Short Radio Waves

**Model description**

This model shows ***short radio waves*** and the principle of propagation of HFW by reflection from the ionosphere. **Electromagnetic waves** with a wavelength of ***λ***** = (10 – 100) m and a frequency (3 - 30) MHz**, which reflect from the ionosphere at a distance of approximately (200 - 1000) km from the Earth's surface, propagate from the ***transmitter***. After the reflection, the radio waves propagate to a receiver, which is represented as the **radio (radio receiver)** in the model.



***Signal A*** propagates from the transmitter to receiver by repeated reflections from the ionosphere and the earth's surface, ***signal B*** demonstrates only **one reflection**.



The ***Wave Item - Detail*** part illustrates two components of an electromagnetic wave. The ***vectors of electrical intensity E***, which are represented by the **red arrows**, are **perpendicular** to the ***vectors of magnetic induction B*** indicated by the **blue arrows**. Both vectors are then simultaneously perpendicular to the direction of propagation of the radio wave, which is illustrated by the ***velocity vector v*** (**green arrow**). The **wavelength** *λ* is indicated by a double-sided **black arrow**.



**Theory**

**Ionosphere** is an ionized part of the atmosphere significantly influencing the propagation of electromagnetic waves. It is composed of neutral gas, ions and electrons. It is of **great importance for the propagation of short-wave radio waves** (**HFW** - High Frequency Waves or **SW** - Short Waves), which mainly reflects from the F-layer (the ionosphere has additional layers D and E) and thus spreads far from the transmitter. However, it also affects the propagation of other signals, eg slows down the **propagation of signals from global GPS or Galileo navigation systems**. It is located in the mesosphere and thermosphere. The lower edge of the ionosphere is at an altitude of about 60 km (day), 95 km (night), in the upper part the ionosphere reaches up to about 700–1000 km.



**The SW radio waves propagate by multiple reflections of the spatial wave from the ionosphere and the Earth's surface, depending on the frequency of the wave and the density of the ionosphere**, which depends primarily on the intensity of cosmic radiation falling from space into the atmosphere of the Earth. The intensity of radiation varies during the day, seasons, solar eleven-year cycles. The signal quality is also affected by the geographical position of the receiver and transmitter.



The **short wave band** is used for **radio communication** at medium and **long distances**. When it was not possible to communicate using satellite systems, SW was the only way to **connect with ocean-going ships and continents**. **With a single reflection from the ionosphere, a connection of about 4000 km can be made**. Shortwave transmitters are also used for the transmission of radio SW or for **amateur radio communication**.



In **isotropic** (homogeneous) environments, electromagnetic waves propagate linearly at the **speed of light** ***c***** = 299 792 458 m∙s**** −¹**. In an **anisotropic** (non-homogeneous) environment, **reflection, refraction, bending and attenuation** (loss of energy) **are manifested**. Radio waves are propagated either by a **direct wave to the radio horizon** (eg very short VHF waves), by a **surface wave** (eg long LW waves) or by a **spatial wave** (by a reflection from the ionosphere).