Q7Antenna And Wave Propagation

(a) Formation of Ionosphere and Its Layers

The ionosphere is formed by the ionizing action of solar radiation (principally extreme ultraviolet and X-ray radiation from the sun) on the neutral gas molecules and atoms present in the upper reaches of the earth's atmosphere (extending roughly from about 60 km to over 600 km altitude) — this high-energy solar radiation has sufficient photon energy to knock electrons free from gas molecules (photoionization), creating a region containing a significant population of free electrons and positive ions, which together constitute the ionosphere. The degree of ionization at any given altitude depends on the balance between the rate of ion-pair production (governed by the intensity of incoming solar radiation, which is itself strongest during daytime and at lower latitudes) and the rate of recombination (electrons and ions recombining back into neutral atoms/molecules, which occurs more rapidly at lower altitudes where atmospheric density, and hence collision frequency, is higher) — this balance produces several distinct layers of varying electron density at different characteristic altitudes.

D-layer (approximately 60-90 km altitude): the lowest ionospheric layer, present only during daytime (disappearing at night due to rapid recombination at this relatively dense, lower altitude), with a comparatively low electron density insufficient to reflect most radio frequencies, but causing significant absorption of medium-frequency (MF) and lower high-frequency (HF) waves passing through it, particularly affecting daytime MF/AM broadcast propagation and lower-HF communication.

E-layer (approximately 90-150 km altitude): present both day and night (though weaker at night), with sufficient electron density to reflect medium-frequency and lower high-frequency radio waves back toward earth, historically important for medium-range HF communication, and also exhibiting a variable, patchy 'sporadic-E' phenomenon that can occasionally support unusually strong VHF propagation over extended distances.

F1 and F2 layers (approximately 150-400+ km altitude): the F-region splits into two sub-layers during daytime (F1 at lower altitude, F2 at higher altitude) due to the different rates of ionization and recombination at these different altitudes under direct solar illumination, merging into a single F-layer at night once solar ionization ceases and recombination reduces the overall electron density — the F-layer (particularly F2) has the highest electron density of all ionospheric layers and is the principal layer responsible for reflecting high-frequency (HF, 3-30 MHz) radio waves back to earth, enabling the long-distance, beyond-line-of-sight skywave (ionospheric) HF communication that has historically been central to shortwave broadcasting, amateur radio, and long-range military/aviation communication.

Ionospheric Variations: Diurnal, Seasonal, Solar-Cycle, and Sporadic-E

The ionosphere's electron density, and hence its critical frequencies and reflection characteristics, undergo several distinct types of systematic and irregular variation that must be accounted for in practical HF link planning. Diurnal (daily) variation arises because solar ionizing radiation is present only during daylight hours, so the D and E layers (and F1) largely disappear at night through recombination, while the F2 layer persists but at reduced electron density, meaning critical frequencies and MUF values are markedly higher during daytime than at night for any given path. Seasonal variation arises from the changing solar elevation angle and day length through the year, causing systematically higher ionization (and higher critical frequencies) in local summer than in local winter for the lower layers, though the F2 layer paradoxically often shows an anomalous seasonal behavior (higher noon critical frequency in winter than summer at many mid-latitude locations, termed the 'winter anomaly'), driven by seasonal changes in the upper-atmosphere neutral composition rather than simple illumination intensity. Solar-cycle variation reflects the approximately 11-year sunspot cycle, during which solar UV and X-ray output (and hence overall ionospheric ionization) rises and falls, so peak usable HF frequencies and achievable communication ranges are substantially higher during solar-maximum years than during solar-minimum years, a factor long-range HF planners must incorporate using published solar-flux-index forecasts. Sporadic-E is an irregular, patchy, and unpredictable occurrence of unusually intense ionization within the E-layer, forming thin, localized clouds of high electron density that can occasionally reflect frequencies well into the lower VHF band (well above the normal E-layer critical frequency), causing sporadic, short-lived long-distance VHF propagation openings that are of particular interest to amateur radio and can also cause unexpected interference to VHF broadcast and other services.

Practical Implications for HF Band Planning

Because of these combined diurnal, seasonal, and solar-cycle variations, practical HF communication systems (broadcast, military, and amateur) must select their operating frequency according to time of day and expected propagation conditions rather than using one fixed frequency around the clock — lower HF bands (around 3-7 MHz) are generally favored for nighttime and winter propagation, when the F2-layer critical frequency and MUF are relatively low, while higher HF bands (14-30 MHz) become usable, and often necessary to avoid excessive D-layer daytime absorption on the lower bands, during daytime and summer conditions when ionization (and hence MUF) is higher — this is precisely why international shortwave broadcasters historically publish separate frequency schedules for different times of day and seasons for the same target service area, and why HF frequency-management authorities issue regularly updated MUF/FOT predictions (based on measured and forecast solar activity) to guide real-time operating frequency selection for reliable long-distance HF links.

(b) Multiple Hop Transmission and Effect of Earth's Magnetic Field

Multiple hop transmission occurs when the distance between a transmitter and a desired receiver exceeds the maximum single-hop range achievable via one ionospheric reflection (which is limited by the maximum practical elevation/take-off angle from the transmitting antenna and the ionosphere's height above ground, as discussed in the corresponding skip-distance/MUF answers elsewhere in this paper) — in such cases, the radio wave, after its first ionospheric reflection, travels back down and reflects (typically with some loss) off the earth's surface itself, then travels back upward and reflects a second time off the ionosphere, and this process may repeat across several successive hops (each combining one ionospheric reflection and one ground reflection) until the wave finally reaches the distant receiver. Each additional hop introduces further signal attenuation (from the imperfect efficiency of both the ionospheric and ground reflections, and from the additional free-space path-length loss over the greater total travel distance), meaning multi-hop propagation paths generally support somewhat lower received signal strength than an equivalent shorter, single-hop path, but multi-hop propagation is nonetheless the fundamental mechanism enabling genuinely long-distance (intercontinental) HF skywave communication, since a single hop's maximum range (typically limited to roughly 2000-4000 km depending on ionospheric height and geometry) is often insufficient to span the full desired communication distance.

Effect of earth's magnetic field on ionospheric wave propagation: the earth's magnetic field permeates the ionosphere, and since the ionosphere consists of free (mobile) electrons, the presence of this magnetic field causes the ionosphere to behave as an anisotropic (direction-dependent) and gyrotropic medium for radio wave propagation, rather than the simple isotropic medium that would exist in the field's absence — an electron moving under the combined influence of the incident wave's oscillating electric field and the steady earth's magnetic field experiences an additional magnetic (Lorentz) force that causes its motion to trace out an elliptical rather than purely linear path, and this magnetically-influenced electron motion causes an incident linearly-polarized radio wave entering the ionosphere to split into two distinct characteristic wave components — the ordinary (O) wave and the extraordinary (X) wave — each propagating with a different phase velocity, refractive index, and hence different effective critical frequency and reflection height within the ionosphere, a phenomenon directly analogous to birefringence in anisotropic optical crystals. This magnetically-induced splitting into ordinary and extraordinary wave components is responsible for several practically important propagation effects, including Faraday rotation (a progressive rotation of the wave's plane of polarization as it traverses the magnetized ionospheric plasma, which must be accounted for and compensated in satellite communication link design) and a modification of the effective maximum usable frequency compared to the simpler non-magnetized-plasma theoretical prediction, making the earth's magnetic field an essential factor that must be incorporated into any complete, quantitatively accurate theory of ionospheric radio wave propagation.