SWR_PWR_meter

SWR and power meter with Wi-Fi interface based on ESP8266 (NodeMCU 0.9).

More information (in Russian): https://ra0sms.com/swr_power_meter/

Features

• Measures forward power (PEP) and SWR in real time
• Web interface with smooth power bar, peak marker and SWR display
• Captive portal for easy first-time Wi-Fi setup
• Saves Wi-Fi credentials to EEPROM (survives reboot)
• Automatic fallback to SoftAP mode when the network is lost
• Automatic reconnect to the saved network
• Configurable power range: 200 / 300 / 1000 / 2000 W
• Peak-hold (PEP) sampling on the device, display ballistics with SSB / CW / FT8 mode selector
• Optional HTTP Basic Auth for all web pages

Specifications

Parameter	Value
Power supply	microUSB 5V 1A
Full-scale power	1 – 1000 W (configurable up to 2000 W)
Frequency range	1 – 30 MHz
Measurement	PEP (Peak Envelope Power)
Wi-Fi range	up to 50 m (entry-level router)
Housing	Gainta G0479
Board	NodeMCU 0.9 (ESP8266)

Hardware

Make sure you use the ESP8266 board v 0.9. Check the pinout!

Pin usage

Pin	NodeMCU	Function
A0	A0	ADC input (forward/reflected detector)
switch_pin (5)	D1	Analog switch control (forward/reflected)
led_pin (16)	D0	Status LED (on = not connected, off = connected)

Gerber files for the PCB: SWR_gerber.zip

Building and flashing

1. Install Arduino IDE (or use arduino-cli).
2. Add the ESP8266 board manager URL: http://arduino.esp8266.com/stable/package_esp8266com_index.json
3. Install the esp8266 core (tested with 3.1.2).
4. Select board NodeMCU 0.9 (ESP8266-12E).
5. Open src/swr_source/swr_source.ino and upload.

Required libraries (installed with the core by default):

• ESP8266WiFi
• ESP8266WebServer
• DNSServer
• EEPROM

Ready firmware

Pre-built binaries for NodeMCU 0.9 (ESP8266-12E) (core 3.1.2):

File	MAX_POWER	Authentication	Size
firmware/swr_source_auth.bin	1000 W	Enabled (admin / swr12345)	330 KB
firmware/swr_source_noauth.bin	1000 W	Disabled	326 KB
firmware/swr_source_noauth_2000W.bin	2000 W	Disabled	326 KB

Flash at address 0x00000 using esptool, Arduino IDE or NodeMCU PyFlasher.

Flashing without Arduino IDE

Download the .bin file you need from the table above, connect the NodeMCU board via USB and follow the steps for your OS.

Windows (NodeMCU PyFlasher)

1. Install the USB-serial driver if Windows does not recognize the board: CH340 or CP2102, depending on the chip on your board (it is printed next to the USB connector).
2. Download NodeMCU-PyFlasher-<version>-x64.exe from NodeMCU PyFlasher releases. It is a single portable program, no installation needed.
3. Run it and set:
   • Serial port — the COM port of the board (click the refresh button if the list is empty; the port number is also shown in Device Manager → Ports (COM & LPT));
   • NodeMCU firmware — the downloaded .bin file;
   • Baud rate — 115200;
   • Flash mode — Dual I/O (DIO);
   • Erase flash — yes for the first flash, no for an update (keeps saved Wi-Fi settings).
4. Click Flash NodeMCU and wait for Firmware successfully flashed in the console.
5. Press the RST button on the board (or replug USB).

