Amazon has the Retivis RA79 for $17.99, which is just a rebadged Quansheng UV-K5, and because of the Beken BK4819 chip there is a wide range of experimental firmwares enabling a lot of extra receive capability. Technically, this also opens up transmit, but there is a lack of filtering and the radios are not clean enough to be used on other frequency bands other than VHF and UHF.
BK4819 is a half-duplex TDD FM transceiver operating within 18 MHz ~ 620 MHz, 840 MHz ~ 1200 MHz band range for worldwide personal radio. Besides speech communication, the BK4819 on-chip FSK data modem supports F2D and F1W emission to be used in both FRS and DPMR band for text message and GPS information exchange.
The BK4819 is a complete, small form factor solution optimized for low-power, low-cost, and highly integrated mobile and portable consumer electronic devices, requiring only a few external decoupling capacitors and an external inductor for input matching.
So having picked up one recently, I finally got around to backing up configuration and calibration data using k5prog-win before flashing egzumer’s custom firmware. And I discovered there is a bug when transmitting if you set the screen brightness to something other than 0 or 10 (and why some alternative firmwares don’t allow this), producing an audible tone that is supposedly picked up by the mic amplifier circuit from the PWM chip. Apparently, the implemented bug fix doesn’t work for my particular radio, but I can try playing with the PWM frequency and compiling the firmware myself, or just leave the radio to use screen brightness 0 or 10. I had discovered the tone when doing a parrot test with my Allstarlink node after I had tweaked the backlight values right after flashing the firmware. It would seem there is a lack of shielding in the design causing the issue, but you can’t really fault that for a $17.99 transceiver, especially when the stock firmware doesn’t allow changing the backlight value. The additional functionality of the firmware is pretty extensive, and there are even more versions of hobbyist firmware to try out even enabling CW reception. So, just in case you play with egzumer’s firmware, be aware that your transmit audio might have an issue if you change the backlight values, and you’ll want to test it against another radio. If you’re running HamVoip there is a parrot mode in the control menu, and there is always node 55553 setup as a parrot node with feedback on audio settings.