Abstract:
An aircraft antenna includes an aerodynamic housing structured for attachment to an outer surface of an aircraft, and the housing contains an electromagnetic radiator and tuned over a first band of frequencies potentially to produce secondary radiations in at least a second band of frequencies, and a suppression filter effective at the frequencies of the secondary radiations.

Description:
RELATED APPLICATIONS 
   This invention relates to U.S. application Ser. No. 60/439,252 filed Jan. 10, 2003 and entitled “Combination Antennas”, and to U.S. application Ser. No. 60/439,381 filed Jan. 10, 2003 and entitled “Combination Antennas”. Applicant claims the benefit of these applications under 35 USC 120. 

   FIELD OF THE INVENTION 
   This invention relates to aircraft antennas, and particularly to aircraft antennas that limit drag on an aircraft while avoiding interference with each other. 
   BACKGROUND OF THE INVENTION 
   Aircraft carry a number antennas for navigational, communication, and other purposes. However, the limited space on the outer surface of an aircraft may require placement of the multiple antennas in close enough proximity to each other to create radio interference between them. In particular, harmonics generated by one antenna may interfere with coincident frequencies in the frequency bands of other antennas. Moreover the various antennas project from the outer surface of the aircraft and introduce drag that interferes with the aerodynamic performance of an aircraft. 
   SUMMARY OF THE EMBODIMENTS OF THE INVENTION 
   According to an embodiment of the invention an aircraft antenna includes an integrated low profile shielded harmonic suppression filter that permits close placement of antennas. 
   According to another embodiment of the invention, two antenna radiators are combined in a single aerodynamic housing to form a single combination antenna and one radiator includes an integrated harmonic suppression filter. 
   These and other features of the invention are pointed out in the claims. Other aspects of the invention will become evident from the following detailed description when read in light of the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic illustration including a side view of an antenna mounted on an airframe and embodying features of the invention. 
       FIG. 2  is a schematic diagram including a front view of the antenna in  FIG. 1  mounted on an airframe and embodying features of the invention. 
       FIG. 3  shows details of the antenna in  FIGS. 1 and 2  mounted on an airframe and embodying features of the invention. 
       FIG. 3A  is a schematic representation of an embodiment of the antenna in  FIGS. 1 ,  2 , and  3  mounted on an airframe. 
       FIG. 4  is a schematic equivalent circuit of an embodiment of the antenna of  FIGS. 1 ,  2 ,  3 , and  3 A. 
       FIG. 4A  is a graph illustrating an embodiment of an operation of the antenna of  FIGS. 1 ,  2 ,  3 ,  3 A, and  4 . 
       FIG. 5  is a schematic equivalent circuit of another embodiment of the antenna of  FIGS. 1 ,  2 ,  3 , and  3 A. 
       FIG. 6  is a plan view of another embodiment of the antenna in  FIGS. 1 and 2 . 
       FIG. 7  is a section  7 — 7  of the embodiment of the antenna in  FIG. 6 . 
       FIG. 8  is a schematic equivalent circuit of an embodiment of the antenna of  FIGS. 6 and 7 . 
       FIG. 9  is a schematic equivalent circuit of another embodiment of the antenna of  FIGS. 6 and 7 . 
       FIG. 10  is a bottom view of an embodiment of a printed circuit board with a filter suitable for the antennas in  FIGS. 1 to 9 . 
       FIG. 11  is a side view of the board in  FIG. 10 . 
       FIG. 12  is a top view of the board in  FIG. 10 . 
       FIG. 13  is a schematic representation of the filter in  FIGS. 10 ,  11 , and  12 . 
       FIG. 14  is a schematic diagram of an equivalent circuit of the filter in  FIGS. 10 ,  11 , and  12 . 
       FIG. 15  is a bottom view of an embodiment of a printed circuit board with a filter suitable for the antennas in  FIGS. 1 to 9 . 
       FIG. 16  is a side view of the board in  FIG. 15 . 
       FIG. 17  is a top view of the board in  FIG. 15 . 
       FIG. 18  is a schematic diagram of an equivalent circuit of the filter in  FIGS. 15 ,  16 , and  17 . 
