SWR digital meter 100mW - 10W

Finally I have made and got working my SWR meter. Wow, aren't there so many designs out there on the web! Parts of the existing offerings I liked were the use of a Bi-core ferrite transformer, I used a BN43-2402 with a turns ratio of 1:10. I have used a couple of the wonderful log detection AD8307 for the Forward and Reflected dBm detection. I worked out that 10W = 22V input, and with the detectors AD8307 maximum input of about 2.5V, I needed the 1:10 ratio.

The output of the AD8307s is fed to the analog A0 & A1 inputs of an Arduino Nano microcomputer. I used an OLED display I have worked with many times, and show just the Forward Power as a bar graph and the VSWR.



The circuit board showing the Transformer, two AD8307s and the Nano



The board and the OLED display



My VFO which outputs 1-60MHz at -10 to +10dBm

The loop antenna I used for testing - the Wonder Wand - The tuning on this is VERY sharp to get a low VSWR, I tested it here on 40m.



I have yet to calibrate the SWR meter, which can be done in software, but it is giving somewhat sensible readings.

Code - Updated V2

This uses a header file I wrote myself which defines a number of displays, for example the Bar graph and the Ultra-large numbers. This header (listed at the bottom below) calls up the open source U8G2 library.

// SWR_2 V2.0
// 18-4-12 Calibrate mWFwd with Power Meter, re-write bar length calculation.

//HEADERS & LIBRARIES
#include "Oled.h"

// CONNECTIONS
// Fwd & Ref analog in, transformer ratio
#define FWDIN A0
#define REFIN A1
#define TRANSRATIO 10

// PARAMETERS
// note AREF is nominally +3300mV. RF in 22V=10W, 10:1 transformer, AD8307 in 2.2V=20dB
#define AREF 3204
#define AMAX 1023

// intercept (dBm), slope (mW/dB), bridge impedance,
// bar display scale, transformer ratio (dB)
#define INTERCEPT_FWD -87.0
#define INTERCEPT_REF -88.0
#define SLOPE_FWD 25.0
#define SLOPE_REF 25.0
#define IMP 51.0
#define BARSCALE 100/20
#define TRATIO 20.0

// GLOBAL VARIABLES
// for display function
double bLFwd;
double vswr;

void setup() {
  analogReference(EXTERNAL);
  // oled init, sets I2C addr to 0x3C
  oled.begin();
}

void loop() {
  int inFwd, inRef;
  double mVFwd, mVRef;
  double VrmsFwd, VrmsRef, dBmFwd, dBmRef, mWFwd, mWRef, vRatio;

  // read inputs FWD & REF
  // is about 1.5-2.5V, calibrate with INTERCEPT value
  inFwd = analogRead(FWDIN);
  inRef = analogRead(REFIN);

  mVFwd = (double)(map(inFwd, 0, AMAX, 0, AREF));
  mVRef = (double)(map(inRef, 0, AMAX, 0, AREF));
  
  // calculations for display
  dBmFwd = (mVFwd / SLOPE_FWD) + INTERCEPT_FWD + TRATIO; // in doubles
  mWFwd = pow(10.0, (dBmFwd / 10.0)); // in double, out double, 0dBm = 1mW
  mVFwd = sqrt((mWFwd * 1000.0) * IMP); // in double, out double

  dBmRef = (mVRef / SLOPE_REF) + INTERCEPT_REF + TRATIO; // in doubles
  mWRef = pow(10.0, (dBmRef / 10.0)); // in double, out double, 0dBm = 1mW
  mVRef = sqrt((mWRef * 1000.0) * IMP); // in double, out double

  // calculatio for bar graph, forward power
  bLFwd = (dBmFwd * BARSCALE) - 100; // tbd 20-30-40dB, 0.1W-1W-10W

