Antenna feed impedance - including radiation resistance, loss resistance and eff
Category: Antennas
When a signal source is applied to an antenna at its feed point, it is found that it presents a load impedance to the source. This is a complex impedance being made up from resistance, capacitance and inductance. In order to ensure the optimum efficiency of the transfer it is necessary to match the antenna to the load, and this requires some understanding of the operation of the antenna in this respect.
The feed impedance of the antenna results from a number of factors including the size and shape of the antenna, the frequency of operation and its environment. The impedance seen is normally complex, i.e. consisting of resistive elements as well as reactive ones. The resistive elements are made up from two constituents, namely the "loss resistance" and secondly the "radiation resistance."
Loss resistance
The loss resistance arises from the actual resistance of the elements in the antenna, and power dissipated in this manner is lost as heat. Although it may appear that the "DC" resistance is low, at higher frequencies the skin effect is in evidence and only the surface areas of the conductor are used. As a result the effective resistance is higher than would be measured at DC. It is proportional to the circumference of the conductor and to the square root of the frequency.
The resistance can become particularly significant in high current sections of an antenna where the effective resistance is low. Accordingly to reduce the effect of the loss resistance it is necessary to ensure the use of very low resistance conductors.
Radiation resistance
The other resistive element of the impedance is the "radiation resistance". This can be thought of as virtual resistor. It arises from the fact that power is "dissipated" when it is radiated. The aim is to "dissipate" as much power in this way as possible. It varies from one type of antenna to another, and from one design to another. It id dependent upon a variety of factors. However a typical half wave dipole operating in free space has a radiation resistance of around 73 Ohms.
Reactive elements
There are also reactive elements to the feed impedance. These arise from the fact that the antenna elements act as tuned circuits that possess inductance and capacitance. At resonance where most antennas are operated the inductance and capacitance cancel one another out to leave only the resistance of the combined radiation resistance and loss resistance. However either side of resonance the feed impedance quickly becomes either inductive (if operated below the resonant frequency) or capacitive (if operated above the resonant frequency).
Efficiency
It is naturally important to ensure that the proportion of the power dissipated in the loss resistance is as low as possible, leaving the highest proportion to be dissipated in the radiation resistance as a radiated signal. The proportion of the power dissipated in the radiation resistance divided by the power applied to the antenna is the efficiency.
A variety of means can be employed to ensure that the efficiency remains as high as possible. These include the use of optimum materials for the conductors to ensure low values of resistance, large circumference conductors to ensure large surface area to overcome the skin effect, and not using designs where very high currents and low feed impedance values are present. Other constraints may require that not all these requirements can be met, but by using engineering judgement it is normally possible to obtain a suitable compromise.
| 
