WSPR-TX Pico Transmitter

WSPR-TX Pico is a well-designed tracker for use in long distance flights of stratospheric balloons:
– low power WSPR transmitter for amateur bands 20m and 30m,
– continuously transmits its position and height during the day,
– the built-in GPS module gives the exact location of the moving cargo,
– ARDUINO compatible microcontroller with open source software
– weight 10.5 grams (without HF antenna),

Parameters:
– transmission frequency: 10 and 14 MHz with a built-in low-pass filter,
– output power: just over 10 mWatt,
– size: 45 x 50 x 25 mm
– power consumption: 0.3W (5V 20mA idle, 60mA during transmission)
– power supply: via solar cells or USB converter to serial port,
– weight: 10.5 g with solar panels

Low output power allows direct operation from the included best-in-class lightweight polymer solar cells.
Up to 10 hours of work a day were observed on the piloted prototypes. Despite the low power, the transmissions will have a good geographical coverage, as the high antenna height compensates for the low power. Broadcasting on two HF bands can increase coverage as propagation sometimes differs between the 20 m and 30 m amateur bands.

Everything in the design has been optimized for weight – from the ultra-thin PCB to all components. Everything that is not needed in flight – like the USB interface – has been removed from the deck. Several generations of prototypes were tested and flown before the final version was adopted. This led to a transmitter with a total weight of just 10.5 grams. This low weight makes it a great proposition for the stratospheric balloon enthusiast. Its strengths are: lowest cost, lowest weight and the easiest to use commercial WSPR tracker.

The on-board tracker includes a stable TXCO to control frequency and further improve frequency accuracy. Each transmitter is individually measured and calibrated during construction. This in turn ensures that operation in a cold environment at high altitudes will have only a minimal effect on the transmission frequency and will ensure that it remains within the narrow 200 Hz WSPR bandwidth.

Transmission is possible on the 30m amateur band or the 20m band, or on both bands. Although the WSPR Pico is a low power transmitter, great care has still been taken to ensure that no unwanted signals are generated and uses a low pass filter at the antenna output for this purpose.

Software – the so-called firmware – is open source and based on Arduino, so the end user can modify them as needed. This software has been actively developed for two years and has proven to work well and to be stable. The same software is used in all ZachTek WSPR transmitter products.

Although the software is modifiable – the typical user does not want to. The author (Harry Zachrisson) had this in mind when developing this and his other WSPR products. They are all “Plug-and-Play” compliant. For example, no software skills are required and configuration is done in easy-to-use Windows software.

To get started, in addition to soldering solar cells, you just need to connect them to your Windows PC and set an amateur call sign. After clicking the “save” button, it is ready to transmit and will do so as soon as it gets the required power. It can be powered from a USB to serial adapter so that the user can carry out a test transmission if necessary before taking it outside and powering it through solar cells. As soon as it has power, and the GPS catches the so-called fixa – will start broadcasting.

An adapter and a USB to serial cable are required for setup, but they are sold separately.

The product ships with a board built, tested and calibrated with frequency.Two solar cells are included, but they are not soldered to the board.Documentation, schema and source code are available for download below:
– scheme
– instruction
– software

remember that to work on amateur bands you must have the so-called Radio license , popularly referred to as the ham radio license.
If you do not have one yourself, look for an amateurs club in the vicinity or invite someone who has permission to join their team.

It is worth buying an adapter USB / SERIAL, which is necessary to configure the sign, select the frequency, etc.