An ionogram is an excellent tool for studying Near Vertical Incedence 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.

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The Basics - NVIS and the Ionogram

• 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".

• Horizontal lines generally indicate stronger NVIS propigation than vertical or sloping lines for any given frequency.

• Multiple reflections, indicated a strong reflecting layer and usually excellent propagation. (These are not actual reflecting layers. They are echos 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 reflecting layers (+500 km.) 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 F2 layer.

• 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.

• QSB (signal fading) is caused by your radio signal taking multiple paths through clouds of ionospheric plasma, and interacting with itself at another stations antenna.
If these paths arrive out of phase they physically cancel each other out. Your signal strength drops.
As the ionosphere shifts, those same waves might combine in phase, making your signal gain strength.

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Some Technical Stuff

• The solid and dashed black lines form the Electron Density Profile (EDP).
The solid black line is the actual height of the F2 layer, but the "apparent" height, displayed as colored radar traces, is much higher because radio waves hitting the ionospheric plasma slow down (like light passing through water). This slowing of the wave makes it appear that the F2 is much higher than it really is.
The dashed line is a calculation of the upper limit of the F2 layer. This upper limit depends on the frequency of the wave.

• "The colors [seen in the ionogram] provide information on polarization, angle-of-arrival, and Doppler. [Echos 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