Abstract:
An apparatus for heating a target comprises a radio frequency heating fork having two substantially parallel tines, the substantially parallel tines electrically connected at a loop end of the radio frequency heating fork, and the substantially parallel tines separated at an open end of the radio frequency heating fork, and a feed coupler connection, the feed coupler connection connecting a power source across the substantially parallel tines of the radio frequency heating fork. The application of power across the substantially parallel tines of the radio frequency heating fork results in induction heating near the loop end of the radio frequency heating fork, and dielectric heating near the open end of the radio frequency tuning fork. A target can be positioned relative to the heating fork to select the most efficient heating method. The heating fork can provide near fields at low frequencies for deep heat penetration.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     [Not Applicable] 
     CROSS REFERENCE TO RELATED APPLICATIONS 
     [Not Applicable] 
     BACKGROUND OF THE INVENTION 
     The present invention relates to radio frequency (“RF”) heating. In particular, the present invention relates to an advantageous and efficient apparatus and method for heating substances of varying conductivities. 
     RF heating can be used in a variety of applications. For example, oil well core samples can be heated using RF energy. These core samples, however, can vary greatly in conductivity, and therefore respond differently to various types of heating. Dielectric heating is efficient and preferable for samples having a low conductivity. Samples with higher conductivity are best heated by inductive heating. Medical diathermy, or the use of heat to destroy abnormal or unwanted cells, is another application that may utilize RF heating. 
     RF heating is a versatile process for suitable for many materials as different RF energies may be used. There can be electric fields E, magnetic fields H, and or electric currents I introduced by the RF heating applicator. Linear applicators, such as a straight wire dipole emphasize strong radial near E fields by divergence of current I. Circular applicators, such as a wire loop emphasize strong radial H fields by curl of current I. Hybrid applicator forms may include the helix and spiral to produce both strong E and H fields. Uninsulated RF heating applicators may act as electrodes to introduce electric currents I in the media. 
     Parallel linear conductors form an antenna in U.S. Pat. No. 2,283,914, entitled “Antenna” to P. S. Carter. Now widely known as the folded dipole antenna, the antenna uses equal direction current flows in the thin wires and a voltage summing action to bring the driving impedance to a higher value. The folded dipole antenna did not, however, include aspects of: antiparallel current flow (opposite current directions or senses), operation with open terminals at one end, induction coupling to a separate feed structure, or capacitor loading. The folded dipole antenna is useful for operation at sizes of about ½ wavelength and above. 
     U.S. Pat. No. 2,507,528 entitled “Antenna” to A. G. Kandoian describes antiparallel (equal but opposite direction) currents flowing on the opposite edges of a slot in a conductive plate. Horizontal polarization was realized from a vertically oriented slot. 
     RF heating may operate by near fields or far fields. Near fields are strong reactive energies that circulate near RF heating applicators. Far fields may comprise radio waves at a distance from the applicator. Both near and far fields are useful for RF heating, and many tradeoffs are possible. For instance, near fields may be more useful for low frequencies, when the applicator is small in size, and for conductive materials. Far fields may be preferred for heating at a distance and for heating low conductivity materials. 
     SUMMARY OF THE INVENTION 
     The present radio frequency heating fork is useful for heating a variety of targets because the heat produced by the radio frequency heating fork includes induction heating and dielectric heating. A particular type of heating can be selected simply by positioning the target relative to the radio frequency heating fork. 
     The present radio frequency heating fork includes a method for heating a target using a radio frequency heating fork, the radio frequency heating fork comprising two substantially parallel tines, the substantially parallel tines electrically connected at a loop end of the radio frequency heating fork, and the substantially parallel tines separated at an open end of the radio frequency heating fork, and a feed coupler connection, the feed coupler connection connecting a power source across the substantially parallel tines of the radio frequency heating fork, the method comprising: positioning a target relative to a radio frequency heating fork; and heating the target by applying power across the radio frequency heating fork using a feed coupler connection. 
     The positioning of the target may further comprise relatively positioning the target between the substantially parallel tines of the radio frequency heating fork. The positioning of the target may further comprise relatively positioning the target on or between the substantially parallel tines of the radio frequency heating fork, and near the loop end of the radio frequency heating fork, where the heating of the target is primarily due to induction heating. Alternatively, the positioning of the target may further comprise relatively positioning the target on or between the substantially parallel tines of the radio frequency heating fork, and near the open end of the radio frequency heating fork, where the heating of the target is primarily due to dielectric heating. 
     The feed coupler connection may be inductively connected to the substantially parallel tines of the radio frequency heating fork near the loop end of the radio frequency heating fork. Alternatively, the feed coupler connection may be electrically connected to the substantially parallel tines of the radio frequency heating fork near the loop end of the radio frequency heating fork. The induction feed coupler connection may include a Balun. Furthermore, the frequency radio frequency heating fork may be tuned using a capacitor placed across the substantially parallel tines of the radio frequency heating fork. 
