TITLE OF THE INVENTION
ANTENNA SYSTEM
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an antenna system employing ferrite bar antenna elements designed to work on multiple frequency bands.
2. Description of the Related Art
A system known as NAVTEX (which stands for Navigational Telex) is operated on an international basis to provide vessels navigating within approximately 300 nautical miles from the coast with maritime safety information, such as navigational warnings and meteorological warnings. In Japan, NAVTEX services are provided by Japan Coast Guard, the services including international NAVTEX broadcasts in English on 518 kHz and national NAVTEX broadcasts in Japanese on 424 kHz.
Resolution MSC.148 (77) adopted by the International Maritime Organization (IMO) requires specific classes of vessels built on and after July 1, 2005 to be equipped with a NAVTEX receiver conforming to performance standards not inferior to those specified in the Resolution, wherein the NAVTEX receiver must be capable of simultaneous reception of international NAVTEX broadcasts on 518 kHz and additional NAVTEX broadcasts either on 490 kHz or 4209.5 kHz.
Two-band active antennas usable on both 518 kHz and 490 kHz are already in practical use. These active antennas currently available include a single whip type electric field antenna and a ferrite bar type magnetic field antenna.
Japanese Patent Application Publication No. 2000312110 discloses an example of an antenna employing a plurality of ferrite bar antenna elements. This antenna is intended for receiving a single-frequency radio signal, such as a signal broadcast from a beacon station (base station) of a Differential Global Positioning System (DGPS) in a 300 kHz band. The ferrite bar antenna elements are individually wound by coils and placed between upper and lower printed wiring boards. The antenna is configured such that the ferrite bar antenna elements are shielded from external electric fields.
To enable reception of the maritime safety information from a long range, it is preferable that the NAVTEX receiver can receive NAVTEX broadcasts on the aforementioned internationally adopted frequency of 4209.5 kHz in the high-frequency (HF) band in addition to 518 kHz in the medium-frequency (MF) band. This capability will be achieved by one of antenna options which include: (1) a single electric field antenna for receiving both MF- and HF-band signals; (2) a single magnetic field antenna for receiving both MF- and HF-band signals; and (3) a dualantenna system including electric and magnetic field antennas.
The aforementioned antenna options as used for receiving the MF- and HF-band signals have their own disadvantages.
Specifically, the electric field antenna, by its very nature, requires good radio frequency (RF) grounding. If this type of antenna is be mounted on a nonmetallic mast, the antenna must be provided with a long grounding wire. When RF grounding conditions of the antenna deteriorate due to corrosion by salty sea water, for example, with the lapse of time, antenna gain for reception dramatically deteriorates. This is because a path for antenna current is significantly reduced as a result of the deterioration of grounding quality. Directional characteristics of the electric field antenna in a horizontal plane vary due to electrostatic coupling between the antenna and any nearby conductor. A NAVTEX antenna should ideally be omnidirectional in the horizontal plane.The electric field antenna picks up unwanted electric field components radiated from nearby noise sources, either onboard or outside a vessel on which the antenna is installed. Since the electric field antenna can not be shielded from external electric fields, a desired signal received by the electric field antenna is masked by a high-intensity pulse signal transmitted by a radar, for example, due to nonlinearity of a tuning circuit and an amplifier circuit of a receiver to which the antenna is connected.
The single magnetic field antenna for multiband application includes a plurality of ferrite bars which are individually wound by coils for receiving signals of different frequency bands. The magnetic field antenna thus configured is prone to a decrease in effective Q-value due to inductive coupling among the coils, resulting in a reduction in power (voltage) thus induced in the individual coils. Mutual induction among the multiple coils would cause deviations of tuning frequencies in both the MF and HF bands. Additionally, as compared to the electric field antenna, the magnetic field antenna by its nature can not have a large effective height.
Compared to the aforementioned first and second antenna options, the third antenna option, or the dualantenna system, can much easily receive 518 kHz (490 kHz) signals as well as 4209.5 kHz signals by use of the magnetic field antenna and the electric field antenna, respectively. The dual-antenna system also has its own disadvantages as stated above. In the MF band, frequencies from 4065 kHz to 4143 kHz allocated to marine radio communications for single side band (SSB) voice communication, for instance, as well as frequencies from 4202.5 kHz to 4207.0 kHz allocated to Narrow Band Direct Printing (NBDP) are used on ships. As the 4209.5 kHz signals can interfere with these frequencies transmitted onboard or from nearby ships, it is necessary that a whip antenna used as the electric field antenna have a relatively small height.On the other hand, however, signal-to-noise ratio (SNR) decreases when the electric field antenna receives unwanted electric field components radiated from nearby noise sources, either onboard or outside the vessel on which the antenna is installed. Accordingly, to achieve a specific level of SNR, the height of the whip type electric field antenna can not be significantly reduced.
SUMMARY OF THE INVENTION
In light of the aforementioned problems of the prior art, it is an object of the invention to provide an antenna system capable of receiving signals in a plurality of frequency bands, the antenna system providing ease of installation, stability over the lapse of time, compatibility with electric field interference and a sufficiently high antenna gain for reception.
In a first principal form of the invention, an antenna system includes at least two ferrite bar antenna elements each of which includes a ferrite bar, a first coil for a first frequency band and a second coil for a second frequency band, the first and second coils being wound on the ferrite bar at positions separated from each other, and a circuit connected to the first and second coils for simultaneously receiving electromagnetic waves in the first and second frequency bands.
