Radio waves
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Corresponding Wikipedia article: Radio wave
Transmission
The radio waves are emitted by the artificial radiation sources in accordance with the radiation laws. The wavelength is determined by the speed of light in a medium and by the field frequency in the source.
The frequency range of the radio waves is limited by the technical capabilities. The radiation of the devices such as magnetron and cyclotron is not, strictly speaking, the radio waves, because they are produced by the natural material particles (electrons) as well as a laser radiation, for example.
Each electric conductor with an AC emits the radio waves with a certain efficiency. When the impedance is active, the current and the charge density are distributed uniformly along a conductor, so the electric field is enclosed within the coductor, the radiation cannot flow beyond it and is absorbed there. The efficiency is significant only when the impedance is reactive. However, the capacitors and the inductors are ineffective, because they do not provide the powerful electric and magnetic vectors at the same point at a right angle. The simple metal wires are effective due to the resonant standing waves, wherein their propagation path is multiple to their wavelength. A standing wave has a reactive phase shift (90°) between the current and voltage waves, and thus the phase coincidence of the magnetic and electric waves along the wire. The current wave distributes the charge along the wire not uniformly, so the electric field is not enclosed within the conductor. This causes radiation in the surrounding ether.
One of the first radio transmitting antennas is considered to be the Hertz experiment. Later, this experiment was repeated by A.S. Popov and G. Marconi. The breakdown of a spark gap between two conductors produces a wideband current pulse, which produces the brief resonant standing waves and, consequently, the radio pulse.
The dipole antenna or the doublet, which is called also the open oscillating circuit, resembles the Hertz device, substituting a spark gap by the feeder between two halves of a metallic rod. The rod length is a half of the wavelength. The maximal energy is emitted into the central plane transverse to the rod.
The folded and ring antennas operate on the same principle, but they have a more compact design. A standing wave emerges in a closed circuit.
The antenna radiation power through any closed surface within the empty space is a constant. So the surface power density decreases with a square of a distance from the antenna, when the absorption or the focusing (as in a parabolic antenna) do not exist.
Receiving
The radio communication and the electromagnetic induction are of the same nature, so both effects are mutually exclusive and cannot be superimposed.
The radio wave receiving is an absorption of the magnetic energy by the inductors, or of the electric energy as the EMF, which is generated in the conductors. The most effective receiving antennas have the same shape as the transmission antennas. The received signal frequency is equal to the source signal frequency (accurate to the Doppler effect).
Speed
The radio signal speed \(c'\), which is measured as the number of its periods \(n\) passed per time \(t\) (frequency \(f\)), is invariant with respect to any frame. The wavelength \(\lambda\) varies with the speed of light, and the longer waves move faster, and the shorter waves move slower: \[f=\frac{c+\Delta c}{\lambda+\Delta\lambda}=const\] \[t=\frac{x}{c+\Delta c}\] \[n=\frac{x}{\lambda+\Delta\lambda}\] \[c'=\frac{n}{t}=const\] The continuous radiolocation is used to measure the velocity due to the Doppler effect, although it is able to measure the distance.
The radio navigation system, such as GPS and GLONASS, is based on a transmission of a continuous information signal, which modulates a carrier frequency. This signal contains the exact real time and the positions of the signal sources (satellites). The receiver simultaneously receives the signals from \(N\) satellites and obtains the primary data of the times \(T_i\) and of the positions \(P_i\) for \(i=1\dotsc N\). The secondary data (the current real time \(T\) and the receiver position \(P\)), in general, are found from the system of equations\[|P-P_i|=c|T-T_i|\;\;\;\;\;i=1\dotsc N\] The values \(T_i\), when the atmospheric interference is negligible, depend only on the current real time \(T\) and the distances from the satellites to the receiver. The radio signal speed, as was mentioned above, is independent on the reference frame, i.e. the speed of light is independent on the reference frame within this task.
The clocks of all the satellites are precisely synchronized and are subjected to the gravitational and kinematic deviations, because their oscillator uses the atomic decay (see "Mass and inertia"). The satellite clocks are continuously observed and corrected by the terrestial stations.
The GPS and GLONASS existence is not a proof of the speed of light invariance with respect to any frame.
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