NVIS (Near Incidence Vertical Skywave) is best accomplished transmitting in the most common NVIS bands (80, 60, ad 20m) with a very high angle of incidence, the radio waves going almost straight up. This is the exact opposite of what we are doing when we're "DXing" (distant communications); as a result, almost all the logic we apply to setting up antennas is turned on its head. We must utilized antennas in ways we typically would avoid, and we'll cover the different ways here. The key is to remember we are attempting to direct radio waves upwards, rather than towards the horizon.
Dipole Antennas
These are the most common antennas in amateur radio. They are easy to make and work well. For DX, all you need to do is get them as high off the ground as you can get so the bulk of the radio signals are radiated towards the horizon. Paradoxically, the lower the antenna is hung the more vertical the radio waves tend to radiate. So, simply dropping the antenna closer to the ground (5-10ft even) the more vertical the waves are directed and the better you will perform in the "Cone Of Communication". Keep in mind the resonant frequency of the antenna will change, so keep an eye on your SWR.
Inverted-V Antennas
These function along the same lines as a dipole. the ends of a dipole antenna slope to the ground from a mast in the middle. The mast would be shorter than you'd ideally want for DX communications, no more than 20 feet tall, with the ends being as low as a couple feet off the ground.
Mobile Antennas
You may have experienced (or seen in movies) military vehicles that have a bumper mount antenna that is then arced over the top of the vehicle, often with a rope pulling the tip towards the bumper at the opposite end of the vehicle. This creates an arch over the vehicle, with the antenna radiating a significant vertical component. A flexible antenna with a coil and a spring base could be utilized in this way. After all, the military has been using it since they've been using radio!
Hamsticks
KK4OHF mentioned using hamsticks for NVIS. Rather than using them as verticals, two of them can be used back-to-back, forming a horizontal dipole, either close to the ground or using the vehicle as a 'counterpoise' or ground. Hamsticks tend to have narrow bandwidth, so keep an eye on your SWR, and a tuner will likely be necessary.
Vertical Antennas
While good for DX communications, verticals make extremely poor NVIS antennas in their typical configuration. On the other hand, if a storm has blown your antenna over, the NVIS gods may be smiling upon you! A vertical antenna, tilted at a 45 degree angle will radiate more signal straight up, rather than at the horizon, giving you better NVIS propagation than if it is vertical. Note: unlike a dipole, simply mounting it closer to the ground will not make a vertical antenna more NVIS friendly, unless you also tilt it.
Elevated radials will tend to make the antenna perform better for NVIS. Also, if the antenna if flexible a rope can be attached to the tip and use to pull the top of the antenna downward (similar to mobile antennas), so the vertical becomes an inverted "L", with a significant horizontal component. Obviously, this could damage an antenna that isn't flexible enough, so be careful.
Crossed Inverted-V Antennas
This is an interesting design I would like to try, based upon a military design (AS-2259/GR). Two dipoles, crossing at 90 degrees of each other at a central mast act as both antenna and stays for the mast! One set of wires cut to about 41ft for 60 meters, and another set of wires cut to 31 ft for 40 meters are wired like a fan dipole would be, the wires acting as guy wires and radiators on those two bands. The 40 meter antenna will probably work at 15 meters (DX, not NVIS) as well. While 15 meters may work, an antenna tuner will likely be necessary because the 60 meter legs will interact enough to alter resonance. This design has potential for being adapted to POTA and SOTA, for 20 and 40 meters (which will probably also work well on 15 and possibly 10m).
Horizontal Loop (Skyloop)
This is a loop antenna just above ground level. It works much the same as a regular loop, except being closer to the ground, it has a higher take-off angle, providing NVIS functionality. Loops have an added benefit of being more noise resistant than other antenna designs. These can be difficult to implement as the antenna will be long and difficult to keep relatively level without a series stand-offs.
