The relationship between the wavelength in free space and the guide wavelength. Furthermore, this experiment will be a way in which to gain experience in using different types of laboratory communications equipment. Introduction What is wavelength? Wavelength of a sinusoidal wave is the distance between identical points in the adjacent cycles of a waveform signal.
Wavelength is commonly designated by the Greek letter lambda (? )Wavelength is inversely correlated to frequency (figure 1. 1), therefore the higher the frequency of the signal, the shorter the wavelength.
vp Is the phase velocity f is the frequency vp Is the phase velocity f is the frequency ?=vpf Figure 1. 1 What is a wave guide? Figure 1. 2 A waveguide is a special form of transmission line consisting of a rectangular (figure 1. 2) or cylindrical metal tube or pipe, through which electromagnetic waves are propagated in microwave and RF communications. It s commonly used in microwave communications, broadcasting, and radar installations.
A waveguide must have a certain minimum diameter relative to the wavelength of the signal and therefore are practical only for signals of extremely high frequency.
Consequently below such frequencies, waveguides are useless as electrical transmission lines. “An electromagnetic field can propagate along a waveguide in various ways. Two common modes are known as transverse-magnetic (TM) and transverse-electric (TE). In TM mode, the magnetic lines of flux are perpendicular to the axis of the waveguide.
In TE mode, the electric lines of flux are perpendicular to the axis of the waveguide. Either mode can provide low loss and high efficiency as long as the interior of the waveguide is kept clean and dry.
” Some disadvantages are: * The high cost, since the material used is special alloy (copper and silver). * It is not possible to pass DC currents along with your RF signal. * The volume and mass particularly are at lower frequencies. Although there is quite a few disadvantages the fact that you can transmit extremely high peak powers and very low loss outweighs it.
Furthermore the Silver plating used on the inside walls of the waveguide decreases the resistance loss making the copper and aluminium waveguides even more efficient. Experimental notes The experiment needs to be conducted to obtain the value of the guide wavelength and thereafter calculate the wave dimension and observe whether it matches the initial result that was measured. The initial result measured was the value.
Cut-off wavelength for the wave guide mode being propagate ? o = Wavelength in free space g = Guide wavelength, known as delta g The equation to measure ? o is already present as it is given by free-space. Although the cut-off wavelength can be calculated by rearranging the formula in figure 1. 1, the experiment will be used to further confirm this mathematical formula. What we will acquire is a range of guide wavelengths throughout the experiment in order to find the Cut-off wavelength. Where is the cut-off wavelength for the waveguide mode being propagated? The dominant mode is being propagated in the rectangular waveguide (figure 1. 2) which means where (a) is the internal broad dimension of the rectangular waveguide.
Block diagram Microwave signal source Isolator Preset attenuator Wave meter Short circuit Calibrated attenuator Tuned SWR amplifier + meter Standing wave detector Microwave signal source Isolator Preset attenuator Wave meter Short circuit Calibrated attenuator Tuned SWR amplifier + meter Standing wave detector Microwave signal source This device is the signal generator where you get 8 to 12 GHz. It is extremely expensive equipment and costs in the region of ? 20,000. This is due to the fact that we are dealing with high frequency signals and not with normal radio waves.
Isolator By terminating one port, a circulator becomes an isolator, which contains the property for energy to flow in one direction only. It samples some of the forward wave power and couples it to a calibrated cavity wave meter for measuring the oscillator frequency. Preset attenuator “Attenuators are essential building blocks when developing test stations for applications” Attenuators are devices used to adjust signal levels which helps to stop the reflected power from reaching the oscillator, control impedance mismatch and to isolate circuit stages. Wave meter
Any device for measuring the free-space wavelengths (or frequencies) of microwaves; usually made of a cavity resonator whose dimensions can be varied until resonance with the microwaves is achieved. The determination is often made indirectly, by measuring the frequency of the wave. Calibrated attenuator The calibrator changes the value if it gets too high. Attenuators are manufactured with high-accuracy calibration, and for utmost precision. They available in standard waveguide were each attenuator is calibrated at the frequency specified at the time of order. Standing wave detector
Standing wave detector detects radio frequency signals along a transmission line or in a waveguide and changes it into a DC voltage for the reason that the waveguide cannot transmit DC currents along with the RF signal. Tuned SWR amplifier + meter The standing wave ratio meter measures the SWR (standing-wave-ratio) which is the ratio of the amplitude of a partial standing wave at a maximum to the amplitude at a minimum in a transmission line. This is an item of radio equipment used to check the quality of the match between the antenna and the transmission line.
Procedure 1. Set up the microwave bench, as indicated. 2. Read the basic instructions for the microwave bench and then obtain oscillations at 8. 5 GHz from the microwave signal source. 3. If it is possible, maximize the deflection on the SWR-meter by using the method outlined in the basic instructions. 4. Measure the frequency, f, using the wave meter. Calculate the free space wavelength, ? o , by using ? o = c/f where c = 2. 997? 1010 cm s-1. 5. Move the standing wave detector (SWD) probe along the slotted line and watch the SWR-meter.