Assignment: Waveguides

Purpose and Discussion

The purpose of this simulation is to demonstrate the characteristics of waveguides. A waveguide is any medium which guides waves. Waveguides are used for frequencies above several giga-hertz where coaxial cable begins to exhibit skin effects and radiation attenuation and losses. The electromagnetic wave that is injected at the input by a signal launcher is bounded by the waveguide and is reflected off of the conducting walls. A wide spacing is provided which allows the transfer of hundreds of thousands of watts of power without breaking down any non-existing dielectric barriers between the conductors as is the case with coaxial cable. This results in very little loss. The signal is then received by a signal absorber. Attention to waveguide dimensions is crucial. The frequency bands of waveguides have assigned designated letters to bands of specified dimensions, frequencies and cutoff wavelengths. The cutoff frequency of a waveguide is the lowest frequency that will propagate through the conducting tube in an actual waveguide. This frequency is normally discussed in terms of wavelength as it is the length of the wave that limits its ability to propagate. Actual waveguides behave much like high pass filters providing high attenuation at frequencies below cutoff.

The characteristic impedance is the impedance that would be measured at the input of an infinite length of waveguide and is given by:

Z_OL = 120π / [1 - (λfs/λ_CL)]½     where  λfs = c/f = freespace wavelength

and λ_CL = 2 (broadwall dimension of the waveguide)

Parts

Resistors: 55 ? Sample Waveguide AC Voltage Source

Test Equipment

• Oscilloscope

Procedure

Figure 21-1 Waveguide Example

1. Connect the circuit illustrated in Figure 21 -1.

2. Calculate the lowest acceptable propagation frequency for a C-Band waveguide. Note your results in the Data section of this experiment. Select the frequency of the AC source = 4.9 GHz.

3. Double-click the sample waveguide. Choose EDIT MODEL. Change the SPICE parameters so that LEN = 4.039 e-002.

4. Double-click the Oscilloscope to view its display. Set timebase to 0.2 ns/Div and Channel A = 1 V/Div. Run the simulation and observe the output waveform. Measure the frequency to verify that the expected signal was propagated by the simulated waveguide. Note your results.

5. Select a frequency of 7.05 GHz for the AC voltage source. Run the simulation again and note your results. Verify that the expected signal was propagated by the simulated waveguide. Run the simulation again and note your results.

6. Calculate the characteristic impedance for the C-Band waveguide.

Expected Outcome

Figure 21-3 Time versus Amplitude of Waveguide Output

Data for Experiment

f_CL = _____

Table 21-1

Z_OL = ___________

Additional Challenge

Repeat all steps for a P-Band waveguide. Verify your data.