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Arctic Radio Propagation

One expert on radio propagation has said that if he wanted to pick the worst place in the world to build a communication system it would be Alaska and western Canada. Permafrost, rugged mountain terrain and the ionospheric effects associated with the aurora combine in Alaska and western Canada to make communication difficult at all radio frequencies.

Frozen ground makes a poor electrical conductor and therefore affects radio communications in two ways. One way has to do with the effectiveness of transmitting and receiving antennas. Many of these work best if set above highly conducting earth. If the ground below the antenna is a poor conductor, it may be necessary to build an artificial ground plane from metal. This is the purpose of the horizontal wire structures one often sees at the base of antennas in the north.

The electrical conductivity of the ground also affects the propagation of radio waves across the surface of the earth. Ordinary AM broadcasting, shore-to-ship communications and aircraft navigational beacons all rely on radio signals that propagate in this fashion--outward from a transmitter and along the ground surface to the receiving antennas. Consequently, an AM broadcast station or similar transmitter located in the north must be more powerful than one at temperate latitude to cover an equal audience area.

The most spectacular quirks and failures of radio communication in the north are those related to the aurora. The frequency range most affected is that above the AM broadcast band because signals in that range normally propagate by reflection off the ionosphere or perhaps by passing through it, to and from satellites flying above the ionosphere. Actually, the upper frequencies commonly used for satellite communications mostly manage to squeak through the ionosphere in pretty good shape, so are not as badly affected as the lower-range frequencies that typically bounce off the ionosphere.

Reasonably reliable long-range communication using signals bounced from the ionosphere takes place at low and middle latitudes because the ionosphere there is relatively stable, though it does change from night to day because sunlight alters the number of ionized particles within the ionosphere.

The number and location of ionized particles in the ionosphere is critical: there must be enough of them there to reflect radio signals; but if there are too many at the wrong altitudes (the range is 40 to 600 km), then the radio waves are absorbed rather than reflected.

Unfortunately, at high latitudes the ionosphere sometimes acts less like a smooth reflecting blanket and more like a stormy sea. The sheets of electrons and protons that rain down into the high atmosphere to create the visible aurora produce profound, localized enhancements in the ionospheric ion density. These enhancements both absorb radio waves and reflect them in strange directions. To make matters even more confusing, strong electric fields accompanying the incoming particle streams cause rapid, mostly horizontal, motions that churn up the auroral zone ionosphere.

One consequence is total loss of desired signals when the ionosphere absorbs rather than reflects radio waves. Other consequences are irregular, often garbled, transmissions and weird signal paths. Sometimes two communicators can do better by pointing their antennas toward the aurora than if they direct them toward each other. And sometimes signals intended to go only a short ways end up propagating halfway around the world.