     The present radio frequency heating fork includes an apparatus for radio frequency heating of a target, the apparatus comprising: a radio frequency heating fork, the radio frequency heating fork having two substantially parallel tines, the substantially parallel tines electrically connected at a loop end of the radio frequency heating fork, and the substantially parallel tines separated at an open end of the radio frequency heating fork, and a feed coupler connection, the feed coupler connection connecting a power source across the substantially parallel tines of the radio frequency heating fork. The application of power across the substantially parallel tines of the radio frequency heating fork results in induction heating near the loop end of the radio frequency heating fork, and dielectric heating near the open end of the radio frequency tuning fork. 
     The feed coupler connection may be inductively connected to the substantially parallel tines of the radio frequency heating fork near the loop end of the radio frequency heating fork. The induction feed coupler connection may include a Balun. Alternatively, the feed coupler connection may be electrically connected to the substantially parallel tines of the radio frequency heating fork near the loop end of the radio frequency heating fork. A capacitor may also be connected between the substantially parallel tines of the radio frequency heating fork. 
     Other aspects of the invention will be apparent to one of ordinary skill in the art in view of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts the present radio frequency heating fork employing a wireless connection. 
         FIG. 2  depicts the present radio frequency heating fork employing a hard-wired connection. 
         FIG. 3  depicts the heating pattern for the radio frequency heating fork with a target. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The subject matter of this disclosure will now be described more fully, and one or more embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are examples of the invention, which has the full scope indicated by the language of the claims. 
     In  FIG. 1 , a radio frequency heating fork  50  includes tines  58  and  59 , and incorporates a wireless, induction feed coupler connection. A coaxial feed  54  is connected at one end to AC power supply  52 , and at the other end to supply loop  56 . The supply loop  56  and the loop end  64  of the heating fork  50  are positioned near each other and overlap, which creates a transformer effect that transfers energy from the supply loop  56  to the heating fork  50 . The induction feed coupler may be adjusted for a fifty Ohm drive resistance or as desired. The amount of overlap and the distance between supply loop  56  and loop end  64  of heating fork  50  can be varied, which in turn varies the resistance and heating. Tines  58  and  59  are electrically connected through loop end  64 . Insulation may be placed over the outside or the heating fork  50  as may be desirable for internal medical diathermy applications. 
     Heating fork  50  may be optionally equipped with capacitor  62  for tuning purposes. Heating fork  50  naturally operates at a frequency of approximately one-quarter of a wavelength. Optional capacitor  62  can reduce this frequency to, for example, one-twentieth or one-thirtieth of a wavelength. RF shielding (not shown), such as a metal box, may be used over the heating for  50  to control radiation. Supply loop  56  advantageously functions as an isolation transformer or Balun which serves as a common mode choke for stray current suppression on the surface of coaxial feed  54 . Although not shown, heating fork  50  may be immersed or otherwise positioned inside a target media to be RF heated. 
     The length L of heating fork  50  is preferentially one-quarter of a wavelength at the operating frequency, although L may be made shortened as desired adding or increasing the capacitance of capacitor  62 . High voltages and high currents are thus easily produced by the heating fork as the hyperbolic tangent function asymptotically approaches zero and infinity through one-quarter of a wavelength, e.g. 90 electrical degrees. 
     Turning now to  FIG. 2 , radio frequency heating fork  100  includes tines  108  and  109 , and incorporates a hardwired feed coupler connection. Coaxial feed  104  is connected at one end to an AC power supply (not shown), and connected at the other end to heating fork  100  at feed coupler connections  106  near loop end  110  of heating fork  100 . Tines  108  and  109  are electrically connected through loop end  110 . When power is applied across heating fork  100 , a strong magnetic field  114  is formed near loop end  110  of heating fork  100 . Conversely, a strong electric field  116  is formed near open end  112  of heating fork  100 . These fields are similarly formed when power is applied to heating fork  50  in  FIG. 1  (not shown). 
     The two different fields provide two different heating qualities. The strong magnetic field  114  formed near loop end  110  of heating fork  100  provides induction heating, which is excellent for heating conductive substances. The strong electric field  116  formed near open end  112  of heating fork  100 , on the other hand, is excellent for heating less conductive, or even non-conductive substances. By positioning target  118  relative to heating fork  100 , the most advantageous form of heating can be used depending on the conductivity of target  118 . For example, a target  118  having a high conductivity may be positioned closer to loop end  110  of heating fork  100 . On the other hand, even a target comprised of distilled water can be heated near the open end of heating fork  100  due to the strong electric field in that area. More even heating may be achieved if target  100  is positioned between tines  108  and  109  of heating fork  100 . 
     The present radio frequency heating fork has a low voltage standing wave ratio (“VSWR”) when operated in an appropriate frequency range. For example, in one embodiment the VSWR approached 1:1 when the radio frequency heating fork was operated at approximately 27 MHz. 
     Heating fork tines  58 ,  59 ,  108  and  109  need not be cylindrical in cross section, and other shapes may be desirable for specific applications. For instance, if used for internal medical diathermy, the fork tines may have a C-shaped cross section to facilitate tissue penetration for positioning the heating fork relative to the target cells. 
     Heating forks  50  and  100  are conductive structures, typically comprised of a metal, having a differential mode electric current distribution with equal current amplitudes on each tine, with currents flowing in opposite directions on each tine. For example, when the AC power supply waveform is sinusoidal the current distribution along heating fork  50  of  FIG. 1  is sinusoidal such that maximum amplitude occurs at the loop end  68 , and a minimum at the open end  68 . The voltage potential across fork tines  58  and  59  is at a minimum at loop end  64  and at a maximum at the open end  66 . The ratio of the voltage E between the tines to the current I along the tines line is the impedance Z is given by:
 