       FIG. 19  is a diagram illustrating another embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   In  FIGS. 1 and 2 , an antenna AN 1  embodying the invention includes a flat, low drag, high speed, antenna housing HO 1  that envelops an antenna system AS 1 , and rests securely on the outer and upper surface of an aircraft AI 1  such as on a wing or fuselage. Suitable means secure the housing HO 1  to the outer surface of the skin of the aircraft AI 1 . A transmitter/receiver system TR 1  that may include one or more transmitters or receivers drives, and receives signals from the antenna system AS 1 . 
     FIGS. 3 ,  3 A, and  4  illustrate an embodiment of details involving the antenna system AS 1  tuned to a VHF band. Here, a base plate BP 1  in the housing HO 1  rests on the surface of the aircraft AI 1 . A connector CN 1  secured on the base plate BP 1  projects from the housing HO 1  through the base plate and the skin of the aircraft AI 1 . The connector CN 1  supports a harmonic suppression filter FI 1 , within a grounded radiation-shielding can CA 1 , and in the housing HO 1 . The connector CN 1  connects the system AS 1  to the aforementioned transmitter/receiver system TR 1 . 
   In the system AS 1 , cable radiator CR 1  extends from the filter FI 1  into the upper end of the housing HO 1  and terminates in an open end. The cable radiator CR 1  contains an inner conductor IC 1  and a sleeve or shield or outer conductor OC 1 . At the lower end of the cable radiator CR 1  the inner conductor IC 1  connects to ground at the grounded shielding can CA 1  and the outer sleeve or outer conductor OC 1  of the cable radiator CR 1  connects to the harmonic suppression filter FI 1 . A capacitor Cp rests under the shielding can CA 1  and is connected across the filter FI 1  from the outer conductor OC 1  to ground. The antenna system AS 1  of  FIG. 3 , when incorporated in the housing HO 1  of  FIGS. 1 and 2  forms a monopole antenna. 
   The shielded harmonic suppression filter FI 1  is, according to various embodiments, a multiple section notch, low pass, or band pass filter built with either distributed or lumped element components. 
     FIG. 4  shows the equivalent circuit for the monopole antenna including the integrated harmonic suppression filter FI 1 . Here, the circuit also includes the monopole antenna radiator element or cable radiator CR 1  appearing as a series RLC circuit with an inductance Ls, a capacitance Cs and a resistance Rs tuned to the center frequency, for example 127.5 MHz, of a VHF band. An inductance Lp represents the “apparent” or “internal” inductance of the open-ended cable radiator CR 1  as seen from the radiator&#39;s lower end across the inner conductor IC 1  and the outer conductor OC 1 . The capacitor Cp connects across the inductance Lp. 
   The capacitor Cp has a value to tune the parallel circuit of capacitor and inductance Lp, to the same center frequency as the cable radiator CR 1  represented by the inductance Ls and capacitance Cs, for example 127.5 MHz. This produces the double-dip VSWR appearing in  FIG. 4A  across a band from 118 MHz to 137 MHz. The capacitor Cp and inductance Lp form an impedance matching network IM 1 . 
   According to different embodiments, the monopole antenna AN 1  of  FIGS. 1 to 4  and  3 A is tuned over various VHF and UHF frequencies (VHF Comm. 118–137 MHz, Orbcomm 137–150 MHz, Wideband VHF/Orbcomm 118–150 MHz, etc.). The integration of a low-profile shielded harmonic suppression filter with the monopole antenna&#39;s VHF/UHF impedance matching network suppresses any harmonic interference such as 12th or 13th harmonics, generated by the transmitter in the transmitter/receiver TR 1  that may be coincident with the frequency band(s) of other antennas tuned to any of GPS (1,575.42 MHz), WSI (1,544.5 MHz), XM Satellite and/or Sirius Satellite (2,332.0–2,345 MHz), Globalstar (2,483.5–2,500 MHz and 1,610.0–1,626.5 MHz), Iriduim (1,616–1,626.5 MHz), Satcom (1,530–1,559 and 1,626.5–1,660.5 MHz), etc. The resultant reduction in Radio Frequency Interference (RFI) from the monopole antenna allows for higher frequency antennas to be placed close to the monopole antenna without the risk of degrading their electrical performance. 