Linux (esptool)

1. Install esptool:

   pip install --user esptool      # or: sudo dnf install esptool / sudo apt install esptool

2. Allow access to the serial port (once, then log out and back in):

   sudo usermod -aG dialout $USER

3. Find the board port — usually /dev/ttyUSB0:

   ls /dev/ttyUSB*

4. Flash the firmware:

   python3 -m esptool --port /dev/ttyUSB0 --baud 115200 write_flash -fm dio 0x00000 swr_source_noauth.bin

   For the first flash you can clear the whole flash beforehand (this also erases saved Wi-Fi settings):

   python3 -m esptool --port /dev/ttyUSB0 erase_flash

5. Press the RST button on the board (or replug USB).

Configuration

Power range

Set MAX_POWER in swr_source.ino to one of: 200, 300, 1000, 2000 (other values fail to compile).

Calibration

The power formula is P = (U + DIODE_OFFSET)² / K (U in ADC counts), where K depends on the power range:

MAX_POWER	K
200 W	5230
300 W	3487
1000 W	1046
2000 W	523

Adjust POWER_K at the top of the sketch to match your hardware (detector sensitivity, ADC divider).

DIODE_OFFSET compensates the detector diode drop (1N5711 ≈ 0.2–0.3 V ≈ 60–90 ADC counts), which makes the pure U²/K formula read low at small power. The default 0 keeps the classic calibration; if you set it, recalibrate POWER_K at the top of the range.

Other measurement constants:

Constant	Default	Meaning
FWD_MIN	50	Forward level (ADC counts) below which power reads 0
SWR_MIN_FWD	100	SWR is calculated only above this forward level
PEAK_BIN_MS	100	Peak-hold window; the reading is the maximum over the last 100–200 ms

SoftAP credentials

The access point SSID is built automatically from the ESP8266 chip ID: SWRmeter_XXXXXX, where XXXXXX is the chip ID in hex (the last 3 bytes of the MAC address). Every device gets a unique name from the same firmware, so nothing needs to be edited per device. The SSID is also printed to the serial port at boot (115200 baud).

The prefix and the password are defined at the top of the sketch:

const char *softAP_prefix = "SWRmeter_";
const char *softAP_password = "1234567890";

Change them before flashing if needed.

Authentication

All web pages can be protected with HTTP Basic Auth. Credentials and the enable flag are defined at the top of the sketch:

const bool AUTH_ENABLED = false;  // Set to true to enable authentication
const char *auth_user = "admin";
const char *auth_pass = "swr12345";

• Set AUTH_ENABLED = true to protect all web pages with a login/password prompt.
• Change auth_user / auth_pass before flashing if needed.
• The browser asks for credentials once; subsequent /xml and /mode requests are authenticated automatically.

How the power reading works

The detector follows the RF envelope (time constant ~1 ms), so in SSB/CW the voltage changes with every syllable and dot. The device samples forward/reflected voltage in close pairs (~0.2 ms apart) several hundred times per second and keeps the maximum over a 100–200 ms window (PEP). SWR is taken from the pair measured at that peak, so forward and reflected values belong to the same moment.

Display ballistics (SSB / CW / FT8)

The /swr page has a SSB / CW / FT8 mode selector. The browser renders at the display frame rate (independent of network timing) with fast attack, hold and slow release, like a peak-reading meter. When the carrier disappears, the reading falls to zero quickly (drop time constant 0.08 s) instead of creeping down along the release curve. Settings are in the MODES table of the page script:

Mode	Attack	Hold	Release	Use case
SSB	0.03 s	700 ms	0.6 s	Readable PEP through speech
CW	0.03 s	300 ms	0.3 s	Bridges gaps between dots/dashes
FT8	0.05 s	0	0.3 s	Constant carrier: fast rise, light smoothing

Also:

• The white marker on the bar shows the peak of the last 2 s and goes to zero together with the reading after TX ends.
• Numbers update 5 times per second; above 100 W the value is rounded to 5 W (above 1000 W — to 10 W).
• SWR keeps its last value for 3 s during pauses instead of jumping to ---.
• If the device stops answering for 2 s, the values dim and the power falls to 0.

The selected mode is kept in RAM only and resets to SSB after reboot (no flash wear).

