Propagations on VHF / UHF / SHF

Aurora

The appearance of auroras (aurora borealis) or northern lights is closely linked to solar activity. During massive geomagnetic storms, high-energy particles flow into the ionosphere near the polar regions, where they ionize the gases of the E layer and higher. This unusual ionization produces spectacular visual auroral displays. Auroral ionization in the E layer scatters radio signals in the VHF and UHF ranges. Aurora-scattered signals are easy to identify. Signals sound very distorted and somewhat wider than normal. At 144 MHz and above, the distortion may be so severe that mostly only CW is useful. This characteristic aurora signal is due to Doppler broadening, caused by the movement of electrons within the aurora.

Sporadic E (ES)

Short skip, familiar on the 10m-band during the summer month, affects the VHF bands up to 144 MHz. Sporadic E (Es), as this phenomenon is called, commonly propagates 28, 50 and 144-MHz radio signals between 500 and 2.300 km. Signals are apt to be exceedingly strong, allowing even small equipped stations to make long distance Es contacts. Unsusual multiple-hop Es has supported contacts up to more than 3.000 km on 144 MHz. At our latitudes Es is most common during May, June and July and events may last only a few minutes up to hours.

Meteor Scatter (MS)

Contacts up to about 2.300 km can be made at 28 MHz through 432 MHz via reflections from the ionized trails left by meteors as they travel through the ionisphere. The kinetic energy of meteors no larger than grains of rice are sufficient to ionize a column of air 20 km long in the E-layer. The particle itself evaporates and never reaches the ground but the ionized column may persist for a few seconds to a minute or more before it dissipates. This is enough time to make very brief contacts by relections from the ionized trails. Millions of meteors enter the Earth's atmosphere every day, but few have the required size, speed and orientation to the Earth to make them useful for meteor-scatter propagation.

Tropospheric Propagations

Tropospheric effects improve with increasing frequency, and 144 MHz is the lowest band at which weather plays an importand propagation role. Weather-included enhancements may extend the normal 400- to 800- km range of well-equipped stations to 1.000 km and more, especially during the summer and early fall. Tropospheric ducting extends this range to 2.000 km and farther over the continent and at least to 4.000 km over some well-known all-water paths, such as that between California and Hawaii.

Rainscatter Propagations

Rainscatter is the use of cloud masses to scatter and reflect signal, giving an enhancement. It works especially well at 10GHz.

The clouds need to be large and dense enough and towering thunder clouds can give good results. It helps to elevate the antenna to aim at the cloud, especially if close by. If both stations can see the same cloud masses, this can give huge signals, but often the rain cells are local to just one station. If you are beaming in the opposite direction to the station you are working, this is termed backscatter.

Spreading of the signal occurs. With forward scatter, the signal sounds a little rough, but SSB is usually easily readable, signal strength permitting, of course. With backscatter, the signal can be very spread out, very much like an auroral signal on 2m, and SSB can be very difficult to understand. Also, there can be significant doppler shift.