In this antenna system, each of the ferrite bar antenna elements is configured such that the first coil for the first frequency band and the second coil for the second frequency band are wound on the ferrite bar at the positions separated from each other. This configuration serves to reduce the degree of inductive coupling between the first and second coils. Consequently, a decrease in effective Q-value lessens and a reduction in power (voltage) induced in the first and second coils becomes smaller, making it possible to achieve a desired level of antenna gain. Also, this configuration of the invention serves to reduce deviations of tuning frequencies in both first and second frequency bands. Also, The circuit connected to the first and second coils for simultaneously receiving electromagnetic waves in the first and second frequency bands without switching between the two bands.
In a second principal form of the invention, an antenna system includes at least two ferrite bar antenna elements each of which includes a ferrite bar, a first coil for a first frequency band and a second coil for a second frequency band, the first and second coils being wound on the ferrite bar at positions separated from each other, and an electrostatic shield for shielding the ferrite bar antenna elements from external electric fields.
In this structure of the invention, the ferrite bar antenna elements are surrounded by the electrostatic shield, so that the antenna system becomes less susceptible to adverse influence of external electric fields acting as noise which are always present in any surrounding environment. Therefore, a desired signal received by the antenna system is not masked or suppressed by a highintensity pulse signal transmitted by a radar, for example, installed in the proximity of the antenna system. Additionally, distributed capacitances are created between the electrostatic shield and the first and second coils. This serves to mitigate inductive coupling between the first and second coils so that the antenna system can simultaneously receive radio signals in the first and second frequency bands in a desirable manner.
In one feature of the invention, the antenna system of the aforementioned second principal form of the invention further includes two printed wiring boards by which the ferrite bar antenna elements are sandwiched, and side printed wiring boards which close a space formed between the two printed wiring boards, wherein the electrostatic shield includes electrically shielding conductor patterns formed on the two printed wiring boards and electrically shielding conductor patterns formed on the side printed wiring boards.
In this structure of the invention, the two printed wiring boards sandwiching the ferrite bar antenna elements electrically shield top and bottom faces of the ferrite bar antenna elements while the side printed wiring boards electrically shield lateral faces of the ferrite bar antenna elements. The exterior of the ferrite bar antenna elements is thoroughly shielded in this way so that the antenna system is less susceptible to adverse influence of external electric fields acting as noise arriving from all directions.
In another feature of the invention, the antenna system further includes a first tuned amplifier circuit for amplifying a signal received in the first frequency band, the first tuned amplifier circuit including a tuning circuit formed of the first coil and a first tuning capacitor for the first frequency band, a second tuned amplifier circuit for amplifying a signal received in the second frequency band, the second tuned amplifier circuit including a tuning circuit formed of the second coil and a second tuning capacitor for the second frequency band, and an output multiplexing circuit for simultaneously outputting an output signal of the first tuned amplifier circuit and an output signal of the second tuned amplifier circuit through a common output terminal.
The antenna system according to this feature of the invention includes the first tuned amplifier circuit for amplifying the signal received in the first frequency band, the first tuned amplifier circuit including the tuning circuit formed of the first coil and the first tuning capacitor for the first frequency band, the second tuned amplifier circuit for amplifying the signal received in the second frequency band, the second tuned amplifier circuit including the tuning circuit formed of the second coil and the second tuning capacitor for the second frequency band, and the output multiplexing circuit for simultaneously outputting the output signal of the first tuned amplifier circuit and the output signal of the second tuned amplifier circuit through the common output terminal as stated above.The antenna system thus structured can simultaneously receive signals in the first and second frequency bands so that the antenna system can receive a specific broadcast in a continuous and reliable manner.
In another feature of the invention, one of the ferrite bar antenna elements is oriented along an x-axis direction and another one of the ferrite bar antenna elements is oriented along a y-axis direction which is perpendicular to the x-axis direction, and the antenna system further includes a first phase-shift adder circuit for adding an output signal of a tuning circuit for the first frequency band including the first coil of the ferrite bar antenna element oriented along the x-axis direction and an output signal of a tuning circuit for the second frequency band including the first coil of the ferrite bar antenna element oriented along the y-axis direction with a phase difference of 90 degrees,and a second phase-shift adder circuit for adding an output signal of a tuning circuit for the second frequency band including the second coil of the ferrite bar antenna element oriented along the x-axis direction and an output signal of a tuning circuit for the second frequency band including the second coil of the ferrite bar antenna element oriented along the y-axis direction with a phase difference of 90 degrees.
The antenna system according to this feature of the invention includes the first phase-shift adder circuit for adding the output signal of the tuning circuit for the first frequency band including the first coil of the ferrite bar antenna element oriented along the x-axis direction and the output signal of the tuning circuit for the second frequency band including the first coil of the ferrite bar antenna element oriented along the y-axis direction with the phase difference of 90 degrees,and the second phase-shift adder circuit for adding the output signal of the tuning circuit for the second frequency band including the second coil of the ferrite bar antenna element oriented along the x-axis direction and the output signal of the tuning circuit for the second frequency band including the second coil of the ferrite bar antenna element oriented along the y-axis direction with the phase difference of 90 degrees as stated above. A plane formed by the x-axis and the y-axis of this antenna system provides a desirable omnidirectional property.
In still another feature of the invention, the antenna system includes at least four ferrite bar antenna elements two of which are oriented along an x-axis direction and other two of which are oriented along a y-axis direction which is perpendicular to the x-axis direction, wherein the first coils for the first frequency band of the two ferrite bar antenna elements oriented along the x-axis direction are connected in series, and the second coils for the second frequency band of the other two ferrite bar antenna elements oriented along the y-axis direction are connected in series.
