DIRECTIONAL DIAGRAM

Background of the Invention

Field of the Invention

[0001] The present invention is related to antennas, and, more particularly, to helical antennas for use in GPS receivers.

Description of the Related Art

[0002] Multipath error is currently one of the most important contributions to the GNSS positioning error budget when a signal reflected from the underlying ground surface is received at the output of the receiving antenna along with the line-of-sight signal. Multipath error is proportional to the ratio

F{9)

[0003] This ratio is typically called Down/Up ratio. Here, ^ is the elevation angle over the local horizon, and -^ ⁺ /-^) j _s the directional diagram (DD) for the antenna at angle & over and under the local horizon respectively. To reduce multipath error, the value should be small. However, to provide stable and reliable operation of a positioning system, reception of all signals over the local horizon is needed.

[0004] Hence, to enhance accuracy of positioning systems, one needs to develop and design receiving antennas with Π-shaped (rectangular) DD providing antenna gain close to a constant value in the whole upper hemisphere and forming a sharp drop when crossing the local horizon downward.

[0005] Navigation signals are received from satellites in the upper hemi-sphere up to elevations 10 ...15 from the horizon. A signal reflected from the ground is fed from the lower hemi-sphere side. FIG. 1 shows a conditional division of space into upper (front) and lower (back) hemi-spheres, as well as a schematic diagram of the direct and reflected waves. To provide both navigation signal reception in the whole upper hemi-sphere and suppression of signals reflected from the ground, the antenna needs a high DD level in the upper hemi-sphere, a low DD level in the lower hemi-sphere, and a sharp drop of DD to the horizon direction.

[0006] A quadrifilar helix antenna is known (see Josypenko, CAPACITIVELY

LOADED QUADRIFILAR HELIX ANTENNA, US 6,407,720), with capacitive elements incorporated in spiral turns as shown in FIG. 2.

[0007] This antenna is produced as a dielectric cylinder 206 with mylar tapes 202, 203,

204, 205 being wound on it. The tapes are both-side-metallized, such that metallization areas 301-302— 321-320 on different sides of the tape would be overlapped, forming capacitors C1-C19.

[0008] US 6,407,720 discloses that the area of capacitor plates is maximum at excitation point 201 and then reduces according to the exponential law to the minimum value at the end of the spiral. One of the embodiments shows that the winding angle is constant and equal to 66.64° (see column 8, line 15 in US 6,407,720).

[0009] In the proposed antenna this angle can be varied.

[0010] Known prior art solutions do not allow obtaining a sharp drop in DD in the direction of the horizon.

[0011] FIG. 4 shows an exemplary DD taken from US 6,407,720. Unlike FIG. 1 the horizon direction is zero of elevation angles. The corresponding angles reading from the horizon (see FIG. 1) are in italics. In this figure, 401 is the directional diagram of a spiral antenna with turns in the form of simple metal tapes; 402 is the directional diagram of the spiral antenna with capacitor spiral turns (the subject matter of US 6,407,720).

[0012] In FIG. 4: Θ = 0° is the direction to the local horizon; Θ = 10°, Θ = -10° are the directions that differed by 10° from the horizon direction up and down respectively. DD values in the mentioned directions are: E(10°) = 0.95, F(-10°) = 0.85. Hence for the given antenna at the elevation of 10°, the Down/Up ratio is as follows: Z)f/(10°)[dB] = 201og[ (-10°)/ (10°)] = -0.97 dB, which is clearly inadequate for GPS applications, where at least -20 dB is required to suppress signals reflected from the ground.

Summary of the Invention

[0013] The present invention is related to a helical antenna that substantially obviates one or several of the disadvantages of the related art. [0014] The main purpose of this invention is to obtain a direction diagram with a sharp drop in the direction of the ground plane (i.e., the horizon direction) and maximum suppression of signals in the lower hemisphere due to selecting capacitive elements of the spiral antenna, spiral winding pitch, spiral diameter and height.

[0015] As such, a quadruple spiral antenna is proposed, where each spiral turn includes a set of capacitive elements. Note that US 6,407,720 confirms that it does not provide a sharp drop of DD in the horizon direction, and US 6,407,720 does not describe a directional diagram with a sharp drop in the horizon direction.

