Patent Number: 05999889&
Section: description

DESCRIPTION OF THE INVENTION The following description is presented solely for the purpose of disclosing how the present invention may be made and used. The scope of the invention is defined by the claims. FIG. 1 diagrams an equivalent circuit for a typical transmitting antenna 15. Transmitter 16 is represented by voltage source Vt in series with impedance Zt to model the reactance and losses in the transmitter. An impedance matching transformer 17 is typically used to match impedance Zt to the antenna impedance. The antenna impedance comprises gross resistance Rg, gross inductance Lg, and gross capacitance Cg. Cg includes the stray capacitive reactance of the antenna coupling to ground and the connecting cables. Lg includes the inductive reactance of the stray inductance in the connecting cables and the series inductance of a tuning coil or variometer typically used for tuning the resonant frequency of the antenna. Rg includes the resistance of the antenna feed cables and the antenna structure causing power to be lost as heat and the radiation resistance of the antenna. The voltage at the antenna feed point is designated as Va, and the current as Ia. The antenna voltage and current are functions of time that characterize the antenna impedance characteristics described above. Antenna voltage Va and current Ia may be measured according to well known techniques when the antenna is operating with a single frequency sinusoidal signal, but in modulated signals such as frequency shift keying each single frequency signal is of such short duration that measuring Va and Ia becomes a significant problem. Referring now to FIG. 2, antenna performance monitor 30 comprises antenna sensor 20, A/D converter 40, and data processor 50. Antenna sensor 20 comprises a capacitive voltage divider 21 connected between the antenna feedpoint at impedance transformer 17 and ground to generate antenna input voltage signal Vm and a current probe 22 connected in series with antenna 15 to generate antenna input current signal Im. The capacitors in voltage divider 21 have a capacitance ratio and a series reactance selected to scale antenna voltage Va to an appropriate voltage signal Vm. Current probe 22 senses antenna current Ia and generates a corresponding voltage signal Im. Current probe 22 may be, for example, a Pearson Electronics Model 310 RF Current Transformer. In operation, voltages Vm and Im are input to A/D converter 40. A/D converter 40 may be, for example, a Rapid Systems Model R2000 Analog to Digital Converter. Signals Vm and Im may be conducted to A/D converter 40 by signal link 23. Signal link 23, may be, for example, a double shielded cable to minimize interference from the magnetic field of the antenna. Alternatively, signal link 23 may be a fiber optic transmitter and receiver link for each of signals Vm and Im, such as a Dymec Model 6721 Fiber Optic Transmitter and Model 6722 Fiber Optic Receiver. A/D converter 40 is initialized and triggered by data processor 50 to digitize signals Vm and Im. The digitized Vm and Im data output from A/D converter 40 are input to data processor 50. A/D converter 40 typically includes anti-aliasing filters to filter out signals having frequencies higher than half the A/D sample rate. The sample rate should be higher than twice the highest transmitter signal frequency. FIG. 3 is a flow chart exemplifying the processing of signals Vm and Im through A/D converter 40 and data processor 50. Signals Vm and Im are anti-aliased by filters 402 and digitized by digitizers 404. The digitized samples are then bandpass-filtered by bandpass filters 406 to remove out-of-band frequencies and base-banded by complex mixers 408. The base-banded complex data is lowpass filtered by lowpass filters 409 and resampled by decimators 410 to a sample rate appropriate to the modulation bandwidth of the radio frequency signal. The decimated data is then subjected to a complex FFT 412. FFT 412 may have, for example, 1024 points. The complex FFT data is averaged over, for example, a 500 point evenly weighted average by averagers 414 resulting in frequency bin averages for Vm and Im at each of the frequencies in the modulation bandwidth of the radio frequency signal. Magnitude functions 416 generate the magnitude of Vm and Im respectively by calculating the square root of the sum of the squares of the real and imaginary coefficients of signals Vm and Im respectively for each frequency bin. Arctangent functions 418 generate the phase angles of Vm and Im by calculating the arctangent of the ratio of the real to the imaginary coefficients for each frequency bin. Impedance magnitude function 420 divides the magnitude of Vm by the magnitude of Im to output the impedance magnitude Zm for each frequency bin. Impedance phase function 422 subtracts the phase angle of Im from the phase angle of Vm to output impedance phase angle Zp for each frequency bin. Rg, Lg, Cg, and the resonant frequency of the antenna may be found from Zm and Zp at two or more frequencies as described in U.S. Pat. No. 5,233,537, included herein by reference thereto. The resonant frequency of the antenna and Rg may also be found by selecting the frequency bin having an impedance phase angle Zp closest to zero. The corresponding impedance magnitude Zm at this frequency is approximately Rg. The reactance for each frequency may also be found by subtracting Rg from Zm of the corresponding frequency bin. The resulting antenna performance parameters may be displayed according to well known techniques or transmitted to a remote location for further processing. Other modifications, variations, and applications of the present invention may be made in accordance with the above teachings other than as specifically described to practice the invention within the scope of the following claims.