![]() Alternatively, the power level can be reduced at the sweeper, but it is usually best to work with a minimum probe depth and maximum source power to maintain a good signal to noise ratio. If it does not, reduce the received power level by reducing the penetration depth of the probe. This can easily be checked by decreasing the power level by 5 db with the variable attenuator and verifying that the power reading indicated on the SWR meter drops by the same amount on the db (bottom) scale. To obtain accurate results with the slotted line, it is critical that the signal level be low enough so the diode is operating in the square-law region. Refer to the procedure in Figure 3-5 of the user's manual (page attached). For small or large values, further adjustment is required. At this point, if the SWR is a moderate value, it can be read directly from the SWR = 1 to 4 scale. Now move the probe carriage to a voltage minimum. Adjust the probe carriage for a maximum reading on the SWR meter, then adjust the gain and range of the meter to obtain exactly a full-scale needle deflection. Connect the "Unknown Load" to the end of the slotted line. Set the SWR meter's range control to 0 db and set the Expand control to NORM. Measurement of SWR: Turn on the modulation function of the sweep generator (the SWR meter will not work unless the input is modulated at 1 khz). (Turn off the modulation during this measurement.) 3. 2ģ Optional: Verify the source frequency with a microwave frequency counter. Do this for two frequencies: 9 GHz and 11 GHz. Since the voltage minima are known to occur at spacings of λ g /2, the guide wavelength can be determined. Note that voltage minima are more sharply defined than voltage maxima, so the minima positions lead to more accurate results. Adjust the SWR sensitivity for a reading near midscale, adjust the carriage position to locate several minima, and record these positions from the scale on the slotted line. Place the blank flange on the load end of the slotted line. Be sure to detune the wavemeter after frequency measurement. ![]() The wavemeter is a tunable resonant cavity, and is used by tuning it until a dip is registered on the SWR meter the frequency is then read from the scale of the wavemeter. Measure the frequency with the wavemeter. Adjust the gain knobs of the SWR meter until the needle moves near maximum (i.e., to the right). Set the source to continuous wave (CW) at 9 GHz with power 0 dbm and enable the RF output and 1 khz modulation. Measurement of Guide Wavelength: First you will measure and verify the signal frequency and wavelength. We are using X-band waveguide, with a=0.9", and a recommended operating range of GHz for dominant mode operation. Setup: Set up the equipment as shown below. Equipment Needed: X-band signal generator with 1 khz pulse modulation capability HP 415E SWR meter coax-to-waveguide adapter waveguide isolator cavity wavemeter precision adjustable attenuator slotted line and detector waveguide matched load waveguide section (1 m long) fixed waveguide attenuator (3 to 10 db) slide-screw tuner blank waveguide flange Unknown load 1Ģ 1. If the power level is too high, the small signal condition will not apply for very low power, the signal will be lost in the noise floor. The detector diode must operate in the square law region for good behavior and accurate results. Make sure that you understand the difference between "guide wavelength" and "wavelength" (see equation 3.87). Before doing the experiment, read The Slotted Line in section 2.4 of the text. While the slotted line is cumbersome to use and gives less accurate results when compared with the automated vector network analyzer, the slotted line is still the best way to learn about standing waves and impedance mismatches. This experiment also introduces the student to common waveguide components such as waveguide-to-coax adapters, isolators, wavemeters, slide-screw tuners, detectors, and attenuators. The 415E has scales calibrated in SWR, and relative power in db. To obtain good sensitivity, the RF signal is modulated with a 1 khz square wave the SWR meter contains a narrowband amplifier tuned to this frequency. This signal is measured with the HP 415E SWR meter. The diode operates in the square-law region, so its output voltage is proportional to power on the line. Slotted lines can be made with any type of transmission line (waveguide, coax, microstrip, etc.), but in all cases the electric field magnitude is measured along the line with a small probe antenna and diode detector. The Slotted Line Introduction: In this experiment we will use a waveguide slotted line to study the basic behavior of standing waves and to measure SWR, guide wavelength, and complex impedance. 1 ECE 584 Microwave Engineering Laboratory Experiments 1. ![]()
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