Low profile dual frequency dipole antenna structure

[0001] This invention relates to antenna structures, and more particularly, to a low profile dipole antenna structure.

[0002] The length of a dipole antenna is related to its operating frequency. A dipole antenna typically has two radiating elements having a common center feed point. The length of the combined dipole radiating elements is typically a multiple of the transmitting or receiving frequency. For example, the dipole radiating elements may have a length that is ¼, ½, or ¾ the wavelength of the radio frequency (RF) energy. In order to operate in two frequency bands, the antenna structure must have two sets of dipole radiating elements with two different lengths.

[0003] In certain applications, such as in an instrument landing system (ILS) of an aircraft, a dual-frequency dipole antenna is used to receive the radio frequencies of the glide slope and localizer radio frequency transmissions. In these applications, the antenna is typically mounted inside the nose cone of the aircraft where space is severely limited. Therefore, it is desirable to provide a dual-frequency dipole antenna that will fit within the confines of available space and not interfere with other equipment on board the aircraft.

[0004] In accordance with an embodiment of the present invention, an antenna includes a first dipole having first and second stripline radiating elements extending in opposite directions from a central feed point and along a generally rectangular outline of the antenna. The first dipole is operable to be resonant at a first frequency. The antenna also includes a second dipole having third and fourth stripline radiating elements extending in opposite directions from the central feed point and generally parallel to the first and second stripline radiating elements. The third and fourth stripline radiating elements generally follow and stay within the rectangular antenna outline. The second dipole is operable to be resonant at a second frequency. The antenna also includes a stripline balun electrically coupled to the central feed point and extending generally parallel with the first and second dipoles and along the rectangular antenna outline.

[0005] In accordance with another embodiment of the present invention, an antenna structure comprises a generally rectangular outline having a width, W, and a length, L, and a center axis bisecting the length of the rectangular outline, and a central feed point lying on the center axis of the rectangular outline. The antenna structure includes a first dipole coupled to the central feed point having first and second radiating elements extending opposite one another along the length of the rectangular outline for a total length less than L. The antenna also includes a second dipole coupled to the central feed point having third and fourth radiating elements extending opposite one another along the length of the rectangular outline for a length equal to L. The third and fourth radiating elements further include short perpendicular segments extending along the width of the rectangular outline operable to extend a total length of third and fourth radiating elements to a predetermined desired length. The third and fourth radiating elements generally stay within the rectangular outline. The antenna structure further includes a balun coupled to the central feed point having a length equal to L.

[0006] In accordance with yet another embodiment of the present invention, a method of forming an antenna structure comprises defining a generally rectangular outline having a width, W, and a length, L, and a center axis bisecting the length of the rectangular outline, and providing a central feed point lying on the center axis of the rectangular outline. The method includes forming a first dipole coupled to the central feed point having first and second radiating elements extending opposite one another along the length of the rectangular outline for a total length less than L. The method also includes forming a second dipole coupled to the central feed point having third and fourth radiating elements extending opposite one another along the length of the rectangular outline for a length equal to L. The third and fourth radiating elements include short perpendicular segments extending along the width of the rectangular outline that are operable to extend a total length of the third and fourth radiating elements to a predetermined desired length. The third and fourth radiating elements generally stay within the rectangular outline. The method further includes forming a balun coupled to the central feed point having a length equal to L.

[0010] The preferred embodiment of the present invention and its advantages are best understood by referring to FIGS. 1 and 2 of the drawings, like numerals being used for like and corresponding parts of the various drawings.

[0011] A multi-band dipole antenna may be formed by coupling a plurality of parallel dipoles to a common feed system. A center-fed dipole antenna provides a low impedance at the dipole resonant frequency and high impedances at other non-harmonic frequencies. Thus, a plurality of center-fed dipoles may be coupled to a common feed point to form a multi-band dipole antenna system. Each dipole may be constructed to resonate at a particular frequency λ.

[0012] FIG. 1 is a simplified schematic diagram of a conventional dual-band antenna system 100 having two dipoles. A first dipole antenna 110 having a resonant frequency f_o1 of wavelength λ₁ is comprised of two radiating elements 110A and 110B of length λ₁/4, respectively. A second dipole 120 having a resonant frequency of f₀₂ of wavelength λ₂ comprises two radiating elements 120A and 120B of length λ₂/4, respectively. Each dipole 110 and 120 is a center-fed dipole antenna and share a common feed point. In the illustrative example, dipole radiating elements 110A and 120A are coupled to an outer shield 130A of coaxial cable 130, and dipole radiating elements 110B and 120B are coupled to an inner conductor 130B of a coaxial cable 130. Each dipole antenna 110 and 120 provides a low feed-point impedance at respective resonant frequency f_o1 and f_o2 (and odd harmonics thereof), and higher impedances at other operational frequencies. When one dipole antenna of a multi-dipole antenna system 100 is resonant, the other dipole provides a higher impedance than the lower-impedance resonating dipole. Thus, the resonating dipole is the natural path for the majority of power flowing through the antenna system.

