EP2923415B1 - Circularly polarized slotted waveguide antenna - Google Patents

Description

Field of the Invention
• This invention is related to slotted waveguide antennas that can receive and transmit electromagnetic waves with circular polarization.
State of the Art
• Circularly polarized slotted waveguide antenna arrays which transmit receive in a directional manner, are realized using slots which are not enabling the control of the aperture amplitude distribution or other slot types which are very hard to manufacture especially at high frequencies. When there is no control over the aperture amplitude distribution of the array, all the elements are excited with equal amplitudes and this causes the side lobe level at the far zone of the antenna to be high. It is also possible to obtain circular polarization by adding a polarizing structure to a linearly polarized antenna. However, losses due to polarizer decrease the antenna efficiency.
  In patent document KR100686606 , slots are used which are opened on the corners of waveguides exciting the cavity at the upper side. On the top side of the cavities, a PCB (printed circuit board) based polarizer is placed. Since the slot parameters are fixed, the excitation of the slots is not possible. Therefore, the invention in patent document KR100686606 does not enable the control of the amplitude distribution of the array.
• In patent document US2007273599 , open ended waveguide which are fed by waveguides are used and this invention does not enable the control of aperture amplitude distribution.
  In patent document US2008266195 , the antenna slots are carved on waveguide broad wall with 45° angle and with a fixed offset from waveguide centerline. The elliptical structures in which the slots are radiating are used to suppress grating lobes. Grating lobes are different than side lobes and they appear due to large inter element spacing in antenna arrays. Suppressing grating lobes does not affect the side lobe level of the antenna and there is no mention of side lobe level reduction in the document.
• In patent document US 2008/117114 A1 an RF feed is provided which is structured as a curved reflector coupled to a sidewall of a waveguide cavity. A radiation source is situated facing the curved reflector. The RF feed may be coupled to a waveguide cavity having radiation elements coupled to top surface thereof, to thereby feed an antenna array. When an antenna array is used, several curved reflector RF feeds may be used, operating in the same or different frequencies.
• The document GB 507473 A and the document US 2006/164315 A1 also discloses some antennas.
Aims For The Development Of The Invention
• The aim of this invention is enabling the reduction of far zone side lobe level of a circularly polarized slotted waveguide antenna array without sacrificing radiation efficiency.
Summary of the Invention
• The realization of circularly polarized slotted waveguide antenna, to accomplish the aims of the invention is shown in the figures:
• Figure 1 . Perspective view of the antena.
• Figure 2 . Cut view of the antenna.
• Figure 3 . The perspective view of the antenna structure having the polarizing waveguide and a single slot.
• The parts in the figures are numbered as follows:
• 1. Antenna
• 2. Rectangular cross section waveguide
• 3. Slot
• 4. Second waveguide
• Θ. Second waveguide placement angle
• The circularly polarized slotted waveguide antenna which is the subject of this invention (1) basically comprises of:
• At least one rectangular cross section waveguide (2) on which at least one slot (3) is carved,
• At least one secondary waveguide (4) which is placed on top of the surface of the rectangular cross section waveguide (2) on which the slots (3) are carved,
• The circularly polarized slotted waveguide antenna which is the subject of this invention comprises of a rectangular cross section waveguide (2) on which slots (3) are carved and which is exciting the slots (3), slots (3) which are carved on the rectangular waveguide (2), secondary waveguide (4) which is placed on top of the rectangular waveguide (2) positioned with a definite angle (θ) with respect to the slots (3).
• In the rectangular waveguide (2), TE10 mode excites the slots (3). The field distribution created on the slots (3) excites the secondary waveguide (4). The fact that secondary waveguide (4) is placed with and angle (θ) causes both TE10 and TE01 modes to be excited. To obtain circular polarization, these two modes should be excited with equal amplitudes and they should have a phase difference of 90° between them.
• The parameter controlling the excitation amplitude ratio of the two modes is the placement angle (θ) of the secondary waveguide (4). Since there is not an analytical expression relating the excitation amplitude ratio to the secondary waveguide (4) placement angle (θ), the angle (θ) causing the excitation of the modes TE10 and TE01 with equal amplitudes is determined using numerical electromagnetic simulations. The determined value is generally between 40° and 50°.
• To obtain circular polarization, the phase difference between TE10 and TE01 modes should be 90° (π/2 radian). If it is assumed that the phase difference of the modes on the slot (3) surface cpo, the modal electric field expressions at a distance "d" away from the slot (3) surface by neglecting the reflections from the aperture at the upper section of the secondary waveguide (4) are given by: $$E_{{\mathit{TE}}_{10}}=E_0{e}^{-{\mathit{j\beta }}_{10}d}$$

