The amateur radio repeater was a complex set of equipment that could easily fill a 19 ″ trunk. There will be a receiver and a separate transmitter, usually redesigned by commercial units, a home-made logic unit with a microprocessor to monitor everything, and a huge set of filters to stop the transmitter output from clogging the receiver. There would then be a set of power supplies to ensure uninterrupted operation during a power outage, possibly with a connected lead-acid cell bank.
Newer repeaters were commercial repeaters. The big radio manufacturers have noticed an amateur radio market, and especially since each of them is pursuing their own digital standards, there has been something like an effort to provide repeater equipment to boost sales of digital transceivers.
But what if you want to set up a simple repeater and you don’t have a shed full of old radios or a hotline for the sales department of a major Japanese manufacturer? If you are [Anton Janovsky, ZR6AIC], You make your own low power repeater using RTL-SDR, low pass filter and Raspberry Pi.
[Anton]The repeater ‘s is an intelligent set of tubes from rtl_sdr making the receipt, csdr demodulation and [F5OEO]’s rpitx performing the transmission. As far as we can see, it doesn’t have a burst tone detector or CTCSS to monitor its transmission, so it’s on full-time, but we suspect that this may be a feature that will be implemented in a timely manner.
With only 10 mW output, this repeater is a toy rather than a useful device, and we would recommend any licensed enthusiast who wants to give it a try to read the fine print in their license schedule before doing so. But this is pure use of Pi and RTL stick and luckily will inspire others in the same vein.