There is a lot to say when it comes to NIVIS, back-pack comms.
Your going in the right direction, continue to think outside the "BOX'.
I run 5 watts, yep 5 watts with a " End Fed Antenna" 1/2 wave long.
It acts like a dipole, but you feed it at the end, so there is no coax to carry. I do some hiking, and like to have a lighter load then my old commo ruck.
Getting the Far end up above 5 feet, and using low frequencies , 10 MHz below, gets me a NIVIS shot, and if you get it up high 10 feet or more, and with the right high band frequencies, you can talk a hell of a lot further then you think.
When you use a short whip, you are asking the tuner to load the antenna, this could cause your antenna to be ineffective for what you are trying to do, but if it works go with it.
I would use the 16 foot whip, on its side, like what has been said. A vertical gives you ground wave, a horizontal antenna gives you sky-wave. Keep the antenna off the ground, so not to complete the path to ground, that's what you are asking the sky to do for you.
There is a great book out, that was written by a NG O,,"Near Vertical Incidence Sky wave Communications" Its a Ham radio book. But then again we are all Amateurs..
I have a lot more to tell you if you want it, email me, etc.
A radio man is the life line to the team, shooting a gun is nice, but I like to get a ride home, not walk..
ASk away.... AL
PS We did a lot of NIVIS stuff while on the GMV, while on the move..