Electrical Engineering Department
BS in Electrical Engineering
Two-way communications using 3 to 30 MHz, high-frequency (HF) radio, also known as shortwave radio, provides worldwide coverage with no infrastructure required between stations. Amateur or “Ham” radio operators apply this advantage to long distance communications and to provide disaster relief communications. Optimum frequencies for long distance propagation are time-of-day variant necessitating an operating frequency range of 3 to 30 MHz. HF half wave dipole antenna length varies from 15 to 150 ft over this range. Fixed-length antenna impedance varies with frequency due to physical dimension dependent capacitance and inductance. Maximum transmitter to antenna power transfer occurs when the transmission line and antenna impedances are matched to the standard 50 Ω radio impedance. For transmitter to antenna impedance matching, antenna length can be adjusted depending on frequency. Alternately, an antenna tuner utilizing a variable inductance and capacitance matching network can compensate for the frequency dependent impedance of a fixed-length antenna. A dipole antenna designed for one frequency can function over a broader frequency range with a manually or automatically adjusted antenna tuner.
The project objective is to develop a microcontroller-based automatic tuner for a fixed length dipole antenna. In high frequency transmission lines, an antenna impedance mismatch causes power reflection back to the transmitter. This corresponds to a greater than unity voltage standing wave ratio (VSWR), the ratio of maximum to minimum transmission line voltage amplitude. A directional wattmeter provides forward and reflected power values to an Atmel ATmega32 microcontroller, which calculates VSWR and adjusts a capacitor - inductor matching network using stepper motors to reduce VSWR to less than 1.5:1 (4% reflected power relative to incident). The antenna tuner system can match loads of up to 26:1 initial VSWR within a frequency range of 3.5 MHz to 30 MHz. The system was tested using a 100 W transceiver connected to a 90 ft, 450 Ω balanced transmission line and a 120 ft dipole antenna 30 ft above ground. Commercial antenna tuners have VSWR matching capabilities from 3:1 to 100:1.