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
Figure 9: Dipole
Receiving Antenna
(Front View)
Figure 10: Dipole
Receiving Antenna
(Side View)
Results
Figure 3: Spark Gap and Transmitting Antenna
Figure 4: Transmitting Antenna Circuit Diagram
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 8001000 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.
Figure 11: Spark Gap and Transmitting Antenna Showing Spark
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
Constructing the stand that held the transmitting and receiving antenna
and reflector was a crucial task of this project. The apparatus had to be
able to be separated to precise, desired distances by casual users so an
optics bench was used. This allowed the antennas and reflector to be
moved easily to accurate distances. Since the optics bench is made of
iron, the apparatus had to be mounted a considerable distance from the
microwave field components because electromagnetic radiation reflects off
conductive materials. The stand had to be stable and rigid so the
apparatus would not move and measurements could be made. PVC was
used to hold the apparatus for its low cost, availability, and nonconductivity. The reliability and design of the stand is crucial because
observations and electric field detection are strongly dependent upon it.
Figure 5: High Voltage Power Supply Circuit Diagram
Figure 1: Theoretical Microwave Electric Field From a Dipole Transmitting Antenna
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 2: Spark Gap and Transmitting Antenna Diagram
Figure 6: Graph of the Charging/Discharging Capacitor in the Spark Gap
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 microwaveinduced alternating current (Figure 12). The signal is then greatly
amplified by a LF356 op-amp where it is then displayed on the multimeter.
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.
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.
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.
Figure 12: Actual Current in Receiving Antenna With Diode in Circuit
References
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.
Figure 7: Initial Receiving Antenna
Figure 8: Receiving Antenna Circuit Diagram
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.
3. A microwave generator, receiver, and reflector. Retrieved June 6, 2007, from
ocw.mit.edu/NR/rdonlyres/Physics/8-022Fall-2004/D9D35AB5-86FA-4651-9B51EA8660685CEE/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
Wilser, S. (2007). Clothespin Spark Gap Microwave Radio Transmitter and Receiver. Poster
presentation, NYSS APS-AAPT Fall Symposium, Syracuse University, September 29, 2008.