   Another embodiment of the antenna AN 1  appears in  FIG. 5 . Here the antenna system AS 1  in the housing HO 1  also includes the monopole antenna radiator element or cable radiator CR 1  also shown as the RLC circuit with inductance Ls, a capacitor Cs and a resistor Rs. It also includes an impedance matching network MI 2 , a wide band resistive T-configuration attenuator AT 1  composed of three resistors RE 1 , RE 2 , and RE 3 , and the shielded multiple section notch, low pass, or band pass filter IF built with either distributed or lumped element components. In the impedance matching network IM 2 , a capacitor CI 1  (33 pf in one embodiment for example) is placed in series with the attenuator AT 1  and a lumped or distributed parallel LI 1  inductor (530 nH in one embodiment for example) across the radiator CR 1 . The This structure enables the antenna AN 1  to tune over wider frequency bandwidths for either transmit or receive applications over frequency bands in the VHF/UHF frequency spectrum (Wideband VHF/Orbcomm 118–150 MHz, etc.). A (120 nH in one embodiment for example) lumped element or distributed inductor Lc in the housing HO 1  and across the input of (in parallel with) the filter FI 1  input compensates for parasitic capacitance (approximately 10 pF) of the filter FI 1  at VHF frequencies and broaden out the VSWR across the desired wideband VHF frequency band. This prevents the parasitic capacitance from adversely affecting the wideband resistive matching network that might otherwise narrow its VSWR bandwidth considerably. The input in  FIGS. 4 and 5  is the transmitter/receiver TR 1  designated as source SO 1 . In  FIG. 5  capacitor CI 1  tunes the impedance matching network IM 2  with the inductance Lp to the frequency of the radiator CR 1 . 
   In another embodiment, the antenna AN 1  lessens the drag of the number of aircraft antennas by incorporating two antenna radiators into the single housing HO 1 .  FIGS. 6 ,  7 , and  8  illustrate the antenna system AS 1  of such a device.  FIG. 6  is a plan view of the antenna system AS 1  and  FIG. 7  a section  7 — 7  of  FIG. 6 . Here, the base plate BP 1  supports a patch radiator PR 1  adjacent a structure containing the capacitor Cp that forms the impedance matching network IM 1  with the inductance Lp, and the harmonic suppression filter FI 1 . The cable radiator CR 1  extends upwardly from the filter FI 1 . The housing HO 1  covers the base plate BP 1 , and encloses the patch radiator PR 1 , the filter FI 1 , the impedance matching network MI 1 , and the cable radiator CR 1 . The shielding can CA 1  of  FIG. 3  encapsulates the filter FI 1 . 
     FIG. 8  shows an equivalent circuit of one embodiment of the antenna system AS  1  illustrated in  FIGS. 6 and 7  and includes the monopole antenna radiator element or cable radiator CR 1  having the RLC circuit with inductance Ls, capacitor Cs, and resistor Rs. It further includes the resonant impedance matching network IM 1  having capacitor Cp parallel to the “internal” inductance Lp of the radiator RC 1 ; and the harmonic suppression filter FI 1  in the form of a shielded multiple-section notch, low pass, or band pass filter built with either distributed or lumped element components. 
   Here again the impedance matching circuit IM 1 , containing capacitor Cp and inductor Lp, is tuned to the same frequency as the cable radiator CR 1  represented by the inductance Ls and capacitance Cs, in one embodiment 127.5 MHz. This produces the double-dip VSWR appearing in  FIG. 4A  across a band from 118 MHz to 137 MHz. 
   The right hand part of the circuit in  FIG. 8  includes the equivalent circuit of the second antenna radiator element, i.e., the patch radiator PR 1 . The latter is represented by equivalent parallel RLC circuit RC 1  with inductance Lm and capacitance Cm and Rm, and amplifier AM 1  (with or without band pass filtering for the application). A bias feed mechanism (not shown) provides DC current to the amplifier AM 1 . The RLC equivalent circuit RC 1  characterizes any of the GPS (1,575.42 MHz), WSI (1544.5 MHz), XM Satellite and/or Sirius Satellite (2,332.0–2,345 MHz), Globalstar (2483.5–2500 MHz and 1610.0–1626.5 MHz), Iriduim (1616–1626.5 MHz), Satcom (1530–1559 and 1626.5–1660.5 MHz), etc. antenna configurations.  FIG. 8  illustrates the mutual coupling between radiators PR 1  and CR 1 . The mutual coupling between of any harmonic RFI (Radio Frequency Interference) from the radiator CR 1  and the second antenna element patch radiator PR 1  is attenuated by the shielded harmonic suppression filter FI 1  in the VHF monopole impedance matching network. 