  

  $$E_{{\mathit{TE}}_{01}}=E_0{e}^{-{\mathit{j\varphi }}_0}{e}^{-{\mathit{j\beta }}_{01}d}$$

  
• In the above equations,
  β₁₀ : TE10 mode propagation constant, β₀₁ : TE01 mode propagation constant,
  d: Height of the secondary waveguide (4) (shown in figures).
• Therefore, the phase difference at a distance "d" from the slot (3) is given by: $${\varphi }_{{\mathit{TE}}_{10}}-{\varphi }_{{\mathit{TE}}_{01}}=\left({\beta }_{01}-{\beta }_{10}\right)d+{\varphi }_0$$

  
• In the above equation,
  φ _{TE 10} : Phase of TE10 mode, φ _{TE 01} : Phase of TE01 mode.
• Propagation constants are expressed as: $${\beta }_{10}=\mathrm{<figure-callout\; id="2"\; label="\pi "\; filenames="imgf0002.png"\; state="\{\{state\}\}">\pi </figure-callout>}\sqrt{{\left(\frac{2\mathrm{<figure-callout\; id="2"\; label="f\; c"\; filenames="imgf0002.png"\; state="\{\{state\}\}">f</figure-callout>}}{c}\right)}^2-{\left(\frac{1}{a}\right)}^2}$$

  

  $${\beta }_{01}=\mathrm{<figure-callout\; id="2"\; label="\pi "\; filenames="imgf0002.png"\; state="\{\{state\}\}">\pi </figure-callout>}\sqrt{{\left(\frac{2\mathrm{<figure-callout\; id="2"\; label="f\; c"\; filenames="imgf0002.png"\; state="\{\{state\}\}">f</figure-callout>}}{c}\right)}^2-{\left(\frac{1}{\mathrm{<figure-callout\; id="2"\; label="b"\; filenames="imgf0002.png"\; state="\{\{state\}\}">b</figure-callout>}}\right)}^2}$$

  

  To obtain circular polarization, the value of d: height of secondary waveguide (4) is given by: $$d=\frac{\pi /2-{\varphi }_0}{\mathrm{<figure-callout\; id="2"\; label="\pi "\; filenames="imgf0002.png"\; state="\{\{state\}\}">\pi </figure-callout>}\left(\sqrt{{\left(\frac{2\mathrm{<figure-callout\; id="2"\; label="f\; c"\; filenames="imgf0002.png"\; state="\{\{state\}\}">f</figure-callout>}}{c}\right)}^2-{\left(\frac{1}{\mathrm{<figure-callout\; id="2"\; label="b"\; filenames="imgf0002.png"\; state="\{\{state\}\}">b</figure-callout>}}\right)}^2}-\sqrt{{\left(\frac{2\mathrm{<figure-callout\; id="2"\; label="f\; c"\; filenames="imgf0002.png"\; state="\{\{state\}\}">f</figure-callout>}}{c}\right)}^2-{\left(\frac{1}{a}\right)}^2}\right)}$$

  
• In the above equation, a and b are the edge lengths of the secondary waveguide (4) cross section (shown in figures), c: speed of the light in vacuum f: frequency. This expression neglects the reflections at the waveguide (4) aperture and therefore, it is approximate. A more accurate value can be obtained via electromagnetic simulations.
• The cross section of the secondary waveguide (4) should be narrow enough to prevent modes other than TE10 and TE01 from propagating. The mode with the lowest cut-off frequency after TE10 and TE01 is the TE/TM11 mode. The cut-off frequency of the TE/TM11 mode, f_c11 is: $$f_{c_{11}}=\frac{\sqrt{{\left(\frac{1}{a}\right)}^2+{\left(\frac{1}{\mathrm{<figure-callout\; id="2"\; label="b"\; filenames="imgf0002.png"\; state="\{\{state\}\}">b</figure-callout>}}\right)}^2}}{2c}$$

  
• Therefore, the highest usable frequency of the antenna, f_max should satisfy: $$f_{\max }<\frac{\sqrt{{\left(\frac{1}{a}\right)}^2+{\left(\frac{1}{\mathrm{<figure-callout\; id="2"\; label="b"\; filenames="imgf0002.png"\; state="\{\{state\}\}">b</figure-callout>}}\right)}^2}}{2c}$$

