As an Amateur Extra I've used a Pi to transmit WSPR 24/7 and was once heard in New Zealand from Ohio. No amplifiers even...

Just to triple-confirm, there was no additional hardware besides the Pi? I'm assuming at least some sort of antenna was involved...

Any links to write-ups/HOWTO's would be tremendously appreciated!

You will need a antenna that is resonant on the frequency you're transmitting on for any hope of the signal leaving the immediate proximity of the Pi. It would also be a very good idea to filter the signal coming out of the Pi to reduce the strength of any frequency spurs accompanying your Pi generated signal and to keep your transmission legal.

This is a pretty good write up of using the Pi as a wspr transmitter: https://github.com/JamesP6000/WsprryPi

Yeah I have a low pass filter from QRP Labs ($8?) and an MFJ tuner as well as a very long wire. If I precisely measure the wire I could skip the tuner.

>but I would love to know the RF quality of the resulting waveform.

Technically it works, but it's very noisy and the transmission range isn't far enough for anything too serious.

A full duplex SDR would be a better investment.

On noise, this is what I suspect, too; thus I am not trading in a couple of HackRF One-s that I am currently playing with. Still would be useful to know the numbers.

On the range, I did not pay much attention (but maybe I should have): the power of HackRF one is low, too; one needs an amplifier to transmit in the open air anyway. Do you think that the rpitx TX power is not suitable for table-top sized prototypes?

There are some details here: http://pe1nnz.nl.eu.org/2013/05/direct-ssb-generation-on-pll...

It says a dynamic range of only 13 dB on the levels.

The one time I tried it without any filter or anything and viewed the signal on my RTL-SDR the quality was horrible. Spurs and distortion and strong harmonics.