Building a Homemade FM Repeater with a Raspberry Pi, Rpitx and RTL-SDR Dongle
A radio repeater is usually a radio tower that receives weak signals from handheld, desktop or other radio, and rebroadcasts the same signal at a higher power over a wide area at a different frequency. This allows communications to be extended over a much greater area.
Repeaters are generally made from expensive professional grade radio equipment, however ZR6AIC has been experimenting with creating an ultra low cost repeater out of a RTL-SDR and Raspberry Pi. In his system the RTL-SDR dongle is set up to receive a signal on the 70 cm (420 – 450 MHz) amateur radio band, and then retransmit it using Rpitx on the 2M (144 – 148 MHz) amateur radio band. He also adds a 2M low pass filter to the output of the Raspberry Pi to keep the signal clean.
Rpitx is software for the Raspberry Pi which we have featured on this blog several times in the past. We’ve also seen the qtcsdr software which also uses Rpitx and an RTL-SDR to create a transceiver. Rpitx allows the Raspberry Pi to transmit radio signals without the need for any transmitting radio hardware at all. It works by modulating signals onto a General Purpose I/O (GPIO) pin on the Raspberry Pi. If the GPIO pin is modulated in just the right way, FM/AM/SSB or other signal modulation approximations can be created at a specified frequency. The signal is however not clean, as this type of modulation generates many harmonics which could be dangerous if amplified. If you use Rpitx, always use appropriate filtering hardware.
ZR6AIC’s post goes into detail about how to install and set up the required software onto the Raspberry Pi and how to set up the script to piece all the programs together into a repeater. He’s also uploaded a video demonstrating the system in action on YouTube.
I know you can make a CB repeater with a remote control.
check out hamvoip.org…..
Proof of concept only. Works in theory only. Not a practical solution. Too many issues to solve before it becomes a useful device. What about all the spurious emissions transmitting a square wave?
For fun and play ok, but… FM repeater needs high dynamics RX (not only weak signal is present), selectivity for near stronger signals, and RTL-SDR don’t have this parameters. In the same price level I can found Chinese radio with cross-band repeating, better RX, higher output power and cleaner output signal (but still with bad parameters in compare with “normal” RPT). Sorry for my EN.
I report two repeaters FM and AM easy to use, construction, availability to resend digital messages and decode them with SDR,
Radio repeater low cost with FM transmitter receive with RTL SDR:
Radio repeater low cost with homemade AM transmitter receive with RTL SDR: