So a number of what we all call SDR receivers are capable of transmitting. The output power in the case of the HackRF one is about 250mV p-p, in other words a very small signal, 0.15mW. This is not much good for transmitting further than the end of the street, but with some amplification you can get that up some. I added the QRPLabs 10w TX to it and was getting close to 0.8W and was being heard over 800km away.
Anyway, my plan for a while has been to use the HackRF as a receiver and make a couple of low power CW transmitters to go with it. By low power i am talking in the 10w kind of range. But this has gotten me thinking, with some VHF and UHF amplification, this might be a really cheap and easy way into SSB and CW on those bands for field day here in VK. It would require a 2nd SDR to act as the receiver, because the HackRF is only half duplex and there is no software out there that can so the switching currently, and it can be locked to a reference signal so it might be somewhat useful.