Antenna system with quality-factor modulation
An electrically small antenna system modulates the Q factor of the antenna during electrical transmission to decrease the Q factor during signal transitions between frequency states thereby affecting an improved trade-off between antenna bandwidth and operating efficiency.
1 . An electrically small antenna system for use with a digitally modulated radio, the system comprising:
an antenna output adapted to connect to and control a current through an antenna radiator;
a radio signal input receiving a radiofrequency signal being a function of an encoding signal defining a series of digital symbols transmitted at symbol times separated at transition times;
an antenna driver circuit operating to apply power to a connected antenna radiator during symbol times and to dissipate power from the antenna radiator during transition times by changing an effective resistance in a path through the antenna radiator to lower an electrical quality factor (Q) of the connected antenna driver circuit and antenna radiator; and
wherein the changing of the effective resistance is synchronized with a transition between digital symbols to lower the electrical Q for at least a portion of that transition and less than an entire symbol time.
2 . The electrically small antenna system of claim 1 wherein the effective resistance is an electrical resistor switched into the path of current flow through the antenna and increasing the effective resistance along that path by at least 10%.
3 . The electrically small antenna system of claim 2 wherein the antenna driver circuit provides a semiconductor switch to insert and remove the electrical resistor into the path of current flow through the antenna to change the electrical Q of the connected antenna driver circuit and antenna radiator.
4 . The electrically small antenna system of claim 3 wherein the semiconductor switch is a four-quadrant switch.
5 . The electrically small antenna system of claim 3 wherein the antenna driver circuit employs a switch using silicon carbide transistors.
6 . The electrically small antenna system of claim 1 wherein the digital encoding signal defines a digital modulation selected from amplitude-shift keying, frequency-shift keying, and phase-shift keying.
7 . The electrically small antenna system of claim 1 further including a radio antenna connected to the antenna output.
8 . The electrically small antenna system of claim 1 wherein the antenna radiator has a largest dimension no more than 1/10 of a maximum wavelength of the driving signal.
9 . The electrically small antenna system of claim 1 wherein the antenna radiator conforms to the following equation:
ka<1
where k=2π/λ where λ is the maximum wavelength of the driving signal and
a is the radius of an imaginary Chu sphere circumscribing the antenna radiator.
10 . The electrically small antenna system of claim 1 wherein including a modulator receiving a carrier signal and the encoding signal to modulate the carrier signal to produce the radio signal input.
11 . The electrically small antenna system of claim 10 wherein the modulator provides digital modulation selected from amplitude-shift keying, frequency-shift keying, and phase-shift keying.
12 . An electrically small antenna system for use with a digitally modulated radio, the system comprising:
an antenna output adapted to connect to and control a current through an antenna radiator;
a radio signal input receiving a radiofrequency signal being a function of an encoding signal defining a series of digital symbols transmitted at symbol times separated at transition times;
an antenna driver circuit operating to apply power to a connected antenna radiator during symbol times and to dissipate power from the antenna radiator during transition times by changing an effective resistance in a path through the antenna radiator to lower an electrical quality factor (Q) of the connected antenna driver circuit and antenna radiator; and
wherein the effective resistance is provided by an amplifier communicating with a current monitor monitoring current through the antenna radiator, and operating during transition times to provide an output voltage that changes as a function of the monitored current to dissipate power in the antenna radiator.
13 . The electrically small antenna system of claim 12 wherein the output voltage changes as a function of the monitored current to oppose the monitored current.
14 . The electrically small antenna system of claim 12 wherein the amplifier monitors current through the antenna radiator to boost power in the antenna radiator during the symbol times.
15 . The electrically small antenna system of claim 12 wherein the amplifier is regenerative to both provide power to the connected antenna and to absorb power from the connected antenna at different times.