  
• By determining the secondary waveguide (4) cross section edge lengths and the height of the secondary waveguide (4) with the given formulae, TE10 and TE01 modes are made 90° out of phase at the aperture of the secondary waveguide (4). Circularly polarized electric field distribution at the aperture of the secondary waveguide (4) radiates circularly polarized waves into free space.
• If it is desired to have elliptical polarizations of arbitrary axial ratios instead of circular polarization, the phase difference between modes should be made different than 90°. In this case, the dimensions of the secondary waveguide (4) should be chosen differently from the values given 90° phase difference.
• Excitation amplitude and phase of the slot (3) is controlled by changing the length of the slot (3) and by changing the distance between the slot (3) and the centerline of the waveguide (2) as it is known from the literature. Additionally, depending on the excitation amplitude and phase, the slots (3) can be modeled as shunt admittances for rectangular cross section waveguide (2). By taking advantage of this fact, the amplitude distribution at the apertures of the designed arrays can be easily controlled. This leads to low side lobe level and low cross polarization level slotted waveguide antenna arrays.
• It is possible to create a linear antenna array by placing secondary waveguides (4) on the rectangular waveguide (2) on which a linear array of slots is carved. Parameters of each slot can be tuned to obtain a desired amplitude distribution at the aperture.
• It is also possible to use a plurality of linear antenna arrays to create planar, circular cylindrical antenna arrays.
• In one embodiment, instead of a rectangular cross section waveguide (2), circular, elliptical or other cross section waveguides with or without ridges can be used.
• In one embodiment, the cross section of the secondary waveguide is elliptical.
• The circularly polarized slotted waveguide antenna (1) which is the subject of this invention is radiating circularly polarized waves without requiring an additional polarizer layer. This enables a fully metal construction which minimizes ohmic losses and increasing antenna efficiency.
• Within the framework of these basic concepts, it is possible to develop a wide variety of embodiments of the inventive circularly polarized slotted waveguide antenna (1). The invention cannot be limited to the examples described herein and it is essentially as defined in the claims.

Claims (7)

1. A circularly polarized slotted waveguide antenna (1) comprises,

   - At least one waveguide (2) on which at least one slot (3) is carved and characterized by,

   - At least one secondary waveguide (4) placed on the face of the waveguide (2) on which slot (3) is carved, which is placed on the slot (3) with an angle (θ) that is between mentioned slot (3) and waveguide (4) so that both TE10 and TE01 modes are excited inside the secondary waveguide (4) with amplitudes of TE10 and TE01 modes equal or near equal, for guiding the radiation emanating from the slot (3).
2. A circularly polarized slotted waveguide antenna (1) as in Claim 1 which is characterized by a secondary waveguide (4) whose height d satisfies the following equality while the φ ₀ is phase difference of the TE10 and TE01 modes on the slot (3) surface, f is frequency, c is speed of the light in vacuum, a and b are the edge lengths of the secondary waveguide (4) cross section; $$d=\frac{\pi /2-{\varphi }_0}{\pi \left(\sqrt{{\left(\frac{2f}{c}\right)}^2-{\left(\frac{1}{b}\right)}^2}-\sqrt{{\left(\frac{2f}{c}\right)}^2-{\left(\frac{1}{a}\right)}^2}\right)}$$

   
3. A circularly polarized slotted waveguide antenna (1) as in Claim 1 which is characterized by a secondary waveguide (4) whose cross section edge lengths a and b satisfies the following inequality while f _max is highest usable frequency and c is speed of the light in vacuum: $$f_{\max }<\frac{\sqrt{{\left(\frac{1}{a}\right)}^2+{\left(\frac{1}{b}\right)}^2}}{2c}$$

   
4. A circularly polarized slotted waveguide antenna (1) as in Claim 1 which is characterized by a waveguide of arbitrary cross section like circular, elliptical or rectangular etc., with or withour ridges inside waveguide (2).
5. A circularly polarized slotted waveguide antenna (1) as in Claim 1 which is characterized by cross section of the secondary waveguide (4) is elliptical.
6. A circularly polarized slotted waveguide antenna (1) array which is characterized by a plurality slots (3) carved on rectangular waveguide (2) as in Claim 1 and a plurality of secondary waveguides (4) placed on the rectangular waveguide (2) such that slots (3) will radiate into secondary waveguides (4) as in Claim 1.
7. An antenna array characterized by a plurality of circularly polarized slotted waveguide antenna (1) arrays as in Claim 1 which are arranged to form planar, circular or cylindrical arrays.

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