The Opposite of DX
There are times when we need an HF antenna that will allow us to reach stations that are near to us. It might be simple, such as a state QSO party or even Field Day. It might be life critical, such as dealing with the devastation of a major hurricane or a large-scale power outage. In either case, the ability of the ionosphere to “bounce” our signal around the earth is not productive.
In the early days of amateur radio, the old heads and Elmers all had this advice: Get your antenna as high as possible! Anything too low they derisively called a “cloud burner.” Well, it turns out that’s exactly what we need for regional emergency communication. This type of close-in propagation even has a name: Near-Vertical Incidence Skywave, or NVIS.
NVIS Basics
The basics): Low antenna height, taking advantage of ground reflection, sends the signal nearly straight upward. The RF wave bounces back down to earth much closer to the station than if it had been launched by a higher antenna at a lower “take-off” angle to be reflected much farther away and possibly make multiple “hops” to reach a faraway station. With the shorter NVIS “one-hop” reflection from the ionosphere, we’re able to reach stations within our own state or within a closer geographic region. Sounds simple! But there is a place where antenna theory and “real life” must meet; in the case of NVIS antennas, that happens to be a moving target.