   Depending upon the values of the capacitor Cp and the structure of radiators CR 1  and PR 1 , this arrangement enables the antenna AN 1  to function over narrow to medium frequency bandwidths for either transmit or receive applications over various frequency bands in the VHF and UHF frequency spectrum (VHF Comm. 180–150 MHz, Orbcomm 137–150 MHz, etc.). At the same time, any harmonic electromagnetic interference that may be generated by the antenna transmitter TR 1  that could adversely affect any of the GPS (1,575.42 MHz), WSI (1544.5 MHz), XM Satellite and/or Sirius Satellite (2,332.0–2,345 MHz), Globalstar (2483.5–2500 MHz and 1610.0–1626.5 MHz), Iriduim (1616–1626.5 MHz), Satcom (1530–1559 and 1626.5–1660.5 MHz), etc. adjacent antennas are suppressed to non-interfering levels. 
   The radiator PR 1  need not be a patch radiator but may be another kind. According to the embodiment in  FIG. 6 , the patch radiator PR 1  is rotated to a diamond position relative to the filter FI 1 . This position tends further to limit the radio frequency interference from the cable radiator CR 1 . However, in other embodiments, the patch radiator PR 1  may be rotated to other positions, such as 90 degrees. 
     FIG. 9  illustrates an equivalent circuit of another embodiment of the antenna system AS 1  illustrated in  FIGS. 6 and 7  and includes the monopole antenna radiator element or cable radiator CR 1  having the RLC circuit with inductance Ls, capacitor Cs, and resistor Rs. It further includes a basic wide band resistive impedance matching network IM 2  with a capacitance CI 1  and an inductance LI 1  and a 3 db attenuator with resistors R 1 , R 2 , R 3  in T-configuration, and a shielded multiple section notch, low pass or band pass filter F 1 , built with either distributed or lumped element components. This enables the antenna to tune over wide frequency bandwidths for either transmit or receive applications over frequency bands in the VHF/UHF frequency spectrum (Wideband VHF/Orbcomm 118–150 MHz, etc.). A compensating inductor Lc (in the form of a lumped element such as 120 nH in one embodiment) or (in the form of a distributed inductor in another embodiment) across the input and parallel to the filter FI 1  input serves to cancel a (10 pF for example) parasitic capacitive reactance of the filter FI 1  that may occur at VHF frequencies. This avoids adversely affecting the wideband resistive matching network that would narrow its VSWR bandwidth considerably. This inductor Lc broadens out the VSWR across the desired wideband VHF frequency band. 
     FIGS. 10 ,  11 , and  12  are bottom, side, and top views illustrating details of an embodiment of an harmonic suppression filter FI 1  in the form of a notch filter NF 1  on a printed circuit board PC 1 .  FIG. 13  is schematic view and  FIG. 14  an equivalent circuit of the notch filter of  FIGS. 11 ,  12 , and  13 . In these figures, a substrate SU 1  supports the filter. Connected conductive traces CT 1 , CT 3 , and CT 5  oppose ground planes to form distributed quarter wave (λ/4) LC circuits L 11  and C 11 , L 31  and C 31 , L 51  and C 51 . The interconnecting conductive traces CT 2  and CT 4  form distributed quarter wave LC impedance inverters II 2  and II 4 . Conductive lands LA 1  spaced from, and surrounding the conductive traces CT 1 , CT 2 , CT 3 , CT 4 , and CT 5  on the top and bottom of the printed circuit board PC 1  provide the ground planes for the conductive traces. Conductive via holes VH 1  connect the lands L 1  on the top with the lands L 1  on the bottom of the printed circuit board PC 1 . A chip capacitor CP 1  (for example 47 pF) is used as part of the resonant impedance matching network IM 1  and corresponds to capacitor Cp in  FIGS. 4 ,  8 , and  14 . The notch filter NF 1  constitutes a third order microstrip notch filter. It has a characteristic low impedance shunt path to ground for resonant in-band (GPS, WSI, XM Satellite and/or Sirius Satellite, Globalstar, Iriduim, Satcom, etc.) harmonic energy. For VHF frequencies the filter FI 1  provides a high impedance path to ground and serves as a low insertion loss transmission line which couples base band energy directly to the antenna impedance matching network and radiator The conductive can CA 1  of  FIGS. 3 and 6  surrounding the PC board PC 1  shields the filter FI 1  to prevent leakage from the microstrip. 
