A Wifi (V)SWR Meter to build yourself
The circuit below serves as a VSWR meter for the VHF-UHF-SHF frequency range. This includes the Wifi (802.11b/g), 2m, 70cm and 13cm Radio Amateur bands.
The maximum usable frequency is determined by the diode (1SS99): 3GHz
The device consists of two parts. One RF head and a (preferably analog) indicator This is the diagram for the RF Head. It is essential to use SMD components and the PCB design below.
Zx is the antenna port.
“RF in” is the Wifi Access Point (or other RF Generator) port.
The high frequency RF Wheatstone bridge design
100Ω – 4
10kΩ – 2
10pF – 2
1nF – 1
50Ω micro dummy (MDUMMY-EMC Super micro resistor 50 ohm 1% 15 GHz, Barend Hendrikse)
N-Type Bulkhead - 2
1SS99 Schottky diode - 1
Use 15x25mm of Epoxy single sided PCB material.
You may either use an etching process or a sharp hobby knife to cut out the copper pattern on the PCB. I always use the knife method, start with the 3mm center micro strip and work from there.
Mount the SMD Capacitors and resistors. Mount the diode last
Assemble the RF Head using the PCB assembly, two flat N-Type bulkhead connectors, four metal 15mm long studs with M3 thread and eight 7mm M3 machine screws.
Solder the PCB in place. Make sure that it is not under mechanical stress. Solder the ground surface to the bulkhead connector at 4 places, making as much contact as possible.
Attach the BNC with a piece of 1mm Aluminum sheet to the assembly. The BNC is used to output the pulsed DC signal to the indicator.
Wifi hardware transmits in short bursts. You need a “peak-hold” circuit to convert the DC pulses from the RF head into a stable DC voltage for the indicating instrument. The design shown here is based on a quad OPAMP, LM324.
OPAMP U1A creates a 'virtual ground' so that the OPAMP can be powered by a single 9 Volt battery.
The voltage from the RF head consists is a pulsed DC signal (so no 2.4GHz present). The pulse width varies between 0.2-0.5ms with a repetition frequency equal to that of the SSID (beacon) interval set in the AP. Voltage is a few volts max and depends on the amount of unbalance in the bridge.
Potentiometer R3 is used to SET the meter at full scale deflection with a fully unbalanced bridge (short circuit plug attached to the Zx port). The worst-case input VSWR of the bridge is 1:2, so you will NOT destroy your AP;-)
OPAMP U1B forms a peak-hold circuit wit the general purpose diode D1, C3 and R5. U1C and U1D buffer the output for the meter and a second (normally unused) output. Output to the meter is limited to +1.2/-0.6 Volt by means of three 1N4148 diodes
Manufacture a scale for the analog indicator as shown in the table below. You will be able to directly read the VSWR from the meter.
Indicator scale (P equals meter deflection, 1 = FSD):
Attach the RF head to the Access Point or transmitter (maximum approximately 200mW) using suitable cable. Set the AP to transmit a beacon at an interval of 100ms or less.
Attach a short circuit to the Zx port on the RF head. Adjust P3 to have full scale deflection on the meter.
Attach a suitable 50Ω dummy load to the Zx port of the RF head. VSWR should read close to 1:1.0.
Note: common VHF/UHF dummy loads do not perform well on this frequency!
Attach a known mismatch dummy load to the Zx port of the RF head. VSWR should be indicated correctly.
Determine antenna VSWR:
Attach the antenna under test to the Zx port of the RF head. Position the RF head as close as possible to the antenna under test, using as little coax cable as possible.
Read the VSWR from the indicator.