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Impedance Matching

 A practical experiment demonstrating the importance of impedance matching in a waveguide system 

This article has been written by Konstantinos Giannopoulos. He has graduated from the University of Portsmouth in UK. He has a BSc degree in Computer Technology and an MSc degree in Communication Networks Planning and Management. For more information you could visit his site here .

The article demonstrates a practical experiment to show the importance of impedance matching in a waveguide microwave system. Matching is always an issue not only in microwaves but also in audio and not only in waveguide systems but also in coaxial. Impedance mismatching introduces standing waves that cause power losses, reduction in power handling capability and frequency instability. Thus, it is very important to match a system and this is demonstrated by this experiment.

The components that were used for the experiment were:

  • Homebrew sensitive current meter (1-100 microamperes)
  • A.P. 63907 T.A.C. crystal holder
  • AEI CS9-B mixer diode used for detection only
  • W.H. Sanders ST16 3-pole tunable filter/attenuator
  • F.X.R. X144A cavity frequency meter
  • C&K DT-435T gunnplexer module used for gunn oscillation only

The circuit setup can be shown in the next photo. The gunn oscillator produces an X-Band carrier signal. The wave meter next to it measures the frequency of this signal by allowing it to pass through it to the detector, only when it resonates to the signal frequency. With the current setup the wave meter resonates at about 9.113GHz. Next to the meter the waveguide filter/attenuator is connected to allow for the settings to be done and finally, the detector is used to detect the carrier and send an amount of current to the current meter.

 

 

 Before switching on the gunn oscillator some final checks were made. The current meter polarity had to be done negative because the AEI CS9-B mixer diode used, has negative polarity. The attenuator of the meter was calibrated to 5Kohm to allow a greater current to be read and to extend the scale of the microampere meter.

The first try was without the filter. The meter read a current of 45 microamperes max (including the 5Kohm attenuator). Then the filter was added with the tuning screws not injected into the waveguide (un-tuned). The current reading had also a value of 45 microamperes.

At this time, the load is not fully matched to the source and there are no tuning screws injected into the system, except of the screw of the gunn oscillator to set it to the right frequency. The crystal holder has a ramp inside that acts as a transformer for the high frequencies for better matching but this is not enough. By making several adjustments to the screws of the filter, a meter current reading of more than 100 microamperes can be achieved. That is because the load has been fully matched now to the source by using the matching screws to vary the capacitance inside the waveguide. The standing waves have been minimized and more power reaches the detector.

The results of the experiment can be summarized in the next table.

 

 

Impedance matching results

 

Operation

 

Current meter reading

Operating without filter or un-tuned filter

< 45 microamperes (5Kohm attenuated)

Operating with tuned filter

>100 microamperes (5Kohm attenuated)

 

            It can be shown from the results that an impedance matched system can be at least 55% more efficient than an un-matched system. In practice, based on the system that the tuning filters are used, this can be translated into more effective radiated output power, a more sensitive receiver and generally a greater amount of useable RF power without adding more components and amplifiers to the system and increasing the noise and cost. Impedance matching is an important issue in radio microwave communications that should always be considered when designing and constructing a system.

 

 
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