  // calculation SWR
  vRatio = mVRef / mVFwd;

  vswr = ((1 + vRatio) / (1 - vRatio));

  delay(200); // helps to slow display flicker

  dispUpdate();
}

// PICTURE LOOP
void dispUpdate() {
  oled.firstPage();
  do {
    dispMsg(50, 0, "SWR_2"); // prog name
    
    dispMsgS(0, 28, "PWR");  // bar title
    dispMsgS(20, 15, "100mW     1W      10W"); // bar scale

    if (bLFwd < 5 || bLFwd > 95) {
      dispBar(20, 28, 5, 0);        // blank bar
      dispMsgL(30, 38, "No Input"); // no input
    }

    else if (vswr > 10) {
      dispBar(20, 28, 5, 0);        // blank bar
      dispMsgL(30, 38, "SWR > 10"); // too big
    }

    else {
      dispBar(20, 28, 5, bLFwd);        // bar value
      dispNumUL(40, 38, abs(vswr), 2);  // always display as positive
    }
  } while (oled.nextPage());
}

Header

// Oled.h
// V2.0 18-4-5 added UL numbers
// defines oled pins, creates "oled" object for 128x64 SH1106 display
// fonts github.com/olikraus/u8g2/wiki

// HEADERS & LIBRARIES
#include "U8g2lib.h"
#include "Wire.h"

// OBJECTS
// oled object, SH1106 controller, 128X64, HW I2C and normal orientation R0
U8G2_SH1106_128X64_NONAME_1_HW_I2C oled(U8G2_R0);

//FUNCTIONS

// BAR
// display bar at x, y, h)eight, l)ength (0-128 pixels)
void dispBar(u8g2_uint_t x, u8g2_uint_t y, byte h, byte l) {
  byte n;

  oled.drawFrame(x, y, 100, h+1);
  for ( n = 0; n < l; n++) {
    oled.drawLine(x + n, y, x  + n, y + h);
  }
}

// FREQ
// display freq at x, y, f (Hz) plus cf (cHz), d)ecimal places (max 3)
void dispFreq(u8g2_uint_t x, u8g2_uint_t y, double f, double cf, byte d) {
  oled.setFont(u8g2_font_10x20_tf); // font
  oled.setFontPosTop(); // origin top

  f = f / 1000.0;
  cf = cf / 100000.0;

  oled.setCursor(x, y);
  oled.print(f + cf, d);
  oled.print("kHz");
}

//MSG
// display small message at at x), y), *m)essage
void dispMsgS(u8g2_uint_t x, u8g2_uint_t y, char *m) {
  // sets font, cursor position and displays message
  oled.setFont(u8g2_font_5x8_tf); // font
  oled.setFontPosTop();
  oled.setCursor(x, y);
  oled.print(m);
}

// display message at at x), y), *m)essage
void dispMsg(u8g2_uint_t x, u8g2_uint_t y, char *m) {
  // sets font, cursor position and displays message
  oled.setFont(u8g2_font_7x13_tf); // font
  oled.setFontPosTop();
  oled.setCursor(x, y);
  oled.print(m);
}

// display large message at at x), y), *m)essage
void dispMsgL(u8g2_uint_t x, u8g2_uint_t y, char *m) {
  oled.setFont(u8g2_font_10x20_tf); // font
  oled.setFontPosTop();
  oled.setCursor(x, y);
  oled.print(m);
}

// display ultra large message at at x), y), *m)essage
void dispMsgUL(u8g2_uint_t x, u8g2_uint_t y, char *m) {
  oled.setFont(u8g2_font_logisoso30_tf); // font
  oled.setFontPosTop();
  oled.setCursor(x, y);
  oled.print(m);
}

// NUM
// display number at x), y), n)umber (double), d)ecimal places
void dispNum(u8g2_uint_t x, u8g2_uint_t y, double n, byte d) {
  oled.setFont(u8g_font_7x14); // fix font for now
  oled.setFontPosTop();
  oled.setCursor(x, y);
  oled.print(n, d);
}

