Re: Serial cable to usb dongle to usb cable
Rick is correct, the USB specification is 5 meters or about 16 feet. Amazon does sell a 20 ft extension cable, with good reviews:
RS232 does not have a specific length specification, however, when using cat 5 cables, 50 feet at 9600 baud is reliable.
I would be more concerned with the RF feedback and the audio quality of the long audio cable runs. That is assuming that you will be using the USB just for the CAT control and running the audio signals separately. If you are also going to remote the USB audio interface, then you are back to the 15 ft limit of USB. I do not know of a way to convert the USB audio to RS232. The 48kbit audio sampling would be hard to do with RS232.
One last comment, if you go the USB route (which is what I would suggest), then I would remote the Audio USB dongle and put ferrites on the USB cable to minimize RF interference. You would also need a powered USB hub for the end closest to the uBITX. One for the audio dongle and one for the uBITX CAT control.
Above are my recommendations. Others may have better ideas. Do the research on the internet and use it at your own risk. Internet searches can be a friend if you validate the results.