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WSPR-TX Mini Documentation page


LP filter build instruction

PC Configuration Software

LP1 Icon.png

WSPR-TX Mini Transmitter is a standalone WSPR transmitter with a built in GPS module and GPS antenna.
It has the the following

  1. Operation with or without a PC. (PC required for initial setup).

  2. Works on any Ham bands from 136kHz to 70MHz

  3. Powered from USB or from a single LiPo battery.

  4. A built in GPS that is used for timing and position calculations.

  5. An Arduino compatible micro-controller runs open source WSPR Beacon software.

  6. Designed to be easy to use and get started with but software can be expanded or changed.

  7. Can be expanded with product #1023 1S 600mA LiPo battery and charging board

  8. Is designed with battery operation in mind and is using several power saving techniques to conserve power and operate for extended periods of time from a single LiPo battery.


  • Frequency:
    136kHz to 70MHz, built for one band as set by on-board low pass filter.

  • Power output:
    10 to 20mWatts in 50ohm.

  • Size:

  • Power usage:
    0.3W (5V 20mA at idle, 60mA at transmit)

  • Weight:
    18g (44g with LiPo add-on)

Theory of operation.

The WSPR-TX Mini transmitter is low power transmitter with an open source Arduino firmware that encodes WSPR packets and transmits them using a Silicon Labs Si5351 PLL.
The output from the PLL is a square wave that is low pass filtered for a single band before going to the antenna jack.

The user builds the on-board low pass filter for a single band of their choice or optionally does the low pass filtering externally.
The RF oscillator is a Si5351 PLL using a 10ppm 25-27MHz crystal as a reference.
The absolute value of the reference crystal is stored in EEPROM at factory setup.

The building blocks of the transmitter.

Parts explained.png


Setting the configuration for the Transmitter.

  • Place the transmitter so that is has visibility of the sky.

  • Connect an antenna to the RF output SMA connector.

  • Connect the USB port to PC computer using a USB A to USB micro cable (not included, purchased separately)

  • Download and install the Configuration software from GitHub :

  • Start the PC configuration and choose a Serial port from the drop down list.

Pick serial.png
  • Click the “Connect” button. If the serial port belongs to a WSPR Transmitter then the software will pull data from it and display in the Software.
    If you have several serial ports in your PC, you may have to try to connect to each of them in order until you find the right one.

  • Set the configuration you want and click the save button.

  • To start the WSPR beacon click the “Start” button in the WSPR Configuration section as shown in the picture.

  • There is also a Signal Generator mode so the WSPR Desktop Transmitter can be used as a piece of test equipment in your shack. It can output a 13dBm square wave from 2kHz to 150MHz. 

  • There is a highest frequency that can be used as set by the low pass filter, if no low pass filter is fitted outputs up to 150MHz can be generated.

  • To stop the WSPR Beacon or the Signal generator click the “stop” button.

  • Save the configuration and operate the transmitter either connected to the computer or standalone with a USB power adapter.

Running the Transmitter standalone.

  • The WSPR Transmitter will either start in WSPR Beacon mode, in Signal generator mode or in idle mode depending on what the start mode is set to in the configuration program, se picture.


​If it is set to WSPR Beacon, it will continuously transmit as set in the configuration.
The Yellow status Led will indicate what it is doing at the moment.

Status information from the LEDs
Yellow LED Off= Waiting/Idling
Yellow LED Single blinking=Acquiring position from the GPS Satellites.
Yellow LED Double blinking=Waiting for top of even minute.
Yellow LED Continuously lit=Transmitting.

Status information in the software.

The status can also be view in the PC software.


The hardware can be divided in to the following blocks:

  1. Powersupply

  2. Micro controller

  3. GPS

  4. RF Oscillator

  5. Low pass filters


All electronics on the mini transmitter is running from a single 3.3V power regulator. The master power is either from the 5V USB connector or from a 3.7V LiPo battery.

The Add-on product #1023 1S LiPo can be attached to the WSPR-TX Mini on the bottom side.

When the 1S LiPo is fitted it has a 600mA battery that will power the transmitter. The USB connector will in this case charge the LiPo battery instead of powering the transmitter.
The LiPo battery will power the WSPR Mini and a power switch that sits on the 1S LiPo board will determine if the transmitter is turned on or not.
The 1S LiPo board has two optional DC inputs for solar or other other charge methods and can accept up to 35V of DC voltage.
Se product number #1023 1S LiPo charger for more information

The transmitter is designed with low power consumption in mind.
It can reduce its power consumption by means of three power saving features that goes in to effect in WSPR Beacon mode:

  1. GPS will be put in power saving modes when the transmission pause is more than a minute. This reduces power consumption with 30 to 40mA.

  2. The power to the Si5351 is turned of during TX pause, this saves about 10 to 15mA.

  3. The microcontroller is put to sleep when the TX pause is a minute or more.
    During sleep it will wakes up every eight second and blinks the Status LED briefly before going to sleep again.
    The microcontroller will come out of sleep 30 seconds before the next transmission block is about to start.


The product #1023 1S LiPo to the left and the #1017 WSPR-TX Mini to the right

1017 next to LiPo sideview.png

WSPR-TX Mini mated to the LiPo battery board. The WSPR-TX  Mini is fully autonomous in this configuration.
Charging can be done from the USB port or using up to two optional DC inputs on the 1S LiPo board. The unit can operate standalone for indefinite periods of time if a charging source like a solar cell is connected to the DC input.

1017 with LiPo sideview.png

Micro controller

The micro controller is a ATMega328P with a Arduino Pro Mini 8MHz 3.3V compatible boot-loader that can be reprogramed from the USB port using the Arduino IDE. It can be accessed and programmed using the USB port and the Arduino IDE.
The micro controller runs the software (sometimes called firmware in the text) that handles all the hardware and encodes the WSPR packets.
The connections to it can be best viewed in the schematics and the software can be modified or replaced by the user.
It comes pre-programmed with a firmware that is open source and can be downloaded from the Github link at the top of the page.



The GPS is on-board and sits below the GPS antenna. When not in use for a few minutes it will be powered down by the software to conserve power.


RF Oscillator

The RF oscillator is a Silicon Labs Si5351 that is using a crystal as a frequency reference for the Si5351 PLL.
I have used both 25, 26 and 27MHz crystals for supply reasons. The frequency is stamped on the component or alternatively can be measured with a frequency counter.
The crystal has 10ppm stability which is sufficient as long as the temperature fluctuations are not to extreme.  The crystal frequency is measured and saved in EEPROM as part of the factory configuration at the time it is built. The firmware use this information for the internal the PLL calculations and this in turn improves the correctness of the output frequency that the Si5351 PLL outputs as part of the WSPR routine.
The user can re-calibrate and save new calibration data if deemed necessary due to crystal aging etc.

When the WSPR beacon software is running the transmitter will automatically pick a random transmit frequency within the 200 Hertz WSPR block and use that frequency for the duration of the 2 minute WSPR block. Subsequent transmission will use another random picked frequency. This avoids ending up on top of another WSPR user and casing interference for extended periods of time.

If the Si5351 PL is not in use for a few minutes it will be powered down by the software to conserve power.

The RF output from the Si5351 goes through a low pass filter (if fitted - at delivery there is no low pass filter, a link wire is soldered in instead, the user will build and mount a low pass filter of his/her choice) to the SMA connector that is is the antenna output, marked "RF" on the circuit board.

The output power is around 12dBm (10-20mW).


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