Propagation: let the wavelength decide

Propagation questions ask what happens between two antennas: how far a signal reaches, why it fades, what stops it, what sends it back. Nearly every one is settled by the wavelength and the size of what is in the way.

One wave, one speed, one ruler

A radio wave is an oscillating electric field with a magnetic field square to it, both square to the line of travel, and in free space every wave moves at the rate light does. So frequency and wavelength say one thing twice: wavelength in metres is about 300 divided by frequency in MHz. Amateur bands are therefore named by their length, not by channel number, and the velocity to know is 300 million metres per second, not miles per hour.

When one transmission reaches you by more than one route, the copies arrive out of step and either add or subtract. At VHF a wavelength is a metre or two, so shifting an antenna a few feet swings the strength sharply, a moving vehicle hears that pattern as a rapid flutter, and a data stream collects delayed copies that overlap the symbols behind, so the error rate climbs. The same holds along a whole path: copies returning from overhead drift in and out of step, and the fading is irregular.

Polarization is the plane the electric field lies in. Over a direct path both ends must share it: turning one antenna square to the other throws most of the signal away. Through the ionised layers the wave is rotated and reshaped and arrives with a foot in both planes, so either orientation recovers it. Those layers are ionised by the sun: they build through the day, decay after dark, and the highest frequency they return rises with solar activity: on an ordinary day it stays well below the top of the shortwave range, and only a strong sunspot cycle lifts it over 28 MHz and, at the peak, past 50 MHz. A patch of intense ionisation low in the layer, or the thin trail of a meteor, works the same way: the thinner the ionisation, the lower the frequency it can still return. Above the ceiling a signal leaves for space, so the higher bands stop at the horizon; the rest is the work of air and terrain.

Where the answers are lost

• Explaining a fade with something exotic. Flutter from a moving station, sharp changes over a few feet, corrupted data and irregular fading are one mechanism, several copies of one transmission combining; distractors offer Doppler shift, water vapour, Faraday rotation, thunderstorms and intermodulation.
• Applying weather everywhere at once. Falling water and dense foliage bite at UHF and above, where the wavelength has shrunk to their scale; distractors put absorption, deflection or extra range on the bands metres long, blame wind, pressure or cold for microwave paths, or have greenery amplifying weak signals.
• Treating polarization as always decisive or never relevant. On a direct path a square mismatch costs a great deal, and distractors soften it into an echo, inverted sidebands or nothing worth noticing; through the ionised layers the ends need not match, yet distractors insist they must, or offer a circular polarization where the conventions are horizontal for weak-signal work and vertical for repeater traffic.
• Letting frequency change the speed. Distractors rank microwaves, UHF and VHF by velocity, have radio outrunning light, put the two fields at differing speeds or side by side instead of square, and swap metres per second for miles per hour.
• Reaching for the sun when the weather is the answer, or the reverse. Regular 300-mile openings on VHF and UHF come from trapping under a temperature inversion, not the D or F2 regions, lightning or sunspots; a sudden strong opening low in VHF is patchy intense E-region ionisation, not backscatter or gray line; and ordinary UHF stops at the horizon by physics, not weak transmitters or dust.
• Running the wavelength formula backwards. Wavelength shortens as frequency rises and the 300 belongs on top; distractors multiply by 300, divide megahertz by 300, hold wavelength constant, or identify amateur bands by channel numbers and letter designators.

Try it yourself