[0013] In practicality, however, parallel coupled dipoles in near proximity with one another may be electrically coupled via mutual inductance therebetween. Mutual inductance may increase the resonant length, e.g. λ₂, of the shorter dipole in a parallel dipole antenna system and may also reduce the operational bandwidth of the shorter dipole 110. Dipoles 110 and 120 may be implemented in a configuration that provides greater separation to enhance the antenna system operation. However, when the available physical confines to accommodate the antenna system are restricted, the aforedescribed problems may be exacerbated.

[0014] With reference now to FIG. 2 a top plan view of a dual-frequency center-fed dipole antenna structure 200 constructed according to an embodiment of the present invention is shown. Antenna structure 200 includes conductive traces or stripline on a printed circuit board (PCB) that is etched, laid down or otherwise formed on a dielectric or non-conductive substrate 202. For example, antenna structure 200 may be formed by pattern etching a copper-plated sheet of synthetic material. Antenna 200 has a first dipole 210 and a second dipole 220 located proximate with one another. First dipole 210 has a first resonant frequency f_o1 corresponding to a first resonant wavelength of λ₁. Second dipole 220 has a second resonant frequency f_o2 corresponding to a second resonant wavelength of λ₂. Therefore, dipole antenna 210 is operable to receive and/or transmit electromagnetic radiation in a first frequency bandwidth, and dipole antenna 220 is operable to receive and/or transmit electromagnetic radiation in a second frequency bandwidth.

[0015] The dipole antennas are generally symmetrical along a center axis 212. Dipole 210 is shown having a linear configuration having radiating elements 210A and 210B with a combined length λ₁/2 or L₁, and is resonant at a frequency f_o1. Dipole 220 may be constructed from multiple straight dipole segments 220A₁-220A₅ and 220B₁-220B₅. It may be seen that in the embodiment shown in FIG. 2, dipole segments 220A₁-220A₅ and 220B₁-220B₅ are generally coupled to neighboring segments at 90° angles and generally confined within a predetermined rectangular outline 272. The radiating elements of dipole 220 are thus bent around the radiating elements of dipole 210 with the dipole segments with a predetermined spacing therebetween. For example, dipole segment 220B₂ is used to turn the direction of radiating element 220B 90° around the end of radiating element 210B and toward the edge of the rectangular outline; dipole segment 220B₃ then turns the direction of radiating element 220B another 90° down the first axis or length of antenna structure 200 adjacent to the rectangular outline; dipole segment 220B₄ then turns the direction of the radiating element 220B another 90° down the second axis or width of antenna structure 200; and dipole segment 220B₅ then turns the direction of the radiating element 220B another 90° back toward the center of the dipole antenna along the first axis. Rectangular outline 272 is compact and limits antenna structure 200 to a predetermined generally rectangular footprint. It may also be seen that an effort has been made to obtain the correct length for dipole 220 while accommodating the real estate occupied by radiating elements of dipole 210.

[0016] Antenna structure 200 further comprises a unique balun 250. Balun 250 is preferably of a compact stripline construction that provides a balanced and high-impedance feed to the antenna. Balun 250 is designed based on the center frequency of the two antenna frequencies (¼ wave length of the center frequency). Balun 250 may be constructed of balun stripline segments 226A coupled to radiating elements 210A and 220A of the respective first and second dipoles, extending perpendicularly with respect to the antenna radiating elements, and coupled to another balun segment 280A₁, substantially parallel with the antenna radiating elements, a shorter balun segment 280A₃ perpendicular to the radiating elements, and then another balun segment 280A₂ parallel with the radiating elements. Balun segment 280A₂ is in turn coupled to a balun segment 280B₂, its symmetrical counterpart on the B side of the antenna. Segment 280B₂ which is coupled to 280B₃ and 280B₁. Balun 250 comprises the inverse T shaped channel formed between these stripline segments. It may be seen that balun 250 comprises two main channel portions 250A and 250B. Balun channel portion 250A is a channel formed generally perpendicularly with respect to the dipole radiating elements. In the embodiment of the present invention, the channel is approximately 0.16″ in width. Balun portion 250B is a channel formed substantially parallel with respect to the dipole radiating elements. In the embodiment of the present invention, the channel is approximately 0.25″ wide and 31.6″ long. Balun portion 250A and 250B thus comprise a continuous channel formed by the stripline and has a resulting configuration of an inverted T. It may be seen that the primary length of the balun is in balun portion 250B which spans nearly the width of antenna 200. It may be seen that the stripline forming balun 250 has substantially the same width, L₂, as the second dipole, and substantially fills in the rectangular antenna outline not already occupied by the first and second dipole antennas. The unique design of balun 250 enables common feed point 260 to be located in close proximity to ground plane 270 while still presenting a balanced, high impedance path to ground from the feed point. Therefore, antenna structure 200 may be formed on a substrate that is planar or one that has some curvature such as the surface of a radome (not shown) on an aircraft. The low profile of antenna structure 200 also enables it to be installed near an edge of the radome without interfering with other radar antennas located nearby.