// display number large at x), y), n)umber (double), d)ecimal places
void dispNumL(u8g2_uint_t x, u8g2_uint_t y, double n, byte d) {
  oled.setFont(u8g2_font_10x20_tf); // font
  oled.setFontPosTop();
  oled.setCursor(x, y);
  oled.print(n, d);
}

// display number ultra large at x), y), n)umber (double), d)ecimal places
void dispNumUL(u8g2_uint_t x, u8g2_uint_t y, double n, byte d) {
  oled.setFont(u8g2_font_logisoso24_tf); // font
  oled.setFontPosTop();
  oled.setCursor(x, y);
  oled.print(n, d);
}

// DATE
// display date
void dispDate(u8g2_uint_t x, u8g2_uint_t y, byte dw, byte da, byte mo, byte yr) {
  oled.setFont(u8g_font_7x14); // fix font for now
  oled.setFontPosTop();
  oled.setCursor(x, y);
  
  switch (dw) {
    case 1:
      oled.print("Mon");
      break;
    case 2:
      oled.print("Tue");
      break;
    case 3:
      oled.print("Wed");
      break;
    case 4:
      oled.print("Thu");
      break;
    case 5:
      oled.print("Fri");
      break;
    case 6:
      oled.print("Sat");
      break;
    case 7:
      oled.print("Sun");
      break;
  }

  oled.print(" ");
  oled.print(da);

  oled.print(" ");
  switch (mo)
  {
    case 1:
      oled.print("Jan");
      break;
    case 2:
      oled.print("Feb");
      break;
    case 3:
      oled.print("Mar");
      break;
    case 4:
      oled.print("Apr");
      break;
    case 5:
      oled.print("May");
      break;
    case 6:
      oled.print("Jun");
      break;
    case 7:
      oled.print("Jul");
      break;
    case 8:
      oled.print("Aug");
      break;
    case 9:
      oled.print("Sep");
      break;
    case 10:
      oled.print("Oct");
      break;
    case 11:
      oled.print("Nov");
      break;
    case 12:
      oled.print("Dec");
      break;
  }
  oled.print(" ");
  oled.print("20");
  oled.print(yr);
}

// TIME
// display time HH:MM:SS at x), y)
void dispTime(u8g2_uint_t x, u8g2_uint_t y, byte h, byte m, byte s) {
  oled.setFont(u8g_font_7x14); // fix font for now
  oled.setFontPosTop();
  oled.setCursor(x, y);
  if (h < 10)
    oled.print("0");
  oled.print(h);
  oled.print(":");
  if (m < 10)
    oled.print("0");
  oled.print(m);
  oled.print(":");
  if (s < 10)
    oled.print("0");
  oled.print(s);
}

// display time HH:MM:SS at x), y)
void dispTimeL(u8g2_uint_t x, u8g2_uint_t y, byte h, byte m, byte s) {
  oled.setFont(u8g2_font_10x20_tf); // font
  oled.setFontPosTop();
  oled.setCursor(x, y);
  if (h < 10)
    oled.print("0");
  oled.print(h);
  oled.print(":");
  if (m < 10)
    oled.print("0");
  oled.print(m);
  oled.print(":");
  if (s < 10)
    oled.print("0");
  oled.print(s);
}

// STEP
// display step at x) y) s)tep
void dispStep(u8g2_uint_t x, u8g2_uint_t y, unsigned int s) {
  oled.setFont(u8g_font_7x14); // fix font for now
  oled.setFontPosTop();
  
  oled.setCursor(x, y);
  switch (s) // display freqStep
  {
    case 10:
      oled.print(" 10Hz");
      break;
    case 100:
      oled.print("100Hz");
      break;
    case 1000:
      oled.print(" 1kHz");
      break;
    case 10000:
      oled.print(" 10kHz");
      break;
    case 100000:
      oled.print("100kHz");
      break;
    case 1000000:
      oled.print(" 1MHz");
      break;
  }
}