The antenna system according to this feature of the invention includes at least four ferrite bar antenna elements two of which are oriented along the x-axis direction and other two of which are oriented along the yaxis direction which is perpendicular to the x-axis direction, wherein the first coils for the first frequency band of the two ferrite bar antenna elements oriented along the x-axis direction are connected in series, and the second coils for the second frequency band of the other two ferrite bar antenna elements oriented along the y-axis direction are connected in series. The antenna system thus structured provides high antenna gain.Since the multiple ferrite bar antenna elements are disposed along the x- and y-axis directions which are perpendicular to each other, a space required for mounting the ferrite bar antenna elements does not proportionally increase with an increase in the number of ferrite bar antenna elements arranged along the two directions, thus providing improved antenna gain without substantial size increase of the antenna system.
These and other objects, features and advantages of the invention will become more apparent upon a reading of the following detailed description in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram showing how ferrite bars of a plurality of ferrite bar antenna elements are wound by coils and how these coils are connected in an antenna system according to a preferred embodiment of the invention; FIGS. 2(A) and 2(B) are diagrams showing how the individual coils of each ferrite bar antenna element are connected; FIG. 3 is a circuit diagram showing how mutual induction occurs between one each MF coil and HF coil of a single ferrite bar antenna element; FIG. 4 is an equivalent circuit diagram showing how an electrostatic shield works when provided around the ferrite bar antenna element of FIG. 3; FIG. 5 is a diagram showing the circuit configuration of the entire antenna system; FIG. 6 is a perspective view of a principal part of the antenna system; FIG. 7 is an exploded perspective view of the principal part of the antenna system;FIG. 8 is a front view of a side printed wiring board; FIGS. 9(A) and 9(B) are a plan view and a side view of the principal part of the antenna system, respectively; FIG. 10 is a cross-sectional diagram showing the entirety of the antenna system of the embodiment; FIG. 11 is a diagram showing a horizontal-plane radiation pattern of the antenna system obtained from measurements at 518 kHz; FIG. 12 is a diagram showing a horizontal-plane radiation pattern of the antenna system obtained from measurements at 490 kHz; and FIG. 13 is a diagram showing a horizontal-plane radiation pattern of the antenna system obtained from measurements at 4209.5 kHz.
DETAILED DESCRIPTION OF THE PREFERRED
EMBODIMENTS OF THE INVENTION
An antenna system according to a preferred embodiment of the present invention is now described with reference to the accompanying drawings.
FIG. 1 is a diagram showing how ferrite bars 7, 8, 9, 10 of a plurality of ferrite bar antenna elements are wound by coils 15xa, 15xb, 15ya, 15yb, 16xa, 16xb, 16ya, 16yb and how these coils 15xa, 15xb, 15ya, 15yb, 16xa, 16xb, 16ya, 16yb are connected in the antenna system. The ferrite bars 7, 8, 9, 10 are cylindrical rods formed by shaping ferrite. Of these ferrite bars 7, 8, 9, 10, the ferrite bars 7, 8 are arranged parallel to an x-axis direction whereas the ferrite bars 9, 10 are arranged parallel to a y-axis direction. In FIG. 1, the ferrite bars 7, 8 oriented along the x-axis direction and the ferrite bars 9, 10 oriented along the y-axis direction are separately illustrated for the sake of explanation.
As shown in FIG. 1, the ferrite bar 7 oriented along the x-axis direction is wound by the coil 15xa for MF and the coil 16xa for HF, while the ferrite bar 8 oriented along the x-axis direction is wound by the coil 15xb for MF and the coil 16xb for HF. On the other hand, the ferrite bar 9 oriented along the y-axis direction is wound by the coil 15ya for MF and the coil 16ya for HF, while the ferrite bar 10 oriented along the y-axis direction is wound by the coil 15yb for MF and the coil 16yb for HF. On each of the ferrite bars 7, 8, 9, 10, one of the MF coils 15xa, 15xb, 15ya, 15yb (hereinafter referred to collectively as the MF coils 15) and one of the HF coils 16xa, 16xb, 16ya, 16yb (hereinafter referred to collectively as the HF coils 16) are wound with a specific distance from each other.
Specifically, each of the cylindrical ferrite bars 7, 8, 9, 10 is 9 mm in diameter and 70 mm in length. The MF coils 15 for MF-band frequencies (490 kHz, 518 kHz) each have 100 turns of winding, while the HF coils 16 for HFband frequency (4209.5 kHz) each have 20 turns of winding. The MF coil 15 and the HF coil 16 wound on each of the ferrite bars 7, 8, 9, 10 are separated by 20 mm from each other.The coils 15xa-15xb (16xa-16xb, 15yb-15yb, 16ya16yb) for the same frequency band wound on each pair of ferrite bars 7, 8 (9, 10) arranged parallel to each other are interconnected and winding directions of these coils 15xa-15xb (16xa-16xb, 15yb-15yb, 16ya-16yb) are determined in such a way that the ferrite bar antenna elements oriented along each direction would efficiently receive magnetic field components of electromagnetic waves incident from the same direction and voltages induced in each pair of coils 15xa-15xb (16xa-16xb, 15yb-15yb, 16ya-16yb) would add up, thereby producing increased antenna outputs to a receiver to which the antenna system is connected.
