Electronic – Measuring output impedance

impedanceRF

Given I have designed and produced an RF circuit with an output for connecting an antenna. I know this antenna has an input impedance of 50Ω. I now wish to measure the actual output impedance of my circuit, before connecting the antenna, to ensure that the impedance matches.

How do I actually measure the output impedance of my circuit?

And additionally, can I do this with hobby friendly priced equipment?

Update: I know the basic theory of impedance matching, but I have never understood how to actually do it. So I am really asking for input on the specific hardware and equipment to use, and how to perform actual tests.

The RF circuit is operating in the UHF range, and transmits about 10dBm, if this helps.

Best Answer

You really need one of a number of options, none of them are hobby friendly if you are above a few MHz. I would guess you are at 2.4GHz, or at 900MHz.

Professional Options

If you might know someone whom has access to equipment that would help you have a few options.

  1. VNA- Vector Network Analyzer. This can give you a smith chart over a frequency range and make sure it shows you at 50 ohms. You can also measure S11 which should be less than -20dB
  2. Spectrum Analyzer- This should allow you to determine your power received at your frequency. This is not ideal, but it should get more output power as you get closer to a proper match.
  3. SWR Meter-As Is noted in another answer and I always forget about, you can use an Standing Wave Ratio meter, but I would still suggest a VNA. They have significantly more features, and can do anything a spectrum analyzer or SWR meter can do.

Hacker style ways to approach it

There is another funny way to approach this, the problem being you will need to build one of a few microwave components.

You could use an isolator and measure its temperature, hotter isolator is equivalent to a worse mismatch. this is not even close to good method as at 10dBm it will be very hard to detect a temperature failure.

You could build your own mixer and feedback your output signal and mix it with your signal returning from the connection. To take away your signal from the feedback you need to use what is called a circulator. This is not fun, but it is doable.

If you would like to try either method, which is similar to creating your own VNA, I can get you more information on how to build these devices.

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