at ANT1 terminal (please see attached my matching network). It gave Za_raw = 133 +j 226 (ohm). RF+ is the input feed line of my antenna. There is a SMA connector at ANT1. It connects the rest of my circuit to the antenna.
Then, if I replace C23 with an inductor L = 47nH, the measured impedance at ANT1 terminal is 60 + j160 (ohm)!!! But using Smith chart, with Zload = Za_raw, a serial added L of 47nH would give Za = 133 + j 496 (ohm). You can see this in the following figure.
All measurements were conducted using a VNA. 1-port calibration was done before measuring. Any suggestions are highly appreciated.
****Modified on 17 Oct.**
Here is my construction. I shorten L2, C8, L5 & C23 using wires. The physical length of the feed line is about 29.5mm.
- The reason for the complicated matching network is that, at first, I would like to make 2 measurement points: at ANT11 and ATN1. ANT1 should be the input impedance of my circuit, while ANT11 should be the antenna input impedance. But later, I thought I don't need to do that. I just need to tune the input impedance of the whole circuit (with all components soldered) at ANT1 to be 50 ohm!
- Why 50ohm? In my measurement, at the resonant frequency (around 915 MHz), the impedance at ANT1 is 50ohm!