More specifically, one end of the MF coil 15xb is grounded, the other end of the MF coil 15xb is connected to one end of the MF coil 15xa, and the other end of the MF coil 15xa is connected to one end of an MF tuning capacitor 17x whose other end is grounded. The MF coils 15xa, 15xb and the MF tuning capacitor 17x are so connected as to deliver an output to a later-described stagger tuned amplifier circuit SAx for MF. Also, one end of the HF coil 16xb is grounded, the other end of the HF coil 16xb is connected to one end of the HF coil 16xa, and the other end of the HF coil 16xa is connected to one end of an HF tuning capacitor 18x whose other end is grounded. The HF coils 16xa, 16xb and the HF tuning capacitor 18x are so connected as to deliver an output to a later-described single-tuned amplifier circuit MAx for HF.
Similarly, one end of the MF coil 15yb is grounded, the other end of the MF coil 15yb is connected to one end of the MF coil 15ya, and the other end of the MF coil 15ya is connected to one end of an MF tuning capacitor 17y whose other end is grounded. The MF coils 15ya, 15yb and the MF tuning capacitor 17y are so connected as to deliver an output to a later-described stagger tuned amplifier circuit SAy for MF. Also, one end of the HF coil 16yb is grounded, the other end of the HF coil 16yb is connected to one end of the HF coil 16ya, and the other end of the HF coil 16ya is connected to one end of an HF tuning capacitor 18y whose other end is grounded. The HF coils 16ya, 16yb and the HF tuning capacitor 18y are so connected as to deliver an output to a later-described single-tuned amplifier circuit MAy for HF.
To add, as can be seen from FIG. 1, two each coils connected in series are wound in opposite directions. For example, the coils 15xa, 16xa are wound in a clockwise direction on the ferrite bar 7 as viewed from each end thereof, whereas the coils 15xb, 16xb are wound in a clockwise direction on the ferrite bar 8 as viewed from each end thereof. This coil winding method serves to prevent undesired inductive coupling between the coils wound on any two parallel ferrite bar antenna elements.
FIG. 2(A) is a circuit diagram of a pair of MF tuning circuits each of which includes two series-connected MF coils of two parallel ferrite bar antenna elements and one MF tuning capacitor. HF tuning circuits can also be expressed by a similar circuit diagram. Two coils for the same frequency band of each pair of two parallel ferrite bar antenna elements are connected in series in this fashion.
The output Sx from one MF tuning circuit including the MF coils of the two parallel ferrite bar antenna elements oriented along the x-axis direction and the output Sy from the other MF tuning circuit including the MF coils of the other two parallel ferrite bar antenna elements oriented along the y-axis direction are added with a phase difference of 90 degrees as shown in FIG. 2(B), whereby a generally omnidirectional antenna response in a horizontal plane is obtained.
While the above-described example of the antenna system is provided with two each ferrite bar antenna elements aligned along the x-axis direction and the y-axis direction as depicted in FIGS. 1, 2(A) and 2(B), the antenna system may be formed of one each ferrite bar antenna element oriented along the x- and y-axis directions.
FIG. 3 is a circuit diagram showing how mutual induction occurs between one each MF coil and HF coil wound on a single ferrite bar. Referring to FIG. 3, the mutual induction occurring between the MF coil and the HF coil wound on the same ferrite bar is explained.
For the sake of the following discussion, the ferrite bar antenna element including the MF coil 15xa and the HF coil 16xa is used as an example. Current il which flows through the MF coil 15xa when the ferrite bar antenna element resonates in an MF band and current i2 which flows through the MF coil 16xa when the ferrite bar antenna element resonates in an HF band are expressed as follows:
i1 = El/rl i2 = E2/r2 where El and E2 are voltages induced in the coils 15xa and 16xa, respectively, and rl and r2 are resistance components of the coils 15xa and 16xa, respectively.
The induced voltages can also be expressed as follows:
EL1 = ilx2 fL1 EL2 = i2x2TT fL2 where Ll and L2 are inductances of the coils 15xa and 16xa, respectively.
Electromotive forces Eml, Em2 induced in the two coils 15xa, 16xa due to the mutual induction therebetween can be calculated by the following equations:
Eml = -Mdi2(t)/dt Em2 = -Mdil(t)/dt These electromotive forces Eml, Em2 induced in the two coils 15xa, 16xa cause deviations of tuning frequencies in both the MF and HF bands and, as a consequence, an effective Q-value would decrease. To prevent this from taking place, the distance between the two coils for the two different frequency bands wound on each ferrite bar is relatively large, so that the aforementioned mutual induction represented by the letter M in FIG. 3 would decrease, resulting in a reduction in the electromotive forces Eml, Em2.
FIG. 4 is an equivalent circuit diagram showing a condition in which the ferrite bar antenna element of FIG. 3 is provided with an electrostatic shield (Faraday shield) shown by broken lines. This configuration produces distributed capacitances Cl-1 to Cl-n between the electrostatic shield and the MF coil 15xa as well as distributed capacitances C2-1 to C2-n between the electrostatic shield and the HF coil 16xa. In this condition, the part of aforementioned resonant current il flows through the individual distributed capacitances Cl-1 to Cl-n while part of the aforementioned resonant current i2 flows through the individual distributed capacitances C2-1 to C2-n.More specifically, the resonant current il flowing through the MF coil 15xa is expressed by the equation il = (sum of distributed currents il-1 to i1-n) + (current i1' flowing through resistive and inductive components of the MF coil 15xa). Likewise, the resonant current i2 flowing through the HF coil 16xa is expressed by the equation i2 = (sum of distributed currents i2-1 to i2-n) + (current i1' flowing through resistive and inductive components of the HF coil 16xa).
