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A radio antenna is like any other form of RF load or signal source. It has a load or source impedance.
In order to obtain the optimum performance the antenna feeder must be matched to antenna to ensure the maximum power transfer.
Accordingly it important to understand the feed impedance of any antenna so that the best performance can be obtained.
Antenna feed impedance basics
This impedance is known as the antenna feed impedance. It is a complex impedance and it is made up from several constituents: resistance, capacitance and inductance.
The feed impedance of the antenna results from a number of factors including the size and shape of the RF 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.
Antenna feed impedance resistive elements
The resistive elements are made up from two constituents. These add together to form the sum of the total resistive elements.
- Loss resistance: The loss resistance arises from the actual resistance of the elements in the aRF ntenna, 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 RF 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 from the RF antenna. The aim is to "dissipate" as much power in this way as possible. The actual value for the radiation resistance varies from one type of antenna to another, and from one design to another. It is dependent upon a variety of factors. However a typical half wave dipole operating in free space has a radiation resistance of around 73 Ohms.
Antenna 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 above the resonant frequency) or capacitive (if operated below the resonant frequency).
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.
It can be seen that the antenna feed impedance is particularly important when considering any RF antenna design. However by maximising the energy transfer by matching the feeder to the antenna feed impedance the antenna design can be optimised and the best performance obtained.