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I've stumbled upon this, but it only works for outputs between 15db and 20dBm. How can I modify it to work for 0dBm sources?

I'm planning to use this to roughly tune my matching circuit for the interface between a RF transmitter and a chip antenna.

Kortuk
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cksa361
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  • That is messier, at 1mW you get much less power, but they are trying to keep their system matched at 50ohm, which is not perfectly efficient. – Kortuk Dec 04 '10 at 21:43

1 Answers1

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The host has another project, Field Strength Sensor for Wifi ISM band (2.4 GHz), that incorporates a preamplifier. WiFi signals are <5mW (<7dBm) broadcast, so this must be picking up <<5mW.

I don't see a reason why you couldn't adapt this preamp stage into the bridge of the SWR meter. The host uses a diode detector in the bridge; you would want to swap out the diode and everything to the right of it in his/her diagram. Resistor values all seem sensible; I'm not sure about the caps.

Disclaimer: I am not an RF engineer! I have made a few GHz amps and played with some microwave structures, but I certainly can't guarantee this will work. (It certainly sounds like fun, though!)

tyblu
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  • I think tyblu is correct here. I was planning to come back later and find a project with a preamp. He has done so, take advantage of it. The major thing here is that you need your SWR meter to have the same input impedance as your chip antenna, otherwise you are tuning the circuit to a missmatch. – Kortuk Dec 04 '10 at 23:18
  • how can I find out the input impedance of my chip antenna cheaply? Is it even possible without a VNA? – cksa361 Dec 06 '10 at 04:43
  • Time for a proper RF engineer, I'd say. I have a hunch that you don't actually have to match the input, just calibrate it properly --I'll take a peek tonight. – tyblu Dec 06 '10 at 06:54
  • kortuk: If you add a preamp, where would you put it? After the directional coupler? So two preamps? – Colonel Panik Jun 09 '21 at 11:32