If the MF coils 15xa, 15xb each have 100 turns of winding as mentioned earlier and together produce a combined inductance LI of 1480 H, capacitance needed for tuning at 500 kHz is 68 pF. If the sum of the distributed capacitances Cl-1 to Cl-n is approximately 53 pF, for example, the MF tuning capacitor 17x must have a capacitance Cl of 15 pF as a target value. Thus, a 30 pF trimmer capacitor is used as the MF tuning capacitor 17x.
Also, if the HF coils 16xa, 16xb each have 20 turns of winding as mentioned earlier and together produce a combined inductance L2 of 74 H, capacitance needed for tuning at 4209.5 kHz is 19 pF. If the sum of the distributed capacitances C2-1 to C2-n is approximately 9 pF, for example, the HF tuning capacitor 18x must have a capacitance C2 of 10 pF as a target value. Thus, a 20 pF trimmer capacitor is used as the HF tuning capacitor 18x.
With this arrangement, the coefficient of coupling between the aforementioned inductances L1 and L2 actually measured was 0.4 or less. This means that adverse influence of the aforementioned mutual induction on the MF and HF tuning circuits can be decreased by the electrostatic shield described above down to a level which is acceptable for practical applications. It is appreciated from the above discussion that the provision of the electrostatic shield serves to mitigate inductive coupling between the MF and HF coils wound on any of the ferrite bars 7, 8, 9, 10 so that the antenna system can simultaneously receive MF and HF radio signals in a much desirable manner.
The electrostatic shield also produces the following advantages:
(1) The electrostatic shield makes the antenna system less susceptible to interference with electric fields radiated from nearby onboard equipment (noise sources), thereby providing an enhanced anti-noise capability.
(2) The electrostatic shield serves to prevent electrostatic coupling between the ferrite bar antenna elements and nearby conductors and achieve uniformity in directivity in the horizontal plane, or omnidirectionality, of the antenna system. The nearby conductors mentioned here may include internal conductors of the antenna system, such as conductor patterns for arranging the MF/HF coils 15, 16 on printed wiring boards and conductor patterns of the amplifier circuits MAx, MAy, SAx, SAy located in central areas of the printed wiring boards, as well as external conductors, such as a mast on which the antenna system is mounted.
(3) The electrostatic shield serves as a reflector for radio waves (3 GHz or 9 GHz) emitted from a radar antenna which may be installed in the proximity of the antenna system, thereby preventing adverse influence of the radio waves from the radar antenna.
Referring now to FIG. 5, the circuit configuration of the antenna system of the present embodiment used as a NAVTEX antenna is described in detail.
A series circuit made of the MF coils 15xa, 15xb of the two ferrite bar antenna elements oriented along the xaxis direction is connected parallel to the MF tuning capacitor 17x to together configure one of the aforementioned two MF tuning circuits. The output of this MF tuning circuit is fed into a gate of a field effect transistor (FET) Qlx in a first stage of the aforementioned stagger tuned amplifier circuit SAx for the MF band. Also, a series circuit made of the MF coils 15ya, 15yb of the two ferrite bar antenna elements oriented along the y-axis direction is connected parallel to the MF tuning capacitor 17y to together configure the other MF tuning circuit.The output of this MF tuning circuit is fed into a gate of an FET Qly in a first stage of the aforementioned stagger tuned amplifier circuit SAy for the MF band.
Similarly, a series circuit made of the HF coils 16xa, 16xb of the two ferrite bar antenna elements oriented along the x-axis direction is connected parallel to the HF tuning capacitor 18x to together configure one of the aforementioned two HF tuning circuits. The output of this HF tuning circuit is fed into a gate of an FET Q2x in a first stage of the aforementioned stagger tuned amplifier circuit MAx for the HF band. Also, a series circuit made of the HF coils 16ya, 16yb of the two ferrite bar antenna elements oriented along the y-axis direction is connected parallel to the HF tuning capacitor 18y to together configure the other HF tuning circuit. The output of this HF tuning circuit is fed into a gate of an FET Q2y in a first stage of the aforementioned stagger tuned amplifier circuit MAy for the HF band.
The stagger tuned amplifier circuits SAx, SAy amplify output signals of the FETs Qlx, Qly, respectively. Sharpness of frequency response of these stagger tuned amplifier circuits SAx, SAy is reduced so that each has a bandwidth of at least 490 kHz to 518 kHz. The stagger tuned amplifier circuits MAx, MAy amplify output signals of the FETs Q2x, Q2y, respectively. Each of these stagger tuned amplifier circuits MAx, MAy has a tuning frequency of 4209.5 kHz.
A second-stage amplifier circuit Am provided in a succeeding stage of the stagger tuned amplifier circuits SAx, SAy amplifies a signal obtained by adding up output signals of the stagger tuned amplifier circuits SAx, SAy. As shown in FIG. 5, an output terminal of the stagger tuned amplifier circuit SAx is connected to a mixing point through a capacitor Cm whereas an output terminal of the stagger tuned amplifier circuit SAy is connected to the mixing point through a resistor Rm, so that the output signals of the stagger tuned amplifier circuits SAx, SAy are added with a phase difference of 90 degrees.
