One aspect of radio antennas that achieves a lot of attention is that of its feed impedance as it can have a major impact on the antenna performance.
A radio antenna is like any other form of RF load or signal source. It has a load or source impedance.
The reason that the antenna impedance is so important is that 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.
When a signal is applied from a signal source, either directly or via a feeder, then it will see a certain impedance and this affects the way in which the signal is accepted by the antenna and power is transferred.
The same is true in the reverse direction for signals that the antenna has 'received from the ether' and need to be transferred either directly or via a feeder or transmission line to the receiver.
Importance of antenna feed impedance
It is easier to visualise power entering from a feeder into the antenna, and although this direction of power transfer will be described, the same occurs in the opposite direction when the antenna is used for reception.
All feeders and indeed RF signal sources have a characteristic impedance, and all antennas have a feed impedance. For the maximum power transfer to occur, the impedances must match.
Take the example of a 50Ω feeder connected to a 50Ω antenna. The feeder will be able to supply power that has the voltage and current with a ratio equivalent to 50Ω
However if a 50Ω feeder is connected to a 75Ω antenna, the the voltage current ratio of the feeder will not match that of the antenna. As the antenna can only accept power that has a ratio of voltage to current corresponding to 75Ω, it cannot accept all the power.
If the feeder is supplying 50 volts at 1 amp, then the antenna can accept the 50 volts but only at 0.66667 amps (which corresponds to the ratio of 75Ω).
The remaining power is reflected back along the feeder and only a proportion of the power is accepted. Also standing waves are set up along the feeder having voltage and current troughs and peaks.
Note on Standing Wave Ratio, SWR & VSWR:
Standing waves are often associated with RF feeders, and they are generated when there is a mismatch between the feeder impedance and the load impedance. At th emismatch, power is reflected and the combined voltages and currents of the forward and reflected power form standing waves along the feeder.
Read more about Voltage Standing Wave Ratio, VSWR.
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 RF 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 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 to give an idea of the sorts of figures that might be expected, a typical half wave dipole operating in free space has a radiation resistance of around 73 Ohms.
Other antenna types will have different levels of radiation resistance, and even the radiation resistance of a dipole will vary considerably according to the presence of nearby objects. For example a dipole is used as the main active or driven element in a Yagi antenna. The parasitic elements either side of the dipole that give the overall antenna its directive pattern will cause the feed impedance of the dipole to fall considerably, often to 20 Ω or less.
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).
Factors affecting antenna impedance
The feed impedance of different antennas can vary widely. Some may present a low impedance whereas others may have a much higher impedance.
Understanding why different antennas have different impedance values enables the right feed methods to be employed and this improves their performance.
There are many reasons why the feed impedance of different antennas changes. We've included some of the main reasons below:
Feed point: The feed point for the antenna has a major impact on the feed impedance that is seen.
Take the example of a half wave dipole. It can be seen that towards the end of the conductor the current is a minimum and the voltage is a maximum. Conversely in the centre the current is a maximum and the voltage a minimum. As a result a centre fed dipole has a low feed impedance (actually 73Ω in free space). This aligns with Ohm's law where a low voltage divided by the current will give a low impedance.
If the antenna is end fed, then it is fed at a voltage point and the feed impedance will be high.
Normally low impedance feed points are preferred because coaxial cable is often used and this has a relatively low impedance. 50Ω is standard for most applications including radio communications, professional usage and the like, but for domestic broadcast, i.e. television and radio, the standard impedance is 75Ω.
Proximity of other objects: It is found that if other objects are close to the antenna, then this will have a major effect on the feed impedance.
For example of a dipole is used in a larger Yagi antenna, then the parasitic elements will bring down the feed impedance dramatically. with a reflector and directors, the feed impedance of the dipole can fall from its free space value of 73Ω to values of 20Ω or less.
Height above ground: One of the major issues for HF antennas in terms of their feed impedance as well as many other aspects of their performance is the height above ground. The ground will have a major impact on the impedance whether it is a horizontal antenna or a vertical antenna. Typically as it gets higher above ground in terms of the number of wavelengths above ground, the effect reduces, but close to the ground it has a major impact.
Type of antenna: The type of antenna also has a major impact on the feed impedance. Not only do elements such as the feed position along the antenna have an impact, but so do other aspects such as whether the antenna is a horizontal dipole or a vertical quarter wave antenna.
There are many factors affecting the feed impedance of an antenna. When installing or designing an antenna, these should be taken into consideration to ensure the optimum feed is provided.
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.
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