As with any other type of antenna, ensuring that a good match between the feeder and the antenna itself are crucial to ensure the performance of the antenna can be optimised.
The impedance of the driven element is greatly affected by the parasitic elements and therefore, arrangements needed to be incorporated into the basic design to ensure that a good match is obtained.
Feed impedance of Yagi driven element
It is possible to vary the feed impedance of a Yagi antenna over a wide range. Although the impedance of the dipole itself would be 73Ω in free space, this is altered considerably by the proximity of the parasitic elements.
The spacing, their length and a variety of other factors all affect the feed impedance presented by the dipole to the feeder. In fact altering the element spacing has a greater effect on the impedance than it does the gain, and accordingly setting the required spacing can be used as one design technique to fine tune the required feed impedance.
Nevertheless the proximity of the parasitic elements usually reduces the impedance below the 50Ω level normally required. It is found that for element spacing distances less than 0.2 wavelengths the impedance falls rapidly away.
Yagi matching techniques
To overcome this, a variety of techniques can be used. Each one has its own advantages and disadvantages, both in terms of performance and mechanical suitability. No one solution is suitable for all applications.
The solutions below are some of the main solutions used and applicable to many types of antenna. There also not the only ones:
- Element spacing: There is a level of impedance variation that can be provided by altering the spacing between the elements. However it is not possible to bring the feed impedance back up to 50Ω needed for most feed applications. .
- Balun: A balun is an impedance matching transformer and can be used to match a great variety of impedance ratios, provided the impedance is known when the balun is designed.
- Folded dipole: One method which can effectively be implemented to increase the feed impedance is to use a folder dipole. In its basic form it raises the impedance four fold, although by changing various parameters it is possible to raise the impedance by different factors.
- Delta match: This method of Yagi impedance matching involves "fanning out" the feed connection to the driven element.
- Gamma match: The gamma match solution to Yagi matching involves connecting the out of the coax braid to the centre of the driven element, and the centre via a capacitor to a point away from the centre, dependent upon the impedance increase required.
It is found that adding parasitic elements to a dipole antenna reduces the feed impedance. Values of 20Ω and lower are experienced. As a result of this it is necessary to take steps to bring the impedance back up to a more convenient level.
Whilst other methods may bring the impedance to the correct region for feeding, adjusting he spacing can trim the impedance to provide the optimum match.
Balun for Yagi matching
The balun is a very straightforward method of providing impedance matching. 4:1 baluns are widely available for applications including matching folded dipoles to 75Ω coax.
Baluns like these are just RF transformers. They should have as wide a frequency range as possible, but like any wound components they have a limited bandwidth. However if designed for use with a specific Yagi antenna, this should not be a problem.
One of the problems with a balun is the cost - they tend to be more costly than some other forms of Yagi impedance matching. They may also be power limited for a given size.
The folded dipole is a standard approach to increasing the Yagi impedance. It is widely used on Yagi antennas including the television and broadcast FM antennas.
The simple folded dipole provides an increase in impedance by a factor of four. Under free space conditions, the dipole impedance on its own is raised from 75Ω for a standard dipole to 300Ω for the folded dipole.
Note on folded dipole:
The folded dipole is a from of dipole that has a higher impedance than the standard half wave dipole - in the standard version it has four times the impedance. However different ratios can be obtained by changing the mechanical attributes.
Read more about the folded dipole.
Another advantage of using a folded dipole for Yagi impedance matching is that the folded dipole has a flatter impedance versus frequency characteristic than the simple dipole. This enables it and hence the Yagi to operate over a wider frequency range.
While a standard folded dipole using the same thickness conductor for the top and bottom conductors within the folded dipole will give a fourfold increase in impedance, by varying the thickness of both, it is possible to change the impedance multiplication factor to considerably different values.
The delta match for of Yagi matching is one of the more straightforward solutions. It involves fanning out the ends of the balanced feeder to join the continuous radiating antenna driven element at a point to provide the required match.
Both the side length and point of connection need to be adjusted to optimise the match.
One of the drawbacks for using the Delta match for providing Yagi impedance matching is that it is unable to provide any removal of reactive impedance elements. As a result a stub may be used.
The gamma match is often used for providing Yagi impedance matching. It is relatively simple to implement.
As seen in the diagram, the outer of the coax feeder is connected to the centre of the driven element of the Yagi antenna where the voltage is zero. As a result of the fact that the voltage is zero, the driven element may also be connected directly to a metal boom at this point without any loss of performance.
The inner conductor of the coax is then taken to a point further out on the driven element - it is taken to a tap point to provide the correct match. Any inductance is tuned out using the series capacitor.
When adjusting the RF antenna design, both the variable capacitor and the point at which the arm contacts the driven element are adjusted. Once a value has been ascertained for the variable capacitor, its value can be measured and a fixed component inserted if required.
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