A second-stage amplifier circuit As provided in a succeeding stage of the stagger tuned amplifier circuits MAx, MAy amplifies a signal obtained by adding up output signals of the stagger tuned amplifier circuits MAx, MAy. As shown in FIG. 5, an output terminal of the single-tuned amplifier circuit MAx is connected to a mixing point through a capacitor Cs whereas an output terminal of the single-tuned amplifier circuit MAy is connected to the mixing point through a resistor Rs, so that the output signals of the stagger tuned amplifier circuits MAx, MAy are also added with a phase difference of 90 degrees.
A low-pass filter LPF connected to the second-stage amplifier circuit Am is a filter which passes MF-band signals (490 kHz, 518 kHz) but cuts off HF-band signals (4209.5 kHz). A high-pass filter HPF connected to the second-stage amplifier circuit As is a filter which passes HF-band signals (4209.5 kHz) but cuts off MF-band signals (490 kHz, 518 kHz). These two filters together constitute an output multiplexing circuit which matches output impedance of the second-stage amplifier circuit Am for the MF-band with output impedance of the second-stage amplifier circuit As for the HF-band and delivers a multiplexed output signal to a coaxial output terminal.This configuration of the antenna system makes it possible to simultaneously output the MF- and HF-band signals fed from the second-stage amplifier circuit Am and the second-stage amplifier circuit As through the single coaxial output terminal without interference between the MF- and HF-band circuits.
The output multiplexing circuit built up of the lowpass filter LPF and the high-pass filter HPF simultaneously outputs the MF- and HF-band signals without switching between the two bands through the single output terminal so that a NAVTEX receiver connected to the coaxial output terminal can receive the MF- and HF-band signals at the same time.
In the antenna system of this embodiment, the aforementioned circuit components other than the ferrite bar antenna elements are disposed on a pair of upper and lower printed wiring boards 1, 2 and the circuit components on the lower printed wiring board 2 are enclosed by shield cases 24 which serves as electrostatic shields.
Effective height he of the ferrite bar antenna system shown in FIG. 1 is given by he = Q Micro e (2 NA/ ) Microe = a Micro m where = wavelength of received signal A = effective cross section area of a ferrite bar r (r = radius) N = total number of turns of a coil Q = overall Q-value (effective Q-value) of the coil, a tuning capacitor and an amplifier under loaded conditions Specifically, the effective height he is calculated as follows.
(1) Effective antenna height for MF band (500 kHz) Provided that N = 200, r = 4.5 mm, Q = 10 and Micro e = 0.8 Microm = 23.2, the effective height he of the ferrite bar antenna system is he = 0.031 (m) for the MF-band signal of 500 kHz.
Indicated by Micro m in the above calculation is permeability (290) of a toroidal core of a ring antenna. This value of m is multiplied by a factor or 0.8 to obtain effective permeability p e of a straight core of a bar antenna.
If output level of a receiver is 6 dBMicro, input voltage of the amplifier is calculated as follows: 6 dBMicro - 15 dB = -9 dBMicro , where 15 dB is overall gain of the amplifier circuits MAx, MAy, SAx, SAy shown in FIG. 5. Thus, a lowest electric field intensity Emu in the MF band receivable by the antenna system is calculated as follows:
Emu = -9(dB ) - Ehe = -9 - (-30.2) = 21.2 (dBMicro /m) where Ehe = 201og(he).
(2) Effective antenna height for HF band (4209.5 kHz) Provided that N = 40, r = 4.5 mm, Q = 10 and Micro e = 0.8 Micro m = 23.2, the effective height he of the ferrite bar antenna system is he = 0.052 (m) for the HF-band signal of 4209.5 kHz.
Thus, a lowest electric field intensity Emu in the HF band receivable by the antenna system is calculated as follows:
Emu = -9 (dBMicro ) - Ehe = -9 - (-25.6) = 16.6 (dBMicro /m) where Ehe = 20 log (he) .
It is appreciated from the foregoing discussion that the antenna system of the embodiment has sufficiently high gain for use in both the MF and HF bands.
The mechanical structure of the antenna system of the present embodiment is now described with reference to FIGS. 6 to 10.
FIG. 6 is a perspective view of a principal part of the antenna system, in which designated by the numeral 1 is the earlier-mentioned upper printed wiring board which is arranged parallel to the lower printed wiring board 2 (hidden by the upper printed wiring board 1 in FIG. 6). Designated by the numerals 3, 4, 5 and 6 are side printed wiring boards which close a space formed between the parallel-arranged upper and lower printed wiring boards 1, 2 on four sides. Conductor patterns formed on the individual printed wiring boards 1-6 are not illustrated in FIG. 6 for simplicity. FIGS. 9(A) and 9(B) are a plan view and a side view of the principal part of the antenna system, respectively. FIG. 7 is an exploded perspective view of the aforementioned principal part of the antenna system, with the four side printed wiring boards 3-6 removed.
Referring to FIGS. 7, 9(A) and 9(B), the ferrite bars 7, 8, 9, 10 of the four ferrite bar antenna elements are attached to a top surface of the lower printed wiring board 2 by means of support members 11, 12, 13, 14 made of electrically conductive rubber. The support members 11, 12, 13, 14 have holes in which ends of the ferrite bars 7, 8, 9, 10 are inserted. Bosses designated by "b" are formed on top surfaces of the individual support members 11, 12, 13, 14. Similar bosses are also formed on bottom surfaces of the individual support members 11, 12, 13, 14.FIG. 7 shows a condition in which the bosses of the support members 11, 12, 13, 14 projecting downward from the bottom surfaces thereof have been fitted into four holes formed in the lower printed wiring board 2 close to four corners thereof with the ends of the ferrite bars 7, 8, 9, 10 inserted into the holes formed in the support members 11, 12, 13, 14.
