Patent Publication Number: US-11394126-B1

Title: Distributed monopole transmitter

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to and claims the benefit of U.S. Provisional Patent Application No. 62/935,533, filed on Nov. 14, 2019, which is incorporated herein by reference as though set forth in full. 
    
    
     STATEMENT REGARDING FEDERAL FUNDING 
     This invention was made under U.S. Government contract N66001-19-C-4018. The U.S. Government may have certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to monopole and dipole antennas. 
     BACKGROUND 
     A monopole antenna has a wire or mast extending a distance from a ground plane and the wire or mast has a length typically less than one half of a desired wavelength. The antenna is typically driven by a single transmitter at the base of the antenna with one side connected to the ground plane and the other to the wire or mast. For the purpose of this disclosure, a “transmitter” is a subsystem that takes in baseband signals and power and delivers a radio frequency signal to an antenna. 
     The primary type of electrically small antenna for transmitting low frequencies (e.g. VLF) is a top-loaded monopole fed at the base of the antenna, as described in Reference [1] below, which is incorporated herein by reference. An electrically small antenna is an antenna much shorter than the wavelength of the signal it is intended to transmit or receive. These top loaded monopole antennas tend to be electrically small and may be less than ⅙ of the extremely long signal wavelength in any dimension. This means that the reactive component of the impedance is much larger than the radiation resistance. In most cases, the antenna is resonated with inductance so that high current can be driven on the antenna to achieve sufficient radiated power. For example, for an amplifier that sources approximately 1 kV, the resonance between the tuning inductor and the capacitive antenna may result in 100 kV at the antenna base. This results in 10,000 times more radiated power than if the amplifier were directly connected to an un-resonated antenna. However, resonating the antenna with inductance results in a bandwidth that is just large enough to accommodate today&#39;s low-data-rate communications signals, and frequency tuning takes substantial time. A broader bandwidth can be achieved by using a non-resonated antenna; however, the radiated power level may be insufficient for many applications. 
     The antenna bandwidth and power handling may be increased by increasing the height or size of the top-load, which may be on the scale of 100s of meters in elevation and square kilometers of area. However, this is not compatible with mobile applications. 
     An alternative is an antenna trailed behind an aircraft. Such an antenna is described in Reference [ 2 ] below, U.S. Pat. No. 4,335,469, issued Jun. 15, 1982, which is incorporated herein by reference. However, this type of antenna is not electrically small and requires a large aircraft and the associated operating cost. 
     Another type of antenna is a waveform synthesis antenna, which directly switches a DC voltage supply in and out of a loop antenna, as described in References [ 3 ] and [ 4 ] below, which are incorporated herein by reference. A waveform synthesis antenna is a loop which is fundamentally different than a monopole antenna. Further, this type of antenna has two issues. First, it is a loop antenna, which inherently has poor radiation efficiency. Second, the RF voltage builds up around the loop but the DC voltage is constant around the loop. Therefore, the voltage is held off by RF chokes and is limited by the breakdown of the components to nearby ground potentials, which ultimately limits the power that can be radiated. 
     In summary, electrically small antennas have been investigated for decades but are limited in power by the voltage handling and voltage breakdown. 
     REFERENCES 
     The following references are incorporated herein by reference as though put forth in full.
     [1] A. D. Watt, VLF Radio Engineering, International Series of Monographs in Electromagnetic Waves, Vol. 14, Permagon Press, New York, 1967.   [2] U.S. Pat. No. 4,335,469, issued Jun. 15, 1982.   [3] Waveform-synthesis method that reduces battery power in an electrically small wideband radiating system, Merenda, J. T., IET Microwaves, Antennas &amp; propagation (2008), 2(1):59.   [4] U.S. Pat. No. 6,229,494, issued Aug. 21, 2012.   

     What is needed is an improved electrically small antenna with more instantaneous bandwidth at high power levels. The embodiments of the present disclosure answer these and other needs. 
     SUMMARY 
     In a first embodiment disclosed herein, a distributed transmitter antenna comprises a plurality of antenna segments, and a plurality of transmitters, wherein a first transmitter of the plurality of transmitters is coupled to a first antenna segment of the plurality of antenna segments, and wherein a second transmitter of the plurality of transmitters is coupled to the first antenna segment and a second antenna segment of the plurality of antenna segments. 