   A capacitor CP 2  (for example 0.5 pf) extends from the central portion of conductive trace CT 3  to ground. The purpose of this capacitor is to adjust the effective length of the conductive trace CT 3 . Conductive traces on printed circuit boards often have insufficient space to follow straight paths and accordingly follow winding paths. However, the board may not provide enough room even for such folded paths. The capacitor CP 2  adjust for this deficiency in  FIGS. 10 ,  11 , and  12 . The Capacitor is necessary only when the length of the trace is inadequate. 
     FIGS. 15 ,  16 , and  17  are bottom, side, and top views illustrating details of an embodiment of an harmonic suppression filter F 12  in the form of a notch filter NF 2  for use with the impedance matching network IM 1  on a printed circuit board PC 1 .  FIG. 18  is an equivalent circuit of the notch filter of  FIGS. 15 ,  16 , and  17 .  FIG. 13  is schematic view of  FIGS. 15 ,  16 , and  17 . In these latter figures, a substrate SU 1  also supports the filter F 12  and the can CA 1  shields the filter. Connected conductive traces CT 1 , CT 3 , and CT 5  also oppose ground planes to form distributed quarter wave (λ/4) LC circuits L 11  and C 11 , L 31  and C 31 , L 51  and C 51 . The interconnecting conductive traces CT 2  and CT 4  form distributed quarter wave LC impedance inverters II 2  and II 4 . Conductive lands LA 1  spaced from, and surrounding the conductive traces CT 1 , CT 2 , CT 3 , CT 4 , and CT 5  on the top and bottom of the printed circuit board PC 1  provide the ground planes for the conductive traces. Conductive via holes VH 1  connect the lands L 1  on the top with the lands L 1  on the bottom of the printed circuit board PC 1 . The chip inductor Lc of  FIG. 9  at the input serves to retune the broadband matching network. Its placement, in parallel to ground with the stray parasitic parallel capacitance to ground of the notch filter, brings them both to parallel resonance and high impedance at VHF frequencies. This effectively cancels detrimental capacitive loading effects of the filter FI 2  and allows the resistive matching network to function properly. 
   The printed circuit board PC 1  in  FIGS. 15 ,  16 , and  17  also uses a capacitor CP 2  from the central portion of the trace CT 3  to ground in order to adjust the effective length of the trace. 
     FIG. 19  illustrates another embodiment of the arrangement in  FIGS. 3 and 3A  as applied to  FIGS. 4 and 8 . Here, a shorted quarter wave stub ST 1  appears across the capacitor Cp of  FIGS. 3 ,  3 A,  4 , and  8 . The quarter wave is for the center frequency of the VHF band at which the cable radiator operates. A short circuit ST 1  for the stub ST 1  is shown at the end opposite the connection to the capacitor Cp. The shorted quarter wave stub ST 1  has the effect of forming a DC ground for the shield or outer conductor OC 1  of the cable radiator CR 1 . The shorted quarter wave stub ST 1  also has the effect applying a high radio-frequency impedance at the center and across the VHF band at which the cable radiator CR 1  operates. The stub ST 1  is, according to an embodiment of the invention, coiled around the can CA 1  in  FIG. 3 . The DC grounding function of the stub ST 1  is not needed in  FIGS. 5 and 9  because the inductor LI 1  serves that purpose. 
   The embodiments of the invention permit close placement of aircraft antennas and combination antennas involving enclosure of multiple antenna radiators in a single aerodynamic housing. While the tuning of VHF/UHF radiators and matching circuits produce bandpasses as shown in  FIG. 4A , the embodiments of the invention suppress harmonic, such as 12th and 13th harmonics, substantially outside the range of the desired VHF/UHF bandpass from affecting nearby higher frequency radiators. 
   According to another embodiment of the invention, the frequencies defined by the inductance and capacitance values of the impedance matching networks IM 1  and IM 2  do not equal frequencies defined by the inductance Ls and capacitance Cs, but are only sufficiently close to widen the bandbass of the radiator CR 1 . 
   While embodiments of the invention have been described in detail, it will be evident to those skilled in the art that the invention may be embodied otherwise.