Waveguide Impedance and Impedance Matching

- details of waveguide impedance, how waveguide impedance is defined, waveguide impedance matching including the use of a waveguide iris or a waveguide post.

Waveguide impedance can be important in a number of applications. In the same way that the characteristic impedance is important for other forms of feeder, the same can be true in a number of instances with waveguides. Techniques including the use of a waveguide iris, or a waveguide post can be used to provide the required level of waveguide impedance matching.

The waveguide impedance needs to be known on a number of instances to ensure the optimum power transfer and the minimum level of reflected power is obtained.

Waveguide impedance definition

There are several ways to define the waveguide impedance - it 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.

All the methods tend to give results that are within a factor of two of the free space impedance of 377 ohms.

Waveguide impedance and reflection coefficient

In just the same that more common coaxial and other feeder systems need to have loads closely matched to the source impedance to obtain the maximum power transfer, the same is true with waveguides. If the waveguide impedance is matched to the source or load, then a greater level of power transfer will occur.

When waveguides are not accurately matched to their loads, standing waves result, and not all the power is transferred.

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

Waveguide impedance matching

In order to ensure the optimum waveguide impedance matching is obtained, small devices are placed into the waveguide close to the point where the matching is needed to change its characteristics.

There are a number of ways in which waveguide impedance matching can be achieved:

  • 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. However gradual changes in impedance do not cause this.

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

The different types of gradual matching using a waveguide horn can be seen in the diagram below:

Diagram showing the waveguide matching using a horn antenna: E-plane; H-plane and pyramid versions shown
E, H plane and pyramid Horn antennas used for waveguide matching

Impedance matching using a waveguide iris

Irises are effectively obstructions within the waveguide that provide a capacitive or inductive element within the waveguide to provide the impedance matching.

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.

The way of using an iris to provide waveguide impednace matching
Impedance matching using a waveguide iris

The waveguide 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 one where there are two, one either side is known as a symmetrical waveguide iris.

Views of Symmetrical and asymmetrical waveguide iris implementations.
Symmetrical and asymmetrical waveguide iris implementations

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.

By Ian Poole

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