Antenna feed impedance
- overview, summary, tutorial about RF antenna or aerial feed impedance and
the importance of matching RF andtennas to feeders. Radiation resistance, loss
resistance, and efficiency are also detailed.
When a signal source is applied to an RF antenna at its feed
point, it is found that it presents a load impedance to the source. This is
known as the antenna "feed impedance" and it is a complex impedance made up from
resistance, capacitance and inductance. In order to ensure the optimum
efficiency for any RF antenna design it is necessary to maximise the transfer of
energy by matching the feed impedance of the RF antenna design to the load. This
requires some understanding of the operation of antenna design in this respect.
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.
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.
Summary
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.
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