First-time setup (Wi-Fi)

1. Power the device via microUSB (5V 1A). Your router must have DHCP enabled.
2. On your phone/computer, connect to the Wi-Fi network SWRmeter_XXXXXX (unique per device, password 1234567890).
3. Open http://192.168.4.1/wifi in a browser.
4. Enter your network name and password, click Connect.
5. The device connects to your network, the SoftAP turns off automatically, and the device gets an IP from your router. Use that IP to access the web interface.

If the connection is lost later, the device automatically re-enables the SoftAP and keeps retrying the saved network every 30 seconds. Automatic retries are non-blocking, so the web server and captive portal stay responsive. Only the first connect (at boot or after saving credentials) waits up to 10 s for the result.

Web interface

URL	Description
/	Start page with connection info
/wifi	Wi-Fi configuration (network scan, connect form)
/swr	SWR / power meter page (with SSB/CW/FT8 mode selector)
/data	JSON data endpoint {"p":123,"s":"1.35"} — used by the meter page
/xml	Legacy XML endpoint (<pwr>, <swr>, <adcpower>) for external integrations
/mode?m=ssb|cw|ft8	Set display smoothing mode (used by the mode selector)

All pages require authentication (if AUTH_ENABLED = true). The /data and /xml endpoints can be used to integrate readings into other systems (e.g., home automation). Both return the peak (PEP) of the last 100–200 ms; s / <swr> is empty when the power is too low for a meaningful SWR.

Changelog

• 2019 — First version
• 2021-03-12 — Added selectable max power (2000/1000/200 W) via #define
• 2025-05-22 — New CSS-based web interface
• 2026-06-24 — Code cleanup: removed unused vars/includes, constexpr, millis() timer
• 2026-10-13 — Reliability & security improvements:
  • Reconnect on connection loss (WL_CONNECTION_LOST / WL_DISCONNECTED)
  • no-cache headers for /xml and /swr
  • SWR clamped at low power, fixed unsigned underflow in SWR calculation
  • Removed dead code, merged flagOn/isWiFiConnected
  • Larger SSID/password buffers (32/63 chars), input validation with error display
  • Compact dark redesign of the /swr page
  • SSB/CW/FT8 display smoothing mode selector (RAM only, no flash wear)
  • HTTP Basic Auth for all web pages (AUTH_ENABLED flag, configurable login/password)
• 2026-10-14 — Wi-Fi fixes:
  • Restored blocking waitForConnectResult() in connectWifi() — fixes first-connect failure (WL_WRONG_PASSWORD) after entering credentials
  • Retry interval 30 s, no retries on wrong password
• 2026-10-15 — Smooth power display in SSB/CW and other improvements:
  • Peak-hold (PEP) sampling on the device: maximum over a 100–200 ms window instead of the last sample
  • Forward/reflected measured in close pairs, SWR taken at the envelope peak
  • Browser ballistics with attack/hold/release per mode, rendering via requestAnimationFrame, peak marker on the bar
  • Sequential polling of the new /data JSON endpoint (no overlapping requests), stale-data indication
  • Wi-Fi modem sleep disabled (removes 100–300 ms random response latency)
  • Non-blocking automatic reconnect retries; millis() overflow-safe timers
  • HTML escaping of SSIDs (XSS from a malicious nearby access point)
  • Pages streamed from PROGMEM in chunks instead of large Strings
  • DIODE_OFFSET calibration constant, static_assert for MAX_POWER
  • Pre-built 2000 W firmware
• 2026-10-16 — SoftAP SSID built from the chip ID (SWRmeter_XXXXXX), no per-device edits needed
• 2026-10-16 — Faster display response:
  • Power reading falls to zero quickly after TX ends in all modes (fast drop + snap to 0), peak marker follows
  • Shorter hold/release in SSB and CW
  • Fast rise in FT8 mode (attack 0.3 s → 0.05 s)
  • Rebuilt pre-built firmware

Note: the buffer size change shifts the EEPROM layout — saved Wi-Fi credentials are cleared once after this update. Re-enter them via the web interface.

License

See LICENSE.