     In another embodiment disclosed herein, a method of providing a distributed transmitter antenna comprises providing a plurality of antenna segments, and providing a plurality of transmitters, wherein a first transmitter of the plurality of transmitters is coupled to a first antenna segment of the plurality of antenna segments, and wherein a second transmitter of the plurality of transmitters is coupled to the first antenna segment and a second antenna segment of the plurality of antenna segments. 
     These and other features and advantages will become further apparent from the detailed description and accompanying figures that follow. In the figures and description, numerals indicate the various features, like numerals referring to like features throughout both the drawings and the description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A, 1B, 1C and 1D  show four configurations of an example 100 m monopole antenna having 1, 2, 4 and 8 distributed transmitters, respectively, radiating at 20 kHz, and  FIG. 1E  shows the radiated power for each of these configurations in accordance with the present disclosure. 
         FIGS. 2A, 2B, 2C and 2D  show four examples of a transmitter in accordance with the present disclosure,  FIG. 2A  shows a baseband signal and electrical power connected to the transmitter with electrical wires,  FIG. 2B  shows a configuration in which data is provided to the transmitter over data bus, which is preferably a wireless link at a frequency different than the transmit frequency and which is converted to baseband by a receiver, and a wirelessly connected power source preferably compressed air that delivers power which is converted to electricity by an energy conversion element, for example a turbine, that converts the power into electrical power,  FIG. 2C  is similar to  FIG. 2A  except with the same power conversion used in  FIG. 2B , and  FIG. 2D  shows an another embodiment where energy is collected from the environment, such as by solar cells or wind turbines, and the data is delivered to the transmitter over a wireless link in accordance with the present disclosure. 
         FIG. 3  shows connections of two distributed transmitters to the monopole or dipole antenna in accordance with the present disclosure. 
         FIG. 4  shows an example embodiment showing three transmitters powered by compressed air with data delivered to the transmitter by a wireless link in accordance with the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth to clearly describe various specific embodiments disclosed herein. One skilled in the art, however, will understand that the presently claimed invention may be practiced without all of the specific details discussed below. In other instances, well known features have not been described so as not to obscure the invention. 
     The present disclosure describes a monopole antenna with a plurality of distributed transmitters that include electrically-floating transmitters connected along the length/height of the monopole. The transmitters are coordinated to produce a desired radiating current in the monopole. Tuning elements, which may be fixed and/or variable inductors, may optionally be included in the transmitters to provide a resonance condition. 
     Another aspect of the present disclosure is the delivery of the power and signal to the transmitters. In some examples, power is delivered by conductors using appropriate filtering. In another example, power may be delivered by mechanical means, such as compressed air. In another example, the transmitters may be powered by energy harvesting (e.g. solar or wind). In some embodiments the signal to be transmitted may be delivered to the transmitter via wires or may be delivered by wireless links, such as short-range radio frequency links at a frequency different than the transmit frequency. 
     The present disclosure describes an electrically-small antenna that can radiate substantially increased bandwidth at higher power levels than prior art antennas. The power handling and bandwidth of prior art VLF antennas is limited by the voltage at the base of the antenna and the quality factor. Therefore, prior art antennas are limited to about 250 kV with about 1% bandwidth. 
     The present disclosure describes a distributed transmitter antenna that enables N times the voltage on the antenna, where N is the number of transmitters, and enables broad bandwidth compared to wideband solutions that do not resonate the antenna. The feed voltage is dropped across N segments of the antenna, which in turn allows approximately N times the amount of radiating current. As discussed further below, the distributed transmitter antenna of the present disclosure also can provide a radiated power greater than or equal to 0.5 times N{circumflex over ( )}2. 
       FIG. 1A  shows a monopole antenna  10  with a mast  12  fed by a single transmitter  14  at its base, which is representative of the prior art. The single transmitter  14  is grounded to ground  16  and connected to the mast  12 . The monopole antenna  10  may be, for example, a 100 m monopole antenna radiating at 20 kHz on an essentially infinite ground plane  16 . The transmitter  14  in this example drives 1 Volt onto the antenna. The power P radiated is limited by the voltage applied and the large reactance of the antenna, as described by the following equations.
 
 P=I{circumflex over ( )} 2* R _radiation
         I=V/X   Therefore P=(V/X){circumflex over ( )}2*R_radiation   where
           I is the antenna current,   R_radiation is the radiation resistance,   V is the voltage applied by the transmitter, and   X is the antenna reactance.   
