An ionogram is an excellent tool for studying Near Vertical Incidence Skywave (NVIS) Propagation. NVIS is useful here in Hawaii, because it provides us with fairly reliable communication between islands on 40, 60, 80 and 160 meters. The ionogram will indicate what bands are currently reflecting NVIS.

Our Ionogram is provided by UML Space Science Lab. Our sincere thanks to Dr. Ivan Galkin and his team for making this tool available to us. The ionosonde is located on Oahu. Below are observations by ham radio operators here in Hawaii.

• The altitude of a reflecting layer for a given frequency is indicated by a black line above that frequency. (Often embedded in a red and/or green line.)
• Horizontal lines generally indicate stronger NVIS propagation than vertical or sloping lines for any given frequency.
• Multiple reflections indicate a strong reflecting layer and usually excellent propagation. (These are not actual reflecting layers. They are echoes produced by multiple bounces between the reflecting layer and the ground.)
• A strong primary reflection with weak secondary reflections may indicate D layer absorption. (The D layer is not a reflecting layer, and is not visible on the ionogram.)
• High altitude (+400 km) reflecting layers produce weaker signals than layers at lower altitudes, as radio waves must travel farther to reach us.
• The E layer (indicated by a thick dark line below 150 km) can produce excellent NVIS propagation even when there is no higher reflecting layer.
• "The colors [sometimes seen in the ionogram] provide information on polarization, angle-of-arrival, and Doppler. [Echoes from ions accelerated by strong geomagnetic fields] are printed in green (with two shades for positive and negative Doppler), vertical echoes in red (two shades), and off-vertical echoes are, N (dark blue), WNW (blue), WSW (brown), S (yellow), ESE (magenta), ENE (violet)." Shing F. Fung - NASA · Goddard Space Flight Centre
• Vertical "rain" indicates a weak reflecting layer with increased atmospheric noise, often indicating a geomagnetic disturbance.
• Vertical artifacts below 150 km are foreign or domestic broadcast stations. Often seen around 4.5 and 8.75 MHz.
• Horizontal antennas almost always provide better NVIS performance than vertical antennas. Height above the ground is not as important as it is when working DX, because NVIS works best with a high "takeoff angle."