HF VHF RF Power/SWR meter with Oled Display

    Presenting here is a simple Arduino based homebrewed HF/VHF RF power & SWR meter. This compact and light weight equipment which is powered by recycled Nokia BL-5C batteries and entire unit is mounted inside a PVC junction box. A DC power socket is provided to recharge the battery using external power supply as we as an option to power the device using external power supply.

    Inspired from the power meter design by ON7EQ . Most of the features are adopted from the parent design as well as few are dropped. Main changes involve in modification of the code to accommodate OLED display instead of LCD display to make it more compact, light weight and low power equipment. Also here we are using two separate dedicated SWR bridges for VHF and HF Bands.

    Since SWR bridge voltage sensors are frequency sensitive it was required to accommodate separate SWR bridges for HF and VHF frequencies. Components are just soldered on a copper clad in dead bug style with both transformers made of 12 turns of 26 SWG as secondary and  secondary winding is just 1 turn or simple pass through of wire as primary turn on a FT 37-43 core. Refer below circuit diagram.

HF SWR Bridge Circuit

    

An assembled HF Bridge

    Talking about VHF SWR Bridge, a micro stripline type voltage sensors are used which are more efficient at VHF range as compared to transformer type sensors. The strips are designed on the copper clad itself and other components on the bridge are soldered on top. Refer circuit diagram below for more details.

    

VHF SWR Bridge

An assembled VHF Bridge

    Both the SWR Bridges are shielded using copper foil and aluminum Foil to prevent RFI to other part of the meter circuitry.  

    The unit is powered by two BL-5C battery with a 2S BMS module to generate 7.4 Volt which is further regulated to 5V by 1117- 5 V low power regulator. An external DC socket is also given to power the device externals as we as to recharge the battery.    

    On the front side we have a Power ON/OFF switch , a Band Switch to change the frequency band from HF/VHF RF power  measurements. A Mode switch is given to set various RF power measurements mode like PEP, dBm and SWR bip mode where speaker will produce tone w.r.t SWR.

Thus the meter can operate in 3 different modes with Vf and Vr voltage common in all display:

1. Measure and display instantaneous forwared power in Watt and dBm along with SWR
2. Measure and display forward PEP power (peak holding with decay)
3. Measure and display instantaneous forward power in Watt along with SWR, and emit 'beeps' corresponding to SWR value.

    An 0.96" OLED display which works on I2C is accommodated in front panel to display the measured values. The circuit is completely assembled on as common PCB and effort was taken to keep it compact. 

Front View

Back View

    TX line input and ANT line output BNC ports are provided on left and right sides of the unit to measure VHF frequency RF power and VSWR. On the Back side of the unit a separate TX and ANT BNC ports are given for dedicated HF RF power/SWR measurement. 

    The forward and reverse DC voltages from both SWR bridges are captured and given to ADC inputs of Atmega 328p as shown in the circuit. Serial interface headers along with preloaded Arduino bootloader are helpful during calibration and testing. Atmega 328 will perform following process when powered ON.

• Measure, calculate and display the maximum measurable RF power by the bridge based on initial calibration and ADC level (5V Max ~ 1022 limit).
• If input RF is within the maximum limit, calculate the power/SWR and display values on the screen.
• Identify the Mode and Band from last saved settings and perform calculations accordingly.
• Save the last settings of Mode and Band in EEPROM.  

Schematic Diagram of Meter

    The most important part is that the power meter needs to be calibrated with a known power source in both HF and VHF before considering actual measurements. I have managed to calibrate same with available power source. Still looking forward for a high power source to fine tune my calibration settings.

   

A completely assembled unit

Calibration Procedure:

• Identify the diode forward voltage from datasheet or by measuring the forward voltage wing DMM and update the values in sketch

            unsigned int Diode = 130;         // diode forward voltage, expressed in mV

• For each band we need to update the calibration factor PowCALXX   (XX= band), using the formula :

                        PowCal=(Vf²/Power) / 1000  (where power from a known source is expressed in Watt and Vf in milli volt)

                        Example : PowCal = 2500 x 2500 / 10 / 1000 = 625

                        Enter this value in the sketch, for the respective bands.

            #define PowCal2m     (625)    // 2m band

       I have taken Nissei RS -102 power meter as reference to check whether my power meter is giving correct value or not (Special Thanks to VU3YEJ  OM Kunal for his unconditional trust on me and lending me his power meter for a week 😇) . The meter readings were satisfactory in comparison with reference meter while checking with available power sources in HF and VHF. Initial calibration and testing were done on QRP level power sources due to limitation in finding high power sources, looking forward for QRO power measurements.

Code will be uploaded Soon.......

73's

VU3ZAG