We haven't used backpack (rucksack) sized radios for at least a decade. If by backpack you mean a typical hiking backpack you can get at REI, then yes I guess they are backpack sized.
Back in the mid-late 90's I had to travel down to Fort Gordon (? I think?) which was home of the Army Signal Corps to give a training class on using our Single Board Computers to the group that built the man-portable radio simulators.
The old guys there (and they were almost all in their 60's+) built simulators that had to weigh and behave exactly like the actual radios for effective training (except that they didn't actually transmit any signals). The radios were huge. I simply could not imagine having to lug that thing around in the field all day.
I attended the Signal Officer Basic Course (SOBC) at Fort Gordon in 2002. Believe it or not, they didn't even have real radios to train us on at the time. We got a powerpoint presentation, which was pretty much useless.
What communication equipment they did train us on was basically obsolete. "We have to train you on the old stuff because the new training hasn't been approved yet. You probably won't see any of this in the field."
