I'm trying to do impedance matching between my PCB antenna and my circuit. My PCB antenna has a measured impedance of 49.4 - j8.1 ohms. The operating frequency is 915 MHz. The goal of matching is to move the circuit impedance as close to 50 ohm as possible.
My process is as follows and it follows the instruction here: (http://wisp.wikispaces.com/WISP+Measurement+Procedures?responseToken=016c891618c82df3ef37a9c2b69cf236)
Step 1: Shorten Lm1 by a small wire (perhaps it's inductance is 2 nH?). Leave other components blank.
Step 2: Measure the input impedance of the circuit afterwards by a Network Analyzer. It gave 0.355 + j42 Ohm.
So Lm1 was replaced by a resistor of 50 Ohm, while Cm2 was replaced by an inductor of 23.5 pH. No Cm1.
Step 4: Start to replace the components as designed in Step 3. Firstly add Cm2, then add Lm1. Do Step 2 after replacing any component (for example, measure the input impedance again after replacing Lm1).
All measurements were performed as 1-port measurement on VNA (Vector Network Analyzer).
There are two problems:
1/ Step 4 problem: Anytime I replaced any component, the measured input impedance was totally different than it should be by being simulated in Smith V4.0. For example, if I add a shunt capacitor of 2 pF, the measured Zin was not be 1.3 + j81.2 Ohm as found by using Smith V4.0 software. I did not record the result so I do not remember the exact result, but I can say that it was totally different both in values and the direction that the impedance should move. I'll get back this situation with numerical values.
2/ Matching performance: Even I finally got the measured Zin ~ 50 Ohm, not by designing in software but by replacing and guessing and replacing again the components, the delivered power after matching was lower than before matching! In theory, the power must be higher because less power mismatch!
My friend got the same 2nd problem when he tried to match his transducer. The signal he got after matching was more noisy than before.
Could anyone help me explain these things? Any suggestion/comment is highly appreciated. Thank you for your time.
------- Updated for response for Mr. Glenn W9IQ-------------------------------
Thank you so much for your comments and your kind explanation @Glenn W91Q. I'll keep in my mind the correct terms. Let me explain more about my measurement set up.
1/ Calibration of the test cable: I used a SHORT-OPEN-LOAD adapter and a coaxial cable (Pic 1). The cable is terminated by a kind of SMA adapter (I don't know how to name it). I need that adapter to connect my antenna/circuit to the cable. So, I attached the SHORT-OPEN-LOAD adapter to the cable through that adapter for calibration. So, it means I calibrate both the cable and that adapter. Am I correct? I followed the calibration procedure of this video https://www.youtube.com/watch?v=GKHHy61qRak&t=18s. But I chose 1 port calibration instead. I used Agilent E5062A VNA model.
2/Set up for antenna input impedance measurement: I connected my dipole antenna to the cable as follows. The SMA connector attached to the antenna is used to connect the antenna to the circuit. The antenna will be a receiving one, not a transmitting one. It was designed using HFSS tool kit. The simulated reflection coefficient is -22 dBm @912 MHz.
By using S11 measurement mode, Smith Chart display tool of the VNA, I found the impedance of (antenna + SMA connector). It must be INPUT impedance, right?
To get the impedance of as close to 50 ohms as possible, the antenna was tuned by cutting its length. I also added a matching circuit (plz see below). The final impedance I got, using this set up, was 49.4-j8.1 ohms.
3/Set up for receiver input impedance measurement: The same set up as above was applied for this.
Can I ask you for more clarifications?
1/ I thought my measurement set up as illustrated above was wrong, because I did not connect the antenna/receiver ground to the shield of the VNA cable as you mentioned earlier. And I still cannot imagine how to do this. If you've got some images for this, I'd highly appreciated it if you can share some.
The VNA must have two connections to the antenna under test. This perhaps is basic but I have seen students completely ignore the ground (shield) connection to the VNA cable or they simply connect it to any convenient ground point on the circuit, treating it like a basic DC ground.
2/ The impedance I got from the set up of measuring input impedance of the receiver was far from 50 ohms. So, I need another matching circuit to move it close to 50 ohms, right?
Thank you again for your time.