What is NVIS?
We have all seen the pictures in films and on TV of military vehicles happily driving about with a massive long antenna mounted on the back, and usually tied to the front bumper by a bit string...why? The simple answer is that it is not a daft idea to stop the antenna being hit by trees and low bridges as it merrily drives along, it is an HF NVIS antenna.
So what is NVIS? - NVIS or Near Vertical Incident Skywave (or Scattering as it is sometimes called by our cousins across the pond), is a technique of transmitting on HF where all the energy is radiated as far as possible vertically from a very low antenna. Normally the golden rule is that an HF antenna should be as high as possible, at least 1/2 wavelength, above the ground, if not a full wavelength. NVIS uses an antenna mounted as low as possible, (ideally <1/20 wavelength) and can even be laid along the ground with suitably insulated wire. the antenna is mounted horizontally to give the best reflection from the ground vertically as opposed to a whip or beam - which propagates towards the horizon.
OK so how and why does it work? Lets imagine a gardener standing in the middle of a garden with a hose watering the lawn and plants. He or she could drag the hose around the garden all day visiting each section of the garden to make sure it all gets watered. But what if they pointed the hose vertically? the jet of water, all things being equal, will fall in a circular pattern around the centre point equally. On HF the signal will travel in a similar way with good solid cover with no gaps out to ~400Kms - 500Km (250mile - 350miles) even at relatively modest power levels of ~10w - 20w, Ideal for many of the longer Emergency Comms operations. and no need for multiple talk-through units etc.
Typical NVIS coverage NVIS vs DX transmission on HF
OK so far? seems good? what is the catch? well there are some...
Firstly NVIS does not really work well above ~40m (7 MHz) it works best on 40m during the day and 80m at night. It will work at 20m but it can/will be marginal.
Secondly, HF antennas for cars are available and many people have them but they tend to be verticals not horizontal and very few ham antennas will fold over to work like the military ones... would you (or the xyl) really want to drive around in your Mini, Fiat 500 or Mondeo with a 15'+ whip tied to the bumper by a bit of nylon string? so this means NVIS is best used whilst stationary.
Thirdly if being used stationary, you will need space for a 10m - 20m dipole at knee height. with your car tent or caravan in the middle. Ok out in the middle of the moors, not so good in the middle of Goose Fair with thousands of people milling about. (Health and Safety Police look away now....)
Finally here in the UK the 2m/70cm bands, our normal operations home are relatively clear. On HF, the bands are very much more crowded and conditions will change according to the time of day, weather and other users, so keeping a clear frequency to work is a lot more challenging. However It is a valid and useful method of working stations over a wide area.
WARNING! WARNING! WARNING!
NVIS antennas operate at low levels and are within easy reach. Apart from a hazard of being cut or tripped by low wires - RF BITES! ALWAYS take care to keep people away from the wires.