[0017] In the exemplary configuration, dipole segments 220A₄, 220A₅, 220B₄, and 220B₅ are each of length L. Thus, dipole 220 has a half-wave resonance length λ₂/2 or (L₂+4L). In the illustrated embodiment, dipole 210 has a half-wavelength λ₁/2 chosen for resonance at a frequency f_o1 that is an odd multiple of a resonance frequency f_o2 of dipole antenna 220. In an embodiment of the present invention, dipole antenna 210 is resonant at a third harmonic of dipole antenna 220. In other words, dipole antenna 210 has a frequency that is three-times the frequency of dipole antenna 220. L₂ is therefore approximately three-times the length of the sum of (L₂+4L). Both dipole antennas 210 and 220 are electrically coupled to a feed line 262 at a common feed point 260. Feed line 262 has an inner conductor that is soldered or otherwise electrically coupled to the A side of dipole antennas 210 and 220 (radiating segment 210A and 220A₁-220A₅), and an outer conductor insulated from the inner conductor that is soldered or otherwise electrically coupled to the B side of the dipole antennas (radiating segments 210B and 220B₁-220B₅). The outer conductor is further electrically coupled ground, thus forming a ground plane 270 in the B side of the dipole antennas as well as striplines 280B₁-280B₃ that form the B side of balun portion 250B. The outer conductor of feed line 262 may be soldered at various points to striplines 280B₁, 280B₂, and/or 280B₃.

[0018] Decoupling elements 240A and 240B are coupled to dipole sections 220A and 220B, respectively. More specifically, decoupling element 240A is coupled to radiating segment 220A₁ and extends in the same general direction thereof; and decoupling element 240B is coupled to radiating segment 220B₁ and extends in the same general direction thereof. Decoupling elements 240A and 240B are operable to prevent dipole antenna 220 from resonating at f_o1 and detuning dipole 210. For example, decoupling elements 240A and 240B eliminate the interaction between the two dipoles when there is a three-to-one frequency relationship therebetween. Therefore, decoupling elements 240A and 240B are operable to direct the radio frequency energy to the proper dipole and minimize the interaction between the dipole elements. In the absence of decoupling elements 240A and 240B, dipole 220 would resonate at odd harmonics of f_o2, for example at f_o1, and would be coupled with dipole 210 during concurrent resonance with dipole 210. Decoupling elements 240A₁ and 240B₁ are approximately λ₁/4 in length, and thereby effectively short dipole sections 220A₁, and 220B₁, when antenna structure 200 operates at 3λ₂/4 (and harmonics thereof). Therefore, the unique design of decoupling elements 240A and 240B “decouples” the two dipole antennas from one another so as to eliminate interference therebetween.

[0019] For the purpose of providing an illustrative example, certain exemplary dimensions and characteristics according to an embodiment of the present invention are provided below:

[0020]

	Dimension/Characteristic	Measurement
	Antenna footprint width	4″
	Antenna footprint length	36″
	L1	14.1″
	L2	30.4″
	L	2.5″
	Width of decoupling element	0.5″
	Spacing between dipole	0.25″
	radiating elements
	Spacing between dipole	0.25″
	radiating element and balun
	f01	330 MHz
	f02	110 MHz

[0021] The stripline balun and dipole elements may be constructed in an integrated assembly with a low profile and small, limited footprint. The entire structure may be etched or formed on a PCB that may be flat or have some curvature. The low profile and limited footprint of antenna structure 200 due to the unique balun and decoupling element designs allow the antenna to be installed in confined spaces without interfering with radiating elements of other structures. For example, in certain applications such as in an instrument landing system (ILS) of an aircraft, antenna structure 200 may be installed on the surface of a radome located in the confined space of the nose cone of the aircraft. Antenna structure 200 would be used to receive the radio frequencies of the glide slope and localizer radio frequency transmissions from a landing site. Therefore, the low profile and limited footprint of antenna structure 200 makes it enable it to fit within the confines of available space and also not interfere with other radar equipment on board the aircraft.

[0022] While the invention has been particularly shown and described by the foregoing detailed description, it will be understood by those skilled in the art that various changes, alterations, modifications, mutations and derivations in form and detail may be made without departing from the spirit and scope of the invention.