 Z   L   =γL  
 
     Where:
         Z L =the impedance along the length of the tines   γ=the complex propagation constant gamma along the fork (including an attenuation constant α and a phase propagation constant β)   L=the overall length of the heating fork from the loop end  64  to the open end  66         

     Continuing the theory of operation with reference to  FIG. 1 , supply loop  56  conveys an electric current I in a curl causing a magnetic field B (not shown). Loop end  64  of heating fork  50  overlaps the magnetic field B of supply loop  56  causing a sympathetic electric current I flow into heating fork  50 . Thus supply loop  56  and loop end  64  essentially form the “windings” of a transformer in region  60 . Bringing supply loop  56  closer to loop end  64  provides a greater load resistance to AC power supply  52 , while moving supply loop  56  further from loop end  64  provides less load resistance to AC supply  52 . The frequency of resonance of heating fork  50  becomes slightly less as supply loop  56  is brought near loop end  64 . 
     The fields generated by heating forks  50  and  100  are now considered. Although skeletal in form, the heating fork structure relates to linear slot antennas, and heating forks  50  and  100  generate three reactive near fields, three middle fields, and two radiated far fields (E and H). The present radio frequency heating forks primarily utilize near-field heating. Without a heating load, the near fields may be described as follows:
 
 H   z   =−jE   0 /2πη[( e   −jkr1   /r   1 )+( e   −jkr2   /r   2 )]
 
 H   ρ   =−jE   0 /2πη[( z−λ/ 4)/ρ)( e   −jkr1   /r   1 )+( z−λ/ 4)/ρ)( e   −jkr2   /r   2 )]
 
 E   φ   =−jE   0 /2π[( e   −jkr1 )+( e   −jkr2 )]
 
     Where:
         p, φ, z are the coordinates of a cylindrical coordinate system in which the slot is coincident with the Z axis   r 1  and r 2  are the distances from the heating fork to the point of observation   η=the impedance of free space=120π   E=the electric field strength in volts per meter   H=the magnetic field strength in amperes per meter       

     There are strong near E fields broadside to the plane of heating forks  50  and  100  during the heating process. The near H fields are strong broadside to the plane of heating fork  50  and  100 , and in between tines  58  and  59  or  108  and  109  as well. 
     The placement of target  118  (see  FIG. 2 ) may significantly modify near field phase and amplitude contours from those present during free space operation, and the derivation of the near field contours involving target  118  may be best accomplished by numerical electromagnetic methods.  FIG. 3  is a profile cut contour plot of the specific absorption rate of heat in watts per kilogram for target  118  being heated by heating fork  100 , with tines  108  and  109  on either side of target  118 . The  FIG. 3  plot was obtained by a method-of-moments analysis. The asymmetry seen is due to meshing granularity and would not be present in symmetric physical embodiments. As can be appreciated, the circular magnetic near fields from each of the antenna fork conductors add constructively in phase as the heating effect is nonzero in the target center. Exemplary operating parameters associated with  FIG. 3  are listed in Table 1 below: 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 Application 
                 Near field RF heating 
               
               
                 Heating fork RF feed 
                 Supply loop 
               
               
                 Target material 
                 Rich Athabasca oil sand, 15% bitumen 
               
               
                 Target size 
                 10.2 cm diameter cylinder, 0.91 meters long 
               
               
                 Target permittivity 
                 5 farads/meter 
               
               
                 Target conductivity 
                 0.0017 mhos/meter 
               
               
                 Target water content 
                 1.1% 
               
               
                 Frequency 
                 6.78 MHz 
               
               
                 Supply loop length 
                 1.05 meter 
               
               
                 Supply loop width 
                 15.2 cm (same as heating fork) 
               
