Waveguide Impedance & Characteristic Impedance Matching

Like other transmission lines & feeder, waveguides have a characteristic impedance which require matching for maximum power transfer.

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The characteristic impedance of a waveguide is very important in many areas of their use.

Like other forms of feeder, waveguides have a characteristic impedance. By matching the waveguide impedance to the source and load, the maximum power transfer occurs on each occasion.

Waveguide impedance definition

There are several ways to define the waveguide impedance - waveguide characteristic impedance is not as straightforward as that of a more traditional coaxial feeder.

  • To determine the waveguide impedance by using the voltage to be the potential difference between the top and bottom walls in the middle of the waveguide, and then take the value of current to be the integrated value across the top wall. As expected the ratio gives the impedance.
  • Measure the waveguide impedance is to utilising the voltage and then use the power flow within the waveguide.
  • The waveguide impedance can be determined by taking the ratio of the electric field to the magnetic field at the centre of the waveguide.

Methods of determining the waveguide characteristic impedance tend to provide results that are within a factor of two of the free space impedance of 377 ohms, i.e. most results for the waveguide impedance fall between about 190 and 750Ω.

Waveguide impedance and reflection coefficient

To obtain the optimum power transfer between a waveguide and its source or load, the impedance of both items at the junction should be the same.

When the impedance of the waveguide is not accurately matched to the load, standing waves result, and not all the power is transferred. Similarly when a source is providing power to the waveguide and there is an impedance mismatch, then it is not possible for all the available power to be transferred.

To overcome the mismatch it is necessary to use impedance matching techniques.

Waveguide impedance matching

There are a number of ways in which waveguide impedance matching can be achieved. The main methods of impedance matching are summarised below:

  • Use of gradual changes in dimensions of waveguide.
  • Use of a waveguide iris
  • Use of a waveguide post or screw

Each method has its own advantages and disadvantages and can be used in different circumstances.

The use of elements including a waveguide iris or a waveguide post or screw has an effect which is manifest at some distance from the obstacle in the guide since the fields in the vicinity of the waveguide iris or screw are disturbed.

Waveguide impedance matching using gradual changes

It is found that abrupt changes in a waveguide will give rise to a discontinuity that will create standing waves as this is seen as an impedance mismatch. However gradual changes in impedance do not cause this as the gradual change is seen as a matching element in the system and not a mismatch.

This approach is used with horn antennas - these are funnel shaped antennas that provide the waveguide impedance match between the waveguide itself and free space by gradually expanding the waveguide dimensions.

There are basically three types of waveguide horn that may be used:

  • E plane
  • H plane
  • Pyramid

Impedance matching using a waveguide iris

Impedance matching within a waveguide can be providing by using a waveguide iris.

The waveguide iris is effectively an obstruction within the waveguide that provides a capacitive or inductive element . In this way this element is able to provide the required matching of the characteristic impedance of the waveguide.

The obstruction or waveguide iris is located in either the transverse plane of the magnetic or electric field. A waveguide iris places a shunt capacitance or inductance across the waveguide and it is directly proportional to the size of the waveguide iris.

An inductive waveguide iris is placed within the magnetic field, and a capacitive waveguide iris is placed within the electric field. These can be susceptible to breakdown under high power conditions - particularly the electric plane irises as they concentrate the electric field. Accordingly the use of a waveguide iris or screw / post can limit the power handling capacity.

Waveguide impedance match using an iris
Inductive and capacitive waveguide iris matching

The waveguide impedance matching iris may either be on only one side of the waveguide, or there may be a waveguide iris on both sides to balance the system.

A single waveguide iris is often referred to as an asymmetric waveguide iris or diaphragm and where there are two: i.e. one iris on each side of the waveguide, it is known as a symmetrical waveguide iris.

Symmetric and asymmetric waveguide iris diaphragms
Symmetric and asymmetric waveguide iris diaphragms

A combination of both E and H plane waveguide irises can be used to provide both inductive and capacitive reactance. This forms a tuned circuit. At resonance, the iris acts as a high impedance shunt. Above or below resonance, the iris acts as a capacitive or inductive reactance.

Impedance matching using a waveguide post or screw

In addition to using a waveguide iris, post or screw can also be used to give a similar effect and thereby provide waveguide impedance matching.

The waveguide post or screw is made from a conductive material. To make the post or screw inductive, it should extend through the waveguide completely making contact with both top and bottom walls. For a capacitive reactance the post or screw should only extend part of the way through.

When a screw is used, the level can be varied to adjust the waveguide to the right conditions.

Ensuring there is a good match between a waveguide and its source and load is essential if the waveguide is to provide optimum operation within and system and ensure that the benefits of its low loss are to be utilised properly. The different methods of providing a good impedance match can be used, the particular approach being dependent upon the system requirements.

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