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Calculate output impedance of transmitter


Mar 23, 2023
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I bought a 433MHz xmtr/rcvr and I'm trying to match an antenna to it. I know I could use a 1/4wave wire, but I have a Nano VNA and I'd like to try and tune the antenna. The manufacturer does not specify the output impedance, but if I sum the two inductors in the VCC path, as shown in the attached image, I calculate the output impedance to be about 20k ohms. That's assuming the total inductance is about 700nH. Does that sound right?
Thank you.


  • 433MHZ SCH.jpg
    433MHZ SCH.jpg
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This is an oscillator and it's not necessary to calculate the output impedance for an antenna.
Because it's not a critical application and the output impedance can be considered as high.
There are 2 inductors and IF I assume each one has 350nH and as a rule of thumb The Impedance of a Choke Inductor has 10 times of Output Impedance//Load Impedance..
The Output Impedance might be approximately 100 Ohm. But this is just a rough approximation. A precise measurement must be done with a sliding tuner.
OK. So I can't get there from here. But can I at least assume the inductance from VCC to the output is about 64nH + 26.4nH = 90.4nH, (see below . I miscalculated previously)? At 433MHz that would be about 246ohm. Shouldn't that be close to the output impedance?
The schematic I attached doesn't indicate the correct values. I measured the dimensions.
L2 = 0.0640 uH (diam 4mm, length 3.2mm, turns 4.5) approx.
L1 = 0.0264 uH (diam 4mm, length 5.4mm, turns 2.5) approx.

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