Patent Application
20130016018
This invention relates to a system for automatically tuning an electrically small transmitting loop antenna and more particularly to a system for automatically tuning a portable antenna, man pack antenna, or an antenna embedded in a garment that radiates at a frequency with a wavelength much greater than the physical size of the antenna.
Radiation of frequencies with wavelengths much larger than the space available for an antenna is often necessary. This results in the use of an electrically small antenna. An electrically small antenna has a very small bandwidth; therefore, small perturbations in the shape of the antenna or in surrounding object movement results in mistuning of the antenna to the extent that it does not radiate electromagnetic energy.
In transmitting antennas which have a narrow tuned bandwidth, it is important to tune the antenna to the transmitted frequency in order to achieve optimum performance in which maximum power is radiated. If the antenna environment changes during the period of a transmission, it is necessary to dynamically adjust the antenna and the antenna matching circuits.
Prior to the present invention, many systems for automatically tuning transmitting antennas were known. In some of these systems, the antenna is automatically tuned to the transmitted frequency using a forward and reflected power measurement. This method enables locating the measurement instruments remotely from the actual radiating element; however, it is difficult to determine if the antenna is above or below resonance. This is because the loop antenna, commonly used, normally returns an inductive reactance from both above and below resonance when only an antenna bandwidth or two in frequency away from the antenna resonance frequency. Additionally, the transmission line separating the antenna from the measurement point transforms the antenna impedance observed by the measuring circuit complicating the determination of the feedpoint impedance.
Prior antenna systems have also used a phase discriminator in the antenna feed line either at the transmitter or at the antenna. Again, as an electrically small loop antenna will not necessarily present a feedpoint capacitive reactance component when tuned off resonance, it is not possible to determine if the loop tuning capacitor should be increased or decreased to achieve antenna resonance.
Not being able to readily determine the radiating element complex impedance means that closing a simple null seeking feedback loop cannot be implemented for resonant frequency tuning and subsequent tracking with movement of the antenna or the surroundings of the antenna.
Additionally, prior art tuning of electrically small loop antennas is accomplished with a mechanical capacitor that is tuned by a rotating actuator such as a direct current motor or a stepping motor. This results in a system that is bulky, heavy and uses considerable primary power.
Radiation of electromagnetic energy from an electrically small antenna is difficult because the bandwidth over which the antenna has a resistive radiation component is extremely small and the radiation resistance and center frequency varies with slight changes in the antenna position, minor loop distortion and changes in the surroundings.
The primary purpose of this invention is to facilitate automatic initial tuning and keeping an electrically small antenna worn on a man's body tuned to the resonant frequency, even with normal body motion changing the terminal impedance of the untuned antenna. The logical extension of this concept is to enable the automatic tuning of an electrically small antenna in any changing physical environment.
A loop antenna is used for this application because a counterpoise is not required for a radiating current distribution. To feed power to a loop, either two points must be tapped on the circumference, constituting an inductive auto-transformer, or a transformer core must encompass the loop at a point on the circumference to inductively transform the real impedance to that of the driving radio frequency transmitter. Either method yields an inductive feedpoint impedance at frequencies well below resonance. Additionally, an inductive feedpoint impedance is also presented at frequencies well above the antenna resonance. This means that it is not possible with a feedpoint measurement to determine if a tunable reactive component intended to resonate the antenna must be increased or decreased in value.
It is, however, possible to track small changes in the antenna impedance caused by antenna movement or movement of the near field surroundings of the antenna with a feedpoint measurement, but only over a bandwidth comparable to that of the very narrow bandwidth of the antenna. This makes practical application of this technique difficult and limited.
Another problem with constructing a continuously tunable system is the lack of availability of a voltage variable reactance that can operate at the currents and voltages incurred in a resonant transmitting loop antenna. At power levels of 10 watts it is not uncommon to create voltages across a tuning capacitor of two thousand zero-to-peak volts.
