RF Circuits

This particular directional coupler above has two ports, each coupled by 20 dB. What that means is, a signal is "coupled" to these ports which is identical to the main signal passing through the directional coupler, only it's attenuated by 20 dB. By placing diode detectors on the outputs of these two ports and comparing the resulting voltages on an oscilloscope, you can quickly determine the integrity of your antenna system in reference to 50 ohms.

SWR Meter Diagram


How It Works

Be sure to measure SWR at the antenna! Feedline loss will attenuate the signal and give you a false SWR reading if you measure it directly at the transmitter's RF output. Don't be like those guys at Field Day who brag about their radio's (internal) SWR reading when connected to a homebrew antenna with 200 feet of Radio Shack RG-58 and banana clips.

Also, don't trust "analog" movement reading SWR meters when dealing with digital data transmitters. The needle response time isn't fast enough to provide an accurate reading.

SWR Meter Pictures



Overview of the hardware components used. The directional coupler is the large blue thing. A N-connector female-to-female adapter is needed to connect the Narda 5 Watt, 50 ohm load. The diode detectors are the silver things on top. They both are identical and house 1N23 point-contact diodes. Their voltage output is via integrated BNC connectors.



Close up view of the diode detectors. They are sometimes referred to as "crystal" detectors. 1N23-style diodes can be hard to find. Fair Radio does carry them. You can also find them used as mixer diodes in some 10 GHz gunnplexer-based automatic door openers.


These particular diode detectors generate a negative output voltage. Remember that when looking at the oscilloscope screen shots.

Source: Simple 2.4 GHz SWR Meter