[0016] The present invention proposes a method of achieving such a sharp drop in the horizon direction due to special selection of capacitive elements as a part of the spiral turns. The operational bandwidth of the antenna is f— 1575 +/- 40 MHz. Note that the antenna in US 6,407,720 can operate at GPS frequencies with scaling, but the directional diagram's shape will be different and will not provide the required directional diagram drop at angles close to horizon.

[0017] Additional features and advantages of the invention will be set forth in the description that follows, and in part will be apparent from the description, or may be learned by practice of the invention. The advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

[0018] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

Brief Description of the Attached Figures

[0019] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

[0020] In the drawings:

[0021] FIG. 1 shows a conditional division of space into the upper and lower hemispheres.

[0022] FIG. 2 shows an appearance of a prior art antenna. [0023] FIGS. 3A, 3B show a spiral turn of a prior art antenna.

[0024] FIG. 4 shows a prior art antenna directional diagram.

[0025] FIGS. 5A-5C show an embodiment of a design of a quadrifilar helix antenna.

[0026] FIG. 6 shows a DD of the proposed antenna with a sharp drop to the horizon direction.

[0027] FIG. 7 shows a Down/Up graph of the proposed antenna.

Detailed Description of the Preferred Embodiments

[0028] Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.

[0029] The proposed invention according to FIGS. 5A-5C is a quadrifilar cylindrical spiral antenna with capacitors soldered into breaks in metal turns.

[0030] The main features of the proposed antenna design are:

1. A quadruple spiral (FIGS. 5A, 5B) with capacitors soldered in- between breaks of spiral turns that is located onto a ground plane;

2. The diameter of the ground plane is selected such that a needed level of suppressing signals reflected from the ground in the nadir direction would be provided. In one of the embodiments, the diameter of the ground plane is 300 mm.;

3. The diameter of the spiral is D = 30 +/- 5 mm.;

4. Total height of the spiral H ₂ is H ₂ = 300 +/- 50 mm.;

5. There is a free area with a height Hi where there are no capacitors Hi = 90 +/- 30 mm.;

6. A variable winding angle is equal to

a(z) = a * z + A, z— 0 ... H ₂ , where (1) a(∑), [deg] is the winding angle;

a = 0.06 + 0.0l[deg/mm];. = 45 + S[deg] are coefficients of the approximation equation for the winding angle;

7. Capacitors are loaded according to the following equation = H ₁ ... H ₂ ' ^where

f

0 is the central frequency of the operational band;

Cn _> [P ^p ] is the capacitance of the n-th capacitor;

z, [mm] is the vertical coordinate varying from zero at the beginning of the spiral and taking on discrete values: z = nh, where n is the number of capacitor position, h = 5...30 [mm] is the pitch of arranging the capacitors along the vertical axis;

b = 0.04 ± 0.01 [Ohm/mm ² ]; B = 1.5 ± 0.3[Ohm/mm] are coefficients in the equation for calculating capacitors.

[0031] These values are optimal values to provide required directional diagram drop at angles close to horizon. The values depend from each other and allow adjusting antenna performance.

[0032] A PCB 509 is used for producing a spiral, with metallization areas 506 that can be manufactured by etching, for example. Between metallization areas there are breaks/slots 507. The produced PCB is then twisted to form a cylinder and fixed in this position.

[0033] Capacitors 507 are soldered in breaks 508 between metallization areas 506. Spiral turns 501, 502, 503, 504 are excited by pins (not shown in figures) passing through holes in the ground plane. The excitation circuit provides excitation of the right-hand circularly-polarized wave. FIG. 6 shows a directional diagram of the pilot antenna, which guarantees Down Up ratio at least -20 dB at elevations 9≥\Q degrees. At this, (10°) = -\ \ .5dB. Similarly to FIG. 4, the angle zero is the zenith direction. The corresponding elevation angles read from the horizon (see FIG. 1) are in italics.

[0034] FIG. 7 presents a graph of Down/Up ratio for the proposed antenna.

[0035] Having thus described a preferred embodiment, it should be apparent to those skilled in the art that certain advantages of the described method and apparatus have been achieved. It should also be appreciated that various modifications, adaptations and alternative embodiments thereof may be made within the scope and spirit of the present invention. The invention is further defined by the following claims.