If the current within the loop antenna, not the feedpoint terminal current, is compared to the feedpoint terminal voltage, a true monotonic indication of the center frequency of the loop is obtained. This is accomplished with a differential current transformer whose reference voltage is modulated by the addition of a sample of the line voltage. The differential current transformer outputs are shifted by plus and minus ninety degrees from the sampled current and these waveforms shift in phase relative to the line voltage as the load impedance changes. Adding a sample of the line voltage to the current samples results in unequal amplitudes of the two waveforms if the load is not resistive. These two waveforms are then fed to matched diode detectors to obtain the waveform magnitude voltages. When the low frequency component of the outputs of the diode detectors are equal, the loop is at resonance with the applied radio frequency power.
Varactors or electrically tuned semiconductor capacitors are seldom if ever used in transmitting antennas because high Q antennas produce a very large voltage across a tuning capacitor, and few diodes that are marketed as varactors can operate at these voltage levels. The peak voltage across an electrically variable capacitance diode is the sum of the direct current bias voltage and the peak value of the radio frequency voltage generated across the diode in the resonant circuit.
All p-n diode junctions display a change in junction capacitance as a reverse voltage is applied. The reason for this is the increase in the width of the junction region that is depleted of carriers as the reverse voltage increases. The doping profile of the junction is often varied to change the relationship between the applied reverse voltage and the capacitance of the junction.
High voltage power diodes have very wide n and p layers so that the layers are fully depleted only at the maximum operating voltage; therefore, the application of a direct current bias voltage determines the effective capacitance of the junction, and the depleted region, varying at the applied radio frequency rate, changes the remainder of the junction between current conducting and depleted states. This means that the diode region outside the bias depleted volume becomes a lossy conductor. The resistance that this adds to the circuit is generally not negligible, but the technique does facilitate dynamic tuning of the antenna with some loss in radiation efficiency.
By combining the above tuning sensor and a high voltage power diode used as a voltage variable capacitance, a feedback loop is implemented. This loop automatically tunes the antenna to resonance, and will maintain the resonance condition even with moderate changes in the antenna characteristics concomitant with physical variations in the loop and the immediate surroundings of the loop.
If the antenna is to be dynamically tuned over a broad frequency range, the varactor must constitute a large part of the tuning capacitance and the antenna degradation due to the undepleted bulk resistance of the varactor will be considerable. If the range or perturbations are small, a shunt low loss capacitor can be used and the bulk resistance loss is decreased. If the instantaneous tuning range is relatively small but the overall tuning range is large, switched low loss capacitors can be placed across a small capacitance range tuning range varactor.
This technique of implementing and tuning an electrically small antenna facilitates the radiation of low frequencies from a body that is very small compared to the radiated radio frequency wavelength. For example, the 7 MHz radio frequency has a nominal wavelength of 40 meters. This frequency of radiation propagates through complex building structures and even to and from caves. A man wearing the small antenna system disclosed in this invention can radiate this low frequency electromagnetic energy for purposes of communication and radio location. The wearer's body motion and the changing of his surroundings will not significantly disrupt the radiation of radio frequency energy.
In the system as shown in
When the output voltage of the instrumentation amplifier 25 indicates the loop antenna 11 is tuned above resonance, the same process produces an increase in the capacitance of the voltage variable capacitor 13 and the resonance frequency is decreased. This process continues at a rate determined by the integration constant of the integrator 27 until a null voltage is obtained at the output of the instrumentation amplifier 25, indicating the loop antenna 11 is tuned to the frequency of the radio frequency transmitter 17.
When the resonant frequency of the loop antenna 11 is automatically adjusted to the frequency of the radio frequency transmitter 17, moderate perturbations in either the loop antenna 11 dimensions or in the environment of the loop antenna 11 will cause compensatory changes in the voltage variable capacitor 13 tuning voltage, causing the loop antenna 11 to maintain resonance and the system to continue radiating radio frequency electromagnetic energy with little or no interruption.
The primary application for this invention is to facilitate the radiation of radio frequencies from electrically small antennas carried on the person of firemen, emergency response personnel, miners and any other persons or object that must be tracked in structures that do not pass the radiation of the typically used higher radio frequencies.
Scaling up the frequencies used for the tracking of personnel in complex environments means that current antenna size in all radio frequency systems can be considerably reduced and the changing environment of these equipments can be accommodated by the automatic tracking of this antenna system. The energy of very high radio frequencies can then be radiated directly from printed circuit boards and even integrated circuit substrates.
