@pancho_hideboo: Thank you for your comment. I'm new in RF, and I do not have any experience in RF measurement. My analog knowledge is not good either. Yes my experiment set up was so funny, but I did not know how to test my circuit board in modules (harvester module, impedance matching module, load module etc. ). So I designed it into different modules with many jumpers and long wire to test and fix one by one. Could you please tell me where I can get the tutorials for RF measurements or how I can test the RF analog modules as good as possible?
@vfone: Thank you for your comment. Yes I need a lot of readings to understand RF measurements. Regarding the capacitor, it is exactly the supercapacitor I put right behind the multiplier circuit. Because there was no space for me to put it on board, so I had to plug it into the breadboard.
Were you saying that the RF ground practically was not connected to the probe's ground with that set up? Should I connect the probe's ground directly to the antenna ground (although I do not imagine how I can do this without any wire)?
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I'm sorry I should add more details in what I'm doing. I'm doing exactly an RFID transponder at 915 MHz. The reader I'm using has the output power of 650mW. At the distance of 15-20cm where I got the result as mentioned above, the received power, measured by a spectrum analyzer, is 3 dBm. The antenna I'm using is a microstrip one with S11 = -(15-20) dBm from Powercast. It's input impedance is around 50 Ohm (measured by VNA).
Please refer to my RF harvester schematic.
Right now I cannot make the impedance matching circuit (L-type) work. So I currently short the L by a small wire and leave the C open.