               At the same time, the 3 dB bandwidth B is given by
       B=1/Q=R_total/X   where Q is the quality factor defined as a ratio of a resonator&#39;s center frequency to its bandwidth, and   where R_total=the real part of the input impedance of the antenna.   
       Therefore the power bandwidth P*B product is proportional to P*B=V{circumflex over ( )}2/X{circumflex over ( )}3*R_radiation*R_total.   

       FIG. 1B  shows a monopole antenna  18  with multiple segments  12  with two transmitters  14  distributed along the length of the antenna  18 , which has two antenna segments  12 , which are referred to as segments  100  and  102  in  FIG. 1B . The bottom transmitter  14  is grounded to ground  16  and connected to segment  100 , and the top transmitter  14  may be floating relative to ground  16 . Each transmitter  14  transmits a voltage V across its output terminals. Therefore the first antenna segment  12  or lowest segment  100  of the antenna  18  is at potential V and the second antenna segment  12  or segment  102  of the antenna  18  is at 2*V. Assuming each transmitter  14 , in the two transmitter example configuration  18  drives V Volts, then the total voltage is 2*V, and one might expect the radiated power to be four times the prior art monopole antenna shown in  FIG. 1A ; however, a method of moments full-wave simulation has shown that the radiated power is about half that, as further described below. The two transmitter configuration  18  also drives more current onto the antenna, as compared to the prior art configuration  10 . 
       FIG. 1C  shows a configuration  22  with  4  transmitters  14  and four antenna segments  12 .  FIG. 1D  shows a configuration  24  with 8 transmitters  14  and eight antenna segments  12 . In each of these configurations the bottom transmitter is grounded to ground  16  and the other transmitters may be floating relative to ground  16 . 
     A method of moments full-wave simulation has been performed with the 1, 2, 4 and 8 transmitters  14  as shown in  FIGS. 1A, 1B, 1C and 1D , respectively, with each transmitter supplying 1 V on a monopole aluminum wire having a diameter of 0.2 meters. As shown in  FIG. 1E , increasing the number of transmitters substantially increases the radiated power by approximately 0.5*N{circumflex over ( )}2 for N&gt;1, where N is the number of transmitters  14 . For the simulated examples, shown in  FIG. 1E , increasing the number of transmitters increased the radiated power by greater than 0.5*N{circumflex over ( )}2 for N&gt;1, where N is the number of transmitters. 
       FIGS. 2A, 2B, 2C and 2D  show details of the transmitter  14  in accordance with the present disclosure. Each transmitter has a positive  30  and negative  32  output terminal that connects to the antenna as shown, for example, in  FIG. 3 . Each transmitter  14  has RF drive electronics  34  that convert a low power baseband signal  40  or  36  into a high power radio frequency signal. The RF drive electronics  34  may include tuning elements, which may be fixed and/or variable inductors, to provide a resonance condition. The baseband signal may be modulated or encoded on an RF waveform. In one example, the RF drive electronics  34  includes an oscillator, a modulator and a power amplifier. However, alternative architectures are possible. 
     It is important that the radio frequency waveform for each respective transmitter  14  be synchronized with the radio frequency waveform for each other transmitter  14 , preferably in phase. One way to synchronize the waveforms is by using precision clock in each transmitter  14  or by providing time from an external precision clock to each transmitter. Another way is to synchronize the transmitters  14  is to synchronize each transmitter to a feature in the baseband signal  40  or  36 , sent over a wired connection  40  or a wireless link  36 , as shown in  FIGS. 2A, 2B, 2C and 2D . 
     If some beam steering is desired then the transmission from each transmitter may be phased to accomplish the beam steering. 
     In  FIG. 2A  a baseband signal  40  and electrical power  42  are provided by means of electrical wires or conductors connected to the transmitter  14 . The energy storage  41  preferably includes capacitors and/or batteries. In this embodiment, the conductors  42  and  40  delivering power and the baseband signal, respectively, may be isolated from the radiating field by means of filters, such as inductive elements, to prevent scattering and unwanted feedback. 
