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Raspberry Pi bareback LF/MF/HF/VHF WSPR transmitter <email@example.com> Makes a very simple WSPR beacon from your RasberryPi by connecting GPIO port to Antanna (and LPF), operates on LF, MF, HF and VHF bands from 0 to 250 MHz. Credits: Credits goes to Oliver Mattos and Oskar Weigl who implemented PiFM  based on the idea of exploiting RPi DPLL as FM transmitter. Dan MD1CLV combined this effort with WSPR encoding algorithm from F8CHK, resulting in WsprryPi a WSPR beacon for LF and MF bands. Guido PE1NNZ extended this effort with DMA based PWM modulation of fractional divider that was part of PiFM, allowing to operate the WSPR beacon also on HF and VHF bands. In addition time-synchronisation and double amount of power output was implemented.  PiFM code from http://www.icrobotics.co.uk/wiki/index.php/Turning_the_Raspberry_Pi_Into_an_FM_Transmitter To use: In order to transmit legally, a HAM Radio License is REQUIRED for running this experiment. The output is a square wave so a low pass filter is REQUIRED. Connect a low-pass filter (via decoupling C) to GPIO4 (GPCLK0) and Ground pin of your Raspberry Pi, connect an antenna to the LPF. The GPIO4 and GND pins are found on header P1 pin 7 and 9 respectively, the pin closest to P1 label is pin 1 and its 3rd and 4th neighbour is pin 7 and 9 respectively, see this link for pin layout: http://elinux.org/RPi_Low-level_peripherals Examples of low-pass filters can be found here: http://www.gqrp.com/harmonic_filters.pdf The expected power output is 10mW (+10dBm) in a 50 Ohm load. This looks neglible, but when connected to a simple dipole antenna this may result in reception reports ranging up to several thousands of kilometers. Example of low-pass filters here: http://www.gqrp.com/harmonic_filters.pdf As the Raspberry Pi does not attenuate ripple and noise components from the 5V USB power supply, it is RECOMMENDED to use a regulated supply that has sufficient ripple supression. Supply ripple might be seen as mixing products products centered around the transmit carrier typically at 100/120Hz. This software is using system time to determine the start of a WSPR transmissions, so keep the system time synchronised within 1sec precision, i.e. use NTP network time synchronisation or set time manually with date command. A WSPR broadcast starts on even minute and takes 2 minutes for WSPR-2 or starts at :00,:15,:30,:45 and takes 15 minutes for WSPR-15. It contains a callsign, 4-digit Maidenhead square locator and transmission power. Reception reports can be viewed on Weak Signal Propagation Reporter Network at: http://wsprnet.org/drupal/wsprnet/spots Frequency calibration is REQUIRED to ensure that the WSPR-2 transmission occurs within the 200 Hz narrow band. The reference crystal on your RPi might have an frequency error (which in addition is temp. dependent -1.3Hz/degC @10MHz). To calibrate, the frequency might be manually corrected on the command line or by changing the F_XTAL value in the code. A practical way to calibrate is to tune the transmitter on the same frequency of a medium wave AM broadcast station; keep tuning until zero beat (the constant audio tone disappears when the transmitter is exactly on the same frequency as the broadcast station), and determine the frequency difference with the broadcast station. This is the frequency error that can be applied for correction while tuning on a WSPR frequency. Do not overclock your RPi as it may make the clock unreliable due to a dynamic clocking feature. DO NOT expose GPIO4 to voltages or currents that are above the specified Absolute Maximum limits. GPIO4 outputs a digital clock in 3V3 logic, with a maximum current of 16mA. As there is no current protection available and a DC component of 1.6V, DO NOT short-circuit or place a resistive (dummy) load straight on the GPIO4 pin, as it may draw too much current. Instead, use a decoupling capacitor to remove DC component when connecting the output dummy loads, transformers, antennas, etc. DO NOT expose GPIO4 to electro- static voltages or voltages exceeding the 0 to 3.3V logic range; connecting an antenna directly to GPIO4 may damage your RPi due to transient voltages such as lightning or static buildup as well as RF from other transmitters operating into nearby antennas. Therefore it is RECOMMENDED to add some form of isolation, e.g. by using a RF transformer, a simple buffer/driver/PA stage, two schottky small signal diodes back to back. Installation / update: Open a terminal and execute the following commands: sudo apt-get install git rm -rf WsprryPi git clone https://github.com/threeme3/WsprryPi.git cd WsprryPi Usage: sudo ./wspr <[prefix]/callsign[/suffix]> <locator> <power in dBm> [<frequency in Hz> ...] e.g.: sudo ./wspr PA/K1JT JO21 10 7040074 0 0 10140174 0 0 where 0 frequency represents a interval for which TX is disabled, wspr-2 or wspr-15 mode selection based on specified frequency. WSPR is used on the following frequencies (local restriction may apply): LF 137400 - 137600 137600 - 137625 (WSPR-15) MF 475600 - 475800 475800 - 475825 (WSPR-15) 160m 1838000 - 1838200 1838200 - 1838225 (WSPR-15) 80m 3594000 - 3594200 60m 5288600 - 5288800 40m 7040000 - 7040200 30m 10140100 - 10140300 20m 14097000 - 14097200 17m 18106000 - 18106200 15m 21096000 - 21096200 12m 24926000 - 24926200 10m 28126000 - 28126200 6m 50294400 - 50294600 4m 70092400 - 70092600 2m 144490400 -144490600 Compile: sudo apt-get install gcc gcc -lm -std=c99 wspr.c -owspr Issues: Two users were reporting that the program never stops transmitting, even when intervals for disabled tx are programmed. The problem was in both cases fixed by flashing a new image on the SD card with a freshly downloaded image: 2013-02-09-wheezy-raspbian.zip. No apt-get upgrade or firmware upgrade was performed. After this WsprryPi TX was running successfully. One user reported his RPi died while in WsprryPi service caused by excessive RF voltage (90V) on GPIO4 created by a 100 watts AM transmitter 50ft away from the antenna. After the damage exessive current was consumed by RPi (1.1A from 5V supply), caused by short-circuiting in the 3.3V logic of the BCM2835 SOC. On his replacement RPi, he is planning to add galvanic isolation and buffering.