Radiation resistance

The radiation resistance is the part of the base point resistance of an antenna that is caused by the radiation of electromagnetic energy. Similar to the wave impedance of a cable, it is determined by geometric factors and allows the calculation to be simplified because the complicated fields in space are calculated back to the feed point. It is independent of the material and does not contain any components such as the ohmic resistance of the antenna wires.

Basics

From the point of view of the energy supplier (generator, oscillator, amplifier, etc.), only the radiation resistance at this point is of interest for the flow of energy. On the other hand, the term allows a separation of the effects of radiation, as well as other losses or energy storage processes. In the frequency range of the radiation resistance is a complex size, so there is an active component and a reactive component .

• The active component describes the beneficial effect of the energy radiation. So it is not to be interpreted as a loss , as is the case with an ohmic resistance, for example. This becomes a damping term in the time domain.
• The reactive component describes energy fluctuations that are only visible in the near field .

Examples

Well-known examples occur with electromagnetic waves in antenna technology and with mechanical waves in acoustics ( acoustic radiation resistance ).

The radiation resistance of an antenna should be thought of at its connection point. The load on the transmitter then consists of the supply line that is terminated with this resistor. The complicated spatial dependence of the radiation field is factored into it and is no longer directly visible.

The radiation resistance of a T antenna with a very large roof capacity is usually very low and amounts to

${\displaystyle R_{S}=80\pi ^{2}\left({\frac {h}{\lambda }}\right)^{2}\,}$

The radiation resistance of a dynamic loudspeaker acts as a counterforce on the chassis membrane. Compared to the other mechanical forces as well as the electromagnetic motor forces, this proportion is usually completely negligible.

The radiation resistance thus enables a well-founded and well-founded approximation to be introduced through exact calculation, which makes the relationships easier to understand than the complete solution of the field equations in addition to the mechanical and electrical equations. Nevertheless, the radiation resistance is not just a mere calculation variable. If this component is missing in the chassis model, for example, then the impedance curve does not match the measurements, so a convincing fit of the measurement curve and the theoretical curve cannot be achieved by varying the other parameters .

literature

1. ↑ Huang, Yi; Kevin Boyle (2008). Antennas: from theory to practice. John Wiley & Sons. pp. 299-301. ISBN 0-470-51028-5