Q7Antenna And Wave Propagation

(a) Effects on Surface Wave Propagation

Effect of frequency: surface (ground) wave attenuation increases markedly with increasing operating frequency, since the ground wave's field is progressively absorbed by the finite (non-zero) resistivity of the earth's surface as the wave travels along it, and this absorption loss mechanism becomes significantly more severe at higher frequencies (the ground behaves as an increasingly lossy medium relative to the wave's own characteristic impedance as frequency rises, and the depth to which the wave's field penetrates into and interacts with the lossy ground also changes with frequency) — this is precisely why surface wave (ground wave) propagation is practically useful and widely exploited only at relatively low frequencies (LF, MF, and lower HF), such as AM broadcast, maritime, and navigation beacon services, and becomes progressively impractical (surviving only over very short distances) at higher HF, VHF, and above, where other propagation mechanisms (sky-wave, space-wave/line-of-sight, tropospheric) must instead be relied upon.

Effect of earth constant (conductivity): the ground wave's attenuation is strongly affected by the electrical conductivity of the earth's surface along the propagation path, with higher ground conductivity producing substantially lower ground-wave attenuation (better, longer-range ground-wave propagation) — sea water, with its very high conductivity, supports by far the best ground-wave propagation range of any common terrestrial surface, which is precisely why maritime MF/LF radio services (and historically, transoceanic longwave communication) have long relied on ground-wave propagation over the highly conductive sea surface; moist/wet soil provides intermediate ground-wave performance; while dry, rocky, or sandy terrain (with comparatively low conductivity) causes substantially greater ground-wave attenuation and correspondingly much shorter useful ground-wave range for the same frequency and transmitter power. This conductivity dependence is why AM broadcast stations situated near or over seawater or salt-marsh terrain can achieve dramatically extended ground-wave coverage compared to an equivalent station operating over dry inland terrain.

Effect of earth curvature: because the earth's surface is curved (rather than flat), a ground wave attempting to follow the surface beyond the geometric line-of-sight horizon must diffract around the earth's curvature to continue propagating — this diffraction process introduces its own additional, progressively increasing attenuation as the propagation distance extends further beyond the horizon, since diffraction efficiency (the fraction of wave energy that can successfully bend around the curved obstacle formed by the earth itself) decreases continuously with increasing distance traveled beyond line-of-sight. This earth-curvature diffraction loss compounds with the ground conductivity-dependent absorption loss discussed above, together setting a fundamental practical maximum range for ground-wave (surface wave) propagation at any given frequency, beyond which the combination of ground absorption and curvature-diffraction loss reduces the received signal below any practically usable level, regardless of transmitter power.

(b) Troposphere and Tropospheric Wave Propagation

Troposphere: the lowest layer of the earth's atmosphere, extending from the ground surface up to an altitude of approximately 10-12 km (somewhat lower at the poles, somewhat higher at the equator), containing essentially all of the atmosphere's weather phenomena (clouds, precipitation, temperature and humidity variation) and the great majority of the atmosphere's total mass. Unlike the ionosphere (which is ionized by solar radiation and located much higher, from roughly 60 km upward), the troposphere is electrically neutral (non-ionized) under normal conditions, and its effect on radio wave propagation arises not from ionization but from the gradual variation of its refractive index with altitude, driven by the normal decrease of atmospheric pressure, temperature, and humidity with increasing height.

Tropospheric wave propagation: because the troposphere's refractive index normally decreases gradually and continuously with increasing altitude (since both atmospheric pressure and, generally, humidity decrease with height, both of which reduce refractive index), a radio wave traveling through the troposphere is subject to a continuous, gentle downward-bending refraction (the wave bends slightly toward the direction of higher refractive index, i.e., downward, toward the earth) — this gradual downward bending causes the wave's effective propagation path to follow the earth's curvature somewhat more closely than a perfectly straight geometric line-of-sight ray would, effectively extending the practical radio horizon distance beyond the simple, purely geometric line-of-sight limit. This effect is conventionally modeled using an effective earth radius factor of approximately 4/3 (the '4/3 earth radius' model), in which the true, curved refractive propagation path is mathematically replaced, for calculation convenience, by an equivalent straight-line ray traveling above a fictitious earth whose radius is scaled up by this factor of 4/3 compared to the true earth radius — since a larger effective earth radius pushes the true geometric horizon further away for the same antenna heights, this model correctly reproduces the practically observed modest extension of usable line-of-sight range due to normal tropospheric refraction.

Justification: Microwave Communication Is Only Due to Tropospheric Propagation

Microwave frequencies (conventionally, roughly above 1-3 GHz and upward) cannot rely on either of the other two major long-distance terrestrial propagation mechanisms available at lower frequencies: ionospheric (sky-wave) propagation is unavailable at microwave frequencies, because sky-wave reflection depends on the operating frequency being below the ionosphere's maximum usable frequency (MUF)/critical frequency (itself set by the ionosphere's peak electron density, as discussed in the corresponding unit V answers of this paper), and microwave frequencies vastly exceed the maximum critical frequency/MUF that even the most heavily ionized ionospheric layers can support (which tops out in the tens of MHz, far below the GHz-range microwave band) — a microwave-frequency wave transmitted toward the ionosphere simply penetrates straight through it into space, without any reflection back to earth whatsoever. Ground/surface-wave propagation is likewise unavailable at microwave frequencies, since surface-wave propagation efficiency requires the operating wavelength to be reasonably comparable to or larger than the physical obstacles/diffraction-relevant length scales involved, and the extremely short wavelengths of microwave frequencies (centimeters or less) are essentially not diffracted at all around the earth's curvature or other terrain obstacles at any practically useful signal level; in addition, ground-wave attenuation itself (discussed in part (a) above) becomes prohibitively severe at these high frequencies. With both the sky-wave and ground/surface-wave mechanisms thus entirely ruled out at microwave frequencies, the only remaining viable terrestrial propagation mechanism is space-wave/line-of-sight propagation, extended somewhat beyond the pure geometric horizon specifically by tropospheric refraction as described above — this is why, in practice, virtually all terrestrial (non-satellite) microwave communication links (point-to-point microwave relay towers, terrestrial radar, and similar systems) are fundamentally dependent on, and limited by, tropospheric space-wave propagation range, fully justifying the statement that microwave communication is essentially only made possible through tropospheric propagation.