Four holes are also formed in the upper printed wiring board 1 close to four corners thereof. The upper printed wiring board 1 is placed on the support members 11, 12, 13, 14 so that the bosses formed on the top surfaces thereof fit into the holes formed in the upper printed wiring board 1, whereby the upper and lower printed wiring boards 1, 2 are assembled together with the ferrite bars 7, 8, 9, 10.
There are formed square-shaped grounding pads G in central areas of top and bottom surfaces of the upper printed wiring board 1 and the lower printed wiring board 2 as illustrated in FIG. 7. Additionally, conductor patterns designated by sl, s2, s3, s4 serving as shields against external electric fields are formed along four sides of each of the upper and lower printed wiring boards 1, 2.
These conductor patterns (hereinafter referred to as electrostatic shield patterns) sl, s2, s3, s4 are made of narrowly separated fine conductor lines running parallel to each side of the upper and lower printed wiring boards 1, 2. The fine parallel conductor lines of each of the electrostatic shield patterns sl, s2, s3, s4 are joined by another conductor line running at right angles to the fine parallel conductor lines at a specific position thereof (a mid-length position in this embodiment so that the fine parallel conductor lines are divided into left-right symmetric halves) and connected to the grounding pad G.While the fine parallel conductor lines of each of the electrostatic shield patterns sl, s2, s3, s4 are electrically connected at the mid-length position as mentioned above, opposites ends of the fine parallel conductor lines are not directly joined but separated by a specific distance from one another. Such conductor patterns made of the conductor lines running parallel to the four sides of the printed wiring boards 1, 2 prevent electric field components oriented parallel to the conductor lines from passing through the conductor patterns, thus serving as a shield against external electric fields. On the other hand, these conductor patterns permit magnetic field components oriented parallel to the conductor lines to pass through, so that the provision of the electrostatic shield patterns sl, s2, s3, s4 does not act to decrease gain of the ferrite bar antenna system.This kind of electrostatic shield is referred to also as a Faraday shield.
The earlier-mentioned shield cases 24 for shielding the individual circuits shown in FIG. 5 are provided in the central areas of both the top and bottom surfaces of the lower printed wiring board 2.
Width of the individual conductor lines and line-toline intervals of the electrostatic shield patterns sl, s2, s3, s4 as well as the distance between the electrostatic shield and the individual coils 15xa, 15xb, 15ya, 15yb, 16xa, 16xb, 16ya, 16yb are determined such that a total value of distributed capacitances produced between the electrostatic shield and any series-connected two coils falls within a range of 1/2 to 2/3 of the capacitance of the relevant tuning capacitor.
Even if the antenna system of the embodiment includes the four ferrite bar antenna elements arranged as described above, the antenna system requires only a little larger installation space than an antenna system including two ferrite bar antenna elements arranged at right angles with each other. Briefly, the ferrite bar antenna system of the embodiment can provide an increased antenna gain without substantial size increase.
FIG. 8 is a front view of one of the side printed wiring boards 3-6. Conductor patterns designated by s5, s6 serving as shields against external electric fields are formed on each of the side printed wiring boards 3-6. These conductor patterns (hereinafter referred to as electrostatic shield patterns) s5, s6 are made of narrowly separated fine conductor lines running parallel to upper and lower sides of each of the side printed wiring boards 3-6. The fine parallel conductor lines of each of the electrostatic shield patterns s5, s6 are joined by another conductor line running at right angles to the fine parallel conductor lines at a specific position thereof (a midlength position in this embodiment so that the fine parallel conductor lines are divided into left-right symmetric halves).The fine conductor lines of the side printed wiring boards 3-6 are formed parallel to the directions of the respective ferrite bars 7-10 of the ferrite bar antenna elements, or at right angles to the direction of a loop formed by the four ferrite bar antenna elements. While the fine parallel conductor lines of each of the electrostatic shield patterns s5, s6 are electrically connected at the mid-length position as mentioned above, opposites ends of the fine parallel conductor lines are not directly joined. These electrostatic shield patterns s5, s6 made of the fine parallel conductor lines serve to protect any circuits on the upper and lower printed wiring boards 1, 2 from external electric fields without causing an increase in electrostatic capacitive components.Therefore, the electrostatic shield patterns s5, s6 do not produce any adverse effects on the circuits of the upper and lower printed wiring boards 1, 2.
Referring to FIG. 8, there are formed four holes designated by "h" in each of the side printed wiring boards 3-6. On the other hand, the upper and lower printed wiring boards 1, 2 each have projecting tabs marked by "p" on side edges as illustrated in FIGS. 6 and 7. The four side printed wiring boards 3-6 are assembled with the upper and lower printed wiring boards 1, 2 in such a manner that the projecting tabs p fit in the holes h. The holes h in the side printed wiring boards 3-6 are individually surrounded by conductor lands for soldering. Additionally, each of the projecting tabs p of the upper and lower printed wiring boards 1, 2 is associated with a conductor pad which is connected to one of the electrostatic shield patterns sl-s4 and thus to the grounding pad G.With the projecting tabs p of the upper and lower printed wiring boards 1, 2 fitted into the holes h in the side printed wiring boards 3-6, the projecting tabs p are soldered to the conductor lands surrounding the respective holes h. As a consequence, the electrostatic shield patterns s5 and s6 on the four side printed wiring boards 3-6 are electrically connected to the grounding pads G of the upper and lower printed wiring boards 1, 2, respectively. Also, the upper and lower printed wiring boards 1, 2 are mechanically joined by the four side printed wiring boards 3-6, together forming a single structure which provides increased stiffness, high resistance to vibration and impact.