     In  FIG. 2B , the data to be transmitted is delivered to the transmitter  14  by a wireless data link  36 , which uses frequency that is different than the transmit frequency. The data link  36  may also be an optical fiber to achieve isolation from the transmitter. The data link  36  may use, for example, IEEE 802.11, Zigbee, Bluetooth, or frequency modulation, and so on. The data to be transmitted may be converted to the desired baseband by receiver  38 . A wireless power source  44 , which may be preferably compressed air, can be used to deliver power to the transmitter  14 , and an energy conversion component  46 , for example a turbine, can be used to convert the power into electrical power. This embodiment has the advantage that the transmitters  14  can easily be at a floating potential relative to ground  16 , making it practical to increase the voltage well above that which is possible in the prior art. The power source  44  may also be a combustible fluid with the energy conversion being, for example, an electrical generator. 
       FIG. 2C  is an example configuration similar to  FIG. 2A  for the baseband signal  40  and similar to  FIG. 2B  for the power source  44  and power conversion and energy storage  46 . 
       FIG. 2D  shows an embodiment where energy  50  is collected from the environment, rather than directly delivered to the transmitter. The energy collection may be by solar cells, wind turbines, or any other method known in the art. The configuration of  FIG. 2D  also shows the use of a data bus  36 , which may be a wireless data link or an optical fiber. 
       FIG. 3  shows the connection of two transmitters  14  to the monopole or dipole antenna. Each transmitter  14  has a positive  30  and a negative  32  output terminal that connects to the antenna  12 . As shown, the bottom transmitter  14  is grounded to ground  16 . 
     A preferred embodiment is shown in  FIG. 4 , which has transmitters  14  in the configuration shown in  FIG. 2B . Shown are three transmitters  14  with power delivered by an air compressor  60 , which is electrically insulated from the transmitters  14  by a compressed air line  62 , which may be an ABS pipe, a Polyethylene pipe, a rubber hose, and so on. In  FIG. 4  data  70  is delivered to the transmitters  14  wirelessly over from a wireless link  64 . 
     In this example, each transmitter  14  creates a potential difference V across transmitter outputs  30  and  32 . Therefore the top of the antenna has a voltage of 3*V. Since the connections to the transmitters are wireless and floating relative to ground, each transmitter  14  may also be floating relative to ground and only needs to withstand and supply voltages on the order of V. In  FIG. 4 , only the bottom transmitter  14  is connected to ground  16 . 
     If N transmitters  14  are used, the voltage applied to the antenna is increased by a factor of N without relying on a narrowband resonance, so the system may have wide bandwidth. As discussed above, the radiated power from the antenna is approximately or greater than 0.5*N{circumflex over ( )}2, where N is the number of transmitters. For 3 transmitters the voltage driven on the antenna is increased by 3 times, and the radiated power is greater than 0.5 times 9 over a prior art antenna as shown in  FIG. 1A . 
     Existing techniques may be used to erect the monopole and dipole antennas with the distributed transmitters  14 , as shown in  FIGS. 1B, 1C and 1D . One of ordinary skill in the art will recognize that the support structure must be designed to stand off a larger voltage, which may mean using longer insulators to connect to guy wires or using non-conductive materials for the guy wires, such as glass fiber, nylon, and so on. 
     Having now described the invention in accordance with the requirements of the patent statutes, those skilled in this art will understand how to make changes and modifications to the present invention to meet their specific requirements or conditions. Such changes and modifications may be made without departing from the scope and spirit of the invention as disclosed herein. 
     The foregoing Detailed Description of exemplary and preferred embodiments is presented for purposes of illustration and disclosure in accordance with the requirements of the law. It is not intended to be exhaustive nor to limit the invention to the precise form(s) described, but only to enable others skilled in the art to understand how the invention may be suited for a particular use or implementation. The possibility of modifications and variations will be apparent to practitioners skilled in the art. No limitation is intended by the description of exemplary embodiments which may have included tolerances, feature dimensions, specific operating conditions, engineering specifications, or the like, and which may vary between implementations or with changes to the state of the art, and no limitation should be implied therefrom. Applicant has made this disclosure with respect to the current state of the art, but also contemplates advancements and that adaptations in the future may take into consideration of those advancements, namely in accordance with the then current state of the art. It is intended that the scope of the invention be defined by the Claims as written and equivalents as applicable. Reference to a claim element in the singular is not intended to mean “one and only one” unless explicitly so stated. Moreover, no element, component, nor method or process step in this disclosure is intended to be dedicated to the public regardless of whether the element, component, or step is explicitly recited in the Claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase “means for . . . ” and no method or process step herein is to be construed under those provisions unless the step, or steps, are expressly recited using the phrase “comprising the step(s) of . . . .”