On portable operations I use a 2 x 13.4 mtrs doublet that Ken GM0AXY described on the sota reflector. I'm very impressed with it on 20m - 10m, it's working OK on 40m and it's working on 80m. I have read that you can improve NVIS performance by laying a wire on the ground beneath the antenna but I've not tried this myself yet.
Here the other day I discovered an article about the antenna by Casey NA7U.
Ya gotta love Field Day !
4 hours ago