1. Clothespin Microwave Transmitter and Receiver By: Steven Wilser wilssm14@mail.buffalostate.edu Adviser: Dr. Dan L. MacIsaac Department of Physics Buffalo State College Funded by the Early Undergraduate Research Program at Buffalo State College Abstract This project studied and mapped the electric field produced by a microwave spark gap antenna using low cost apparatus and supplies. A spark gap microwave transmitting antenna made from a household clothespin was driven by a high voltage current limited power supply. A receiving antenna made from household wiring flex (Romex) was used to detect the electric field where the intensity of the field was displayed on a multi-meter. A low cost reflector was used to create standing waves within the electric field which enabled measurement of the wavelength of the radiation by measuring the distance between nodes. Linear arrays of wire were used as polarizers to analyze the signal. Turning the receiving antenna ninety degrees perpendicular to the transmitting antenna will cause the receiving antenna to pick up zero signal. There is also no signal propagating directly out from the ends of the antenna. Introduction This project was designed to be integrated into the Physics 112 and other electricity and magnetism courses as a demonstration or laboratory, to have students map and reflect the electric field (Figure 1) in a standard lab activity. Methods continued References The spark gap is driven by a constructed high voltage power supply with limited current (Figure 5). A low voltage, moderate current laboratory power supply provides 20 volts which is rapidly switch on and off through a MJE3055T bipolar transistor. The 20 volts is then stepped up to 800- 1000 volts through a step-up transformer. By the law of conservation of energy as the voltage increases the current must decrease. Megaohm resistors further limit the current to keep the spark gap safe for user skin contact. The capacitor on the transmitting antenna is slowly charged and rapidly discharges across the spark gap resulting in slow charge/rapid discharge bursts. This is an example of a “relaxation oscillator.” The charge resonating back and forth through the dipole antenna is analogous to the ringing of a bell. After the charge is produced its intensity decays exponentially with time, like the sound of a bell becoming fainter with time after it has been hit. See Figure 6, this was experimentally verified. Results The wooden clothespin was most efficient compared to the plastic clothespin because it was more stable and rigid. The plastic clothespin could not be tuned to the precise distance the spark gap needed to be in order to conduct. The wooden clothespin being more rigid could be tuned to precise gap distances. The wooden clothespin was also able to maintain the desired gap distance for a longer period of time without having to retune. The second, “pigtail” version of the receiving antenna was more efficient because it was noise resistant and was more sensitive to the electric field. This version of receiving antenna was able to detect more electric field signal and block out extraneous signals. The “pigtail” antenna are also stronger, durable, and can withstand extensive use in the lab. Discussion The wooden clothespin was most efficient compared to the plastic clothespin because it was more stable and rigid. The plastic clothespin could not be tuned to the precise distance the spark gap needed to be in order to conduct. The wooden clothespin being more rigid could be tuned to precise gap distances. The wooden clothespin was also able to maintain the desired gap distance for a longer period of time without having to retune. The second, “pigtail” version of the receiving antenna was more efficient because it was noise resistant and was more sensitive to the electric field. This version of receiving antenna was able to detect more electric field signal and block out extraneous signals. The “pigtail” antenna are also stronger, durable, and can withstand extensive use in the lab. Without the rectifying diode in the receiving antenna circuit, the signal would not display on the voltmeter. The diode must also be put in the correct orientation. 1. Pine, J., & King, J., & Morrison, P., & Morrison, P., (1996). Zap!: Experiments in electrical currents and fields. Boston, MA: Jones and Bartlett Publishers, Inc.. 2. Experiment 3: Hertz’s microwave experiment. Retrieved June 6, 2007, from web.mit.edu/8.022/www/labs/lab3.pdf. 3. A microwave generator, receiver, and reflector. Retrieved June 6, 2007, from ocw.mit.edu/NR/rdonlyres/Physics/8-022Fall-2004/D9D35AB5-86FA-4651-9B51- EA8660685CEE/0/lab3.pdf. 4. Electricity and Magnetism Experiments from Kits. Retrieved July 11, 2007, from http://ocw.mit.edu/rdonlyres/FBEC1B73-A8D1-4307-A02B- 48D399852FA7/0/802x.pdf. Figures 3,7,9,10 and 11, photos by Steven Wilser Figures 1,2,4,5,6,8 and 12 from Massachusetts Institute of Technology Figure 1: Theoretical Microwave Electric Field From a Dipole Transmitting Antenna Figure 3: Spark Gap and Transmitting Antenna Figure 4: Transmitting Antenna Circuit Diagram Figure 11: Spark Gap and Transmitting Antenna Showing Spark Figure 5: High Voltage Power Supply Circuit Diagram Figure 6: Graph of the Charging/Discharging Capacitor in the Spark Gap Methods A spark is produced when a high electric field causes the air to conduct. When the spark occurs a current is established and the charges in an antenna accelerate. This acceleration causes the charges to radiate electromagnetic waves. Each subsequent spark produces a burst of radiation. The spark gap is improved at radiating when the current in the spark oscillates along a dipole antenna. Dipole antennae are most efficient when the antenna is a half wavelength long. See Figures 2, 3 and 4. Different materials were experimented with in the construction of the transmitting antenna such as plastic and wood clothespins. Reliably mounting and adjusting the geometry of transmitting, receiving and reflecting components proved unexpectedly and considerably challenging. The receiving antenna is a resonant dipole that is the same length as the transmitting antenna. The radiation the receiving antenna detects generates a voltage which is passed through a rectifying diode. This effectively cuts out the negative portion of the received microwave- induced alternating current (Figure 12). The signal is then greatly amplified by a LF356 op-amp where it is then displayed on the multi- meter. In the first version of receiving antennas, the rectifying diode was placed between the two brass arms of an antenna dimensionally matched to the transmitting antenna, and was connected to a coaxial cable to provide a grounded shield to minimize the pickup of unrelated signals. See Figures 7 and 8. In the second “pigtail” version, the receiving antenna is made of a single piece of wire with a one-turn loop in the center. Each end is ¼ wavelength long and the microwave radiation received is rectified by a diode. The resulting direct current runs through the op-amp and through a shielded cable. The loop in the center of the antenna provides stability and acts as a one-turn inductor which prevents low frequency fields from affecting observations. See Figures 9 and 10. Figure 2: Spark Gap and Transmitting Antenna Diagram This experiment and lab activity is modeled after some work at the Massachusetts Institute of Technology (MIT) (See Figures 1,2,4,5,6,8 and 12) and California Institute of technology (CalTech) physics courses. Various types of transmitting and receiving antenna were constructed and experimented with along with different types of stands to hold the apparatus. This lab activity would be important for students studying electricity and magnetism because sparks and electric fields are topics analyzed in these courses. Figure 7: Initial Receiving AntennaFigure 8: Receiving Antenna Circuit Diagram Figure 9: Dipole Receiving Antenna (Front View) Figure 10: Dipole Receiving Antenna (Side View) Figure 12: Actual Current in Receiving Antenna With Diode in Circuit