Since the electrostatic shield patterns s5 and s6 of each of the side printed wiring boards 3-6 are electrically isolated from each other as illustrated in FIG. 8, the electrostatic shield patterns sl-s4 and the grounding pads G of the upper printed wiring board 1 are electrically isolated from the electrostatic shield patterns sl-s4 and the grounding pads G of the lower printed wiring board 2.
In this configuration, the individual conductor patterns and electrodes do not produce any loop of conductive material surrounding the ferrite bar antenna elements so that the antenna gain of the ferrite bar antenna system is not reduced.
The principal part of the antenna system shown in FIG. 6 is configured as thus far described. Since the support members 11, 12, 13, 14 located between the four corners of the upper printed wiring board 1 and those of the lower printed wiring board 2 for supporting the ferrite bars 7, 8, 9, 10 are made of the electrically conductive rubber, the corners of the upper and lower printed wiring boards 1, 2 are electrically shelled by the support members 11, 12, 13, 14. Consequently, surrounding areas of the individual ferrite bar antenna elements are almost entirely shielded from external electric fields so that the above-described configuration of the embodiment provides excellent shielding effects.
In the antenna system of the present embodiment, the upper and lower printed wiring boards 1, 2 are joined by the four side printed wiring boards 3-6 by fitting the projecting tabs p of the upper and lower printed wiring boards 1, 2 into the holes h in the side printed wiring boards 3-6 and soldering the projecting tabs p to the conductor lands surrounding the respective holes h as discussed above. The antenna system of the embodiment may be modified such that the upper and lower printed wiring boards 1, 2 are joined by the four side printed wiring boards 3-6 by just fitting (and not soldering) the projecting tabs p of the upper and lower printed wiring boards 1, 2 in the holes h in the side printed wiring boards 3-6, both mechanically and electrically.
FIG. 10 is a cross-sectional diagram showing the entirety of the antenna system of the embodiment including an antenna housing showing by hatching. The antenna housing includes a dome-shaped cover 22 and a base 23 on which the principal part of the antenna system is mounted. The cover 22 is internally threaded along a peripheral part thereof while the base 23 is externally threaded along a peripheral part thereof. The cover 22 is screwed onto the base 23 so that the cover 22 and the base 23 are joined with good waterproofness. The base 23 has on an upper surface thereof receptacles into which the aforementioned bosses of the support members 11-14 projecting downward from the bottom surfaces thereof are fitted. On the upper surface of the base 23 there are also formed bosses to which the upper and lower printed wiring boards 1, 2 are together fixed by screws.
NAVTEX stations for international services transmit electromagnetic waves from a vertical antenna so that an electric field component E transmitted from each NAVTEX station is a vertically oriented wave and a magnetic field component H is a horizontally oriented wave. Therefore, the electric field component E (which is vertically polarized) of the electromagnetic waves transmitted from the NAVTEX stations can pass through the horizontally extending fine conductor lines of the electrostatic shield patterns sl-s6, whereas electromagnetic interference, or noise, which is not vertically polarized is mostly reflected by the electrostatic shield patterns sl-s6 so that electric field components of radio waves other than from the NAVTEX stations are substantially suppressed.
FIGS. 11 to 13 are diagrams showing horizontal directivity of the antenna system of the embodiment obtained from actual measurements.
Specifically, FIG. 11 is a diagram showing a horizontal-plane radiation pattern obtained at 518 kHz, FIG. 12 is a diagram showing a horizontal-plane radiation pattern obtained at 490 kHz, and FIG. 13 is a diagram showing a horizontal-plane radiation pattern obtained at 4209.5 kHz. These horizontal-plane radiation patterns show measurement results obtained with three different samples of the antenna system numbered (1) to (3) in FIGS. 11 to 13. The measurement results indicate that the degree of uniformity in the directivity in the horizontal plane of the samples of the antenna system in the MF band (490 kHz, 518 kHz) is less than 2 dB against a target value of 1 dB as expressed in terms of antenna gain, so that all of the samples (1) to (3) satisfy requirements for horizontalplane omnidirectionality in the MF band.The degree of uniformity in the directivity in the horizontal plane of the samples (1) to (3) in the HF band (4209.5 kHz) is less than 3 dB against a target value of 1.5 dB as expressed in terms of antenna gain, so that the samples (1) to (3) satisfy requirements for horizontal-plane omnidirectionality in the HF band as well.
While the antenna system of the foregoing embodiment is configured to receive signals in the two frequency bands, that is, the MF band and the HF band, the invention is not limited to the antenna system for these frequency bands but is applicable to antenna systems for receiving signals in any two frequency bands which are so separated that can not be received by ferrite bar antenna elements each of which wound by a single coil.
WHAT IS CLAIMED IS:
1. An antenna system comprising:
at least two ferrite bar antenna elements each of which includes a ferrite bar, a first coil for a first frequency band and a second coil for a second frequency band, the first and second coils being wound on the ferrite bar at positions separated from each other; and a circuit connected to said first and second coils for simultaneously receiving electromagnetic waves in the first and second frequency bands.
2. An antenna system comprising:
at least two ferrite bar antenna elements each of which includes a ferrite bar, a first coil for a first frequency band and a second coil for a second frequency band, the first and second coils being wound on the ferrite bar at positions separated from each other; and an electrostatic shield for shielding said ferrite bar antenna elements from external electric fields.