               
                 Supply loop spacing from 
                 0.190 m center to center 
               
               
                 heating fork 
               
               
                 Transmitter power 
                 1 kilowatt RMS 
               
               
                 VSWR 
                 Under 2.0 to 1 
               
               
                 Heating fork length 
                 3.1 meters 
               
               
                 Spacing between fork 
                 15.2 cm 
               
               
                 conductors 
               
               
                 Fork conductor diameter 
                 2.28 cm 
               
               
                 Capacitor location 
                 1.33 meters from loop end 
               
               
                 Capacitor capacitance 
                 317 pf 
               
               
                 SAR rate in target 
                 5-10 watts/kilogram 
               
               
                 H field amplitude in target 
                 0.1 to 0.4 amperes/meter 
               
               
                 E field amplitude in target 
                 ~8 kilovolts/meter 
               
               
                   
               
             
          
         
       
     
     The present radio frequency heating fork has been tested and found effective for the heating of petroleum ores, such as Athabasca oil sand in dielectric pipes. Referring to  FIG. 2 , in a large scale application heating fork tines  108  and  109  may comprise hollow metallic pipes to permit the withdrawal of radio frequency heated materials such as hydrocarbon ores or heavy oil, e.g. heating fork tines  108  and  109  may be comprised of solid wall or perforated wall well piping. 
     Frequency and electrical load management for the present radio frequency heating fork will now be discussed in reference to  FIGS. 1 and 2 . It may be preferred that heating fork  100  be operated at resonance for impedance matching and low VSWR to AC power source  102 . Two methods for such operation involve variable frequency and fixed frequency operation. In the variable frequency method, AC power supply  102  is changed in frequency during heating to track the dielectric constant changes of target  118 . This may be accomplished, for example, with a control system or by configuring AC power source as a power oscillator with heating fork  100  as the oscillator tank circuit. A second loop similar to supply loop  56  (see  FIG. 1 ) may be used as tickler to drive the oscillator. 
     In a fixed frequency method, AC power source  52  may be held constant in frequency by crystal control, and the value of capacitor  62  varied to force a constant frequency of resonance from heating fork  50 . The fixed frequency approach may be preferred if it is desired to avoid the need for shielding from excess RF radiation. For example, the fixed frequency approach may avoid the need for shielding by use of a RF heating frequency allocation. In the United States this may be in an Industrial, Scientific and Medical (ISM) band, e.g., at 6.78 Mhz, 13.56 Mhz, and other frequencies. 
     It is preferential to space tine  58  from tine  59  of RF heating fork  50 , and tine  108  from tine  109  of RF heating fork  100 , by about 3 or more tine diameters to avoid conductor proximity effect losses between the tines. Conductor proximity effect is a nonuniform current distribution that can occur with closely spaced conductors that increases loss resistance. Litz conductors may be useful with the present invention in low frequency embodiment of the present invention, say below about 1 MHz. The RF heating forks  50  and  100  may be operated in a vacuum or dielectric gas atmosphere such as sulfur hexafluoride (SF 6 ) to control corona discharges from open ends  66  and  112  at very high power levels. When uninsulated and in contact with a target media  118  that is conductive, heating forks  50  and  100  apply electric currents directly into the target media. Open ends  66  and  112  can function as electrodes if so configured. 
     Target  118  may comprise a heating puck, a dielectric pipe, or even a human patient undergoing a medical treatment. A method of the present invention is to place RF heating susceptors in the RF heating target for increased heating speed, or for selectively heating a specific region of the target. A RF heating susceptor is a material that heats preferentially in the presence of RF energies, such as, for example, graphite, titanates, ferrite powder, or even saltwater. 
     The present RF heating fork may also be useful for generating far fields and as an antenna when RF heating targets are not used. The orientation of the radiated far electric field is opposite that of heating fork orientation, e.g. a horizontally oriented heating fork produces a vertical polarized wave. The present RF heating forks are therefore useful for both near and far field heating, and for communications. 
     The present RF heating fork has multiple applications as a tool for RF heating, such as food and material processing, component separation and upgrading hydrocarbon ores, heat sealing and welding, and medical diathermy. The present RF heating fork may be operated at low frequencies for sufficient penetration, and by near fields for controlled radiation, thereby providing a selection of energy types E, H, and I. 
     Although preferred embodiments of the invention have been described using specific terms, devices, and methods, such description is for illustrative purposes only. The words used are words of description rather than of limitation. It is to be understood that changes and variations may be made by those of ordinary skill in the art without departing from the spirit or the scope of the present invention, which is set forth in the following claims. In addition, it should be understood that aspects of the various embodiments may be interchanged either in whole or in part. Therefore, the spirit and scope of the appended claims should not be limited to the description of the preferred versions contained herein.