Now it was time to test the PI-Zero as a WSPR beacon. Yes, a Raspberry PI can act as a transmitter (almost) without any additional hardware except an antenna. The RF signal is generated directly from the systems clock oscillators and it is available at a GPIO pin. The main problem is that this output signal is a square wave. This means loads of harmonics and other unwanted signals that a good radio amateur does not want to transmit. It would not only be bad conduct but violate spectrum regulations in most countries. Usually low pass filters are used to reduce these spurious emissions. In this case the very narrow frequency response of a magnetic loop antenna helps to further reduce unwanted transmissions.
I am using a 7 pole Chebyshev low pass following the design used in Janny Lists "Australia Project". An additional 100nF capacitor between the filter and the loop is needed to avoid a DC short circuit that may destroy the Raspberry Pi.
The software on the Pi Zero that creates the time synchronized WSPR signal is called "Wsprry Pi" and can be found on Github.
For the experiments the Pi Zero was powered from a Power Bank. I connected to it via SSH over a WiFi connection. The antenna was sitting in my office on the ground floor.
Because the PiZero WSPR system does not receive I had to use my Icom IC-R10 scanner/receiver for tuning. For minimum sensitivity the R10s rubber duck antenna had been removed and the attenuator enabled. Because loop antennas have minimum field strength on the front/back axis it was not too difficult to find a spot where the scanner showed only a small S-Meter reading. Now it was possible to tune the loop for maximum output.
The result was astonishing: Spots from Finland all the way down to the Mediterranean with just 10mW and an indoor loop antenna.