There are several ways of transmitting a WSPR signal that don't require a computer and I've tinkered with most of them. Each has its own advantages and disadvantages:
If you have no interest in discovering elusive openings on the higher bands, it seems ideal and is often used in conjunction with SOTA-Beams DXplorer website that plots the stations that receive you on a time vs. distance graph. My own experience with testing reception reports and this website lead me to question the resolution of points needed in order to be plotted. You can read more about that here.
I've detailed the steps necessary to converting a $30 R-Pi to a 20m WSPR transmitter here. Set up properly, the RPi generates 100mW on 20 meters. It requires the TAPR shield which is both a LP filter and an RF amplifier that plugs into the GPIO pins on the RPi. Time synchronization is maintained via your router and NTP.
So far there are no TAPR shields available for other bands.
- Time sync maintained via GPS with optional module
- Operation on any band from 2m to 2200m
- Up to 6 bands, one after the other, may be used automatically in sequence
- Can be configured to transmit from once to 30 times per hour
- Other modes available as well
- Optional 5 watt amp available
The U3S produces 200mW out and can be purchased with or w/o a case and with as few or as many LP filters as needed. It operates from 5V.
My own experiences with building and using this transmitter can be found here.