PROPAGATION

As explained many times in this column, the ionosphere is an essential player in propagating an HF (high frequency; 3-30 MHz, also known as shortwave) radio signal beyond line-of-sight distances between a radio transmitter and a radio receiver. Radio signals in the HF spectrum are bent (actually, a combination of mostly refraction and seldom reflection) by the ionosphere, allowing a signal to skip from one location to another. Like a flashlight’s beam of light, reflected off a wall-mounted mirror in a dark room, you can see the beam arriving and departing the mirror at an angle — a radio wave can be reflected in a similar way off the ionosphere.

When the ionosphere is highly energized by solar activity, higher HF frequencies are refracted. But, when solar activity is low, as it is right now because we’re at the beginning of Solar Cycle 25, the ionosphere is weakly to moderately energized. Only the mid-HF frequencies and lower are being efficiently refracted (as a general rule; even the 10-meter band has openings now, when the 10.7-cm Radio Flux rises–TH). The Technician operator who longs to talk to the world by using SSB on HF is generally cut off from the world (but not from regional coverage). The operator must move down to lower bands to work radio stations around the world.

The 10-meter band is useful for shorter distances because of several common types of propagation. These include, for instance, Sporadic-E (E_s). And, if shorter distances are desired on HF communications, there is a technique used to target closer areas, using the F-and E-regions. This is known as Near Vertical Incidence Skywave, or NVIS, and it is highly effective.

NVIS is pronounced as “niv-iss.” Another loving. This radio propagation mode involves using antennas that radiate most of the radio energy at very high radiation angles, approaching or reaching 90° (straight up at and into the ionosphere), at a frequency below the critical frequency of the ionosphere at the point of entry by that radio wave ( ).