JP7793882B2 - Microstrip Antenna - Google Patents

Description

The present invention relates to a microstrip antenna, particularly for use in wireless power transmission.

Transmitters used for wireless power transmission emit higher-power radio waves than transmitters used for wireless communication. For this reason, when a transmitter used for wireless power transmission emits radio waves with frequencies outside the frequency band used for that wireless power transmission, radio wave interference becomes a particular problem. For this reason, filters installed in transmitters used for wireless power transmission to reduce the emission of radio waves with frequencies outside the frequency band used and to reduce radio wave interference are required to have better characteristics, i.e., greater attenuation.

In conventional transmitters, increasing the number of filter stages inserted between the amplifier and antenna and enlarging the circuit area achieves increased filter attenuation. However, there are many limitations on the circuit area of a transmitter.

On the other hand, it is acceptable for the transmitter antenna to occupy a relatively large space in order to achieve the required antenna gain. Therefore, adding a filtering function to the antenna allows the filter inserted between the amplifier and antenna to be made smaller, thereby making more effective use of the volume within the transmitter.

Non-Patent Document 1 discloses a circular microstrip antenna equipped with a patch having a slit aligned with the fundamental mode current line. In this microstrip antenna, the patch surface current distribution of higher-order modes that occur near the harmonic frequencies is disrupted, suppressing the occurrence of those higher-order modes and lowering the resonant frequency of those higher-order modes, thereby suppressing harmonic radiation. In this way, a filtering function is added to this microstrip antenna.

Patent document 1 also discloses a microstrip antenna equipped with an H-shaped microstrip patch and having high radiation efficiency.

Patent No. 6764163

Shahriar et al., "Considerations on Circular Microstrip Antennas with Slits Used as Rectenna Elements," IEICE Transactions on Electronics, Information and Communication Engineers, Japan, February 2001, Vol. J84-B, No. 2, pp. 244-253

However, in the microstrip antenna disclosed in Non-Patent Document 1, in order to obtain a large filter attenuation, it is necessary to increase the unloaded Q of the radiating element acting as a resonator and achieve steep resonance characteristics. However, if the dielectric substrate of the microstrip antenna is made thinner in order to increase the unloaded Q, the radiation efficiency decreases.

For example, if the fundamental frequency is in the 2.45 GHz band, a return loss of 0.18 dB is obtained at the second harmonic frequency, in other words, a filter attenuation of 14 dB. However, in this case, the thickness of the dielectric substrate is 0.006 λ, where λ is the wavelength of the fundamental wave, the unloaded Q is 107, and the radiation efficiency is approximately 58%.

Furthermore, the microstrip antenna disclosed in Patent Document 1 has high radiation efficiency, but does not have very good filtering functionality.

The object of the present invention is to provide a microstrip antenna that can achieve both high radiation efficiency and large filter attenuation.

The microstrip antenna of the present invention comprises a ground conductor, a patch conductor facing the ground conductor and having a first portion, a second portion, and a third portion, a first insulating layer formed between the ground conductor and the patch conductor, and a distributed constant filter connected to the first portion and formed of a first conductor pattern, wherein the first portion is formed to extend between the second portion and the third portion, and is shorter than the second portion and the third portion in a direction perpendicular to the extension direction of the first portion, and the distributed constant filter passes a fundamental wave propagating through the first portion in the extension direction of the first portion and attenuates unwanted waves propagating through the first portion in the extension direction of the first portion.

The present invention makes it possible to realize a microstrip antenna that achieves both high radiation efficiency and large filter attenuation.

Fig. 1A is a plan view of a microstrip antenna 10 according to a first embodiment of the present invention, and Fig. 1B is a cross-sectional view of the microstrip antenna 10 taken along the line AA. FIG. 2 is a graph illustrating the frequency characteristics of the return loss of the microstrip antenna 10 and the microstrip antenna of the first comparative example. FIG. 3 is a graph illustrating the second harmonic attenuation and radiation efficiency of the microstrip antenna 10, the microstrip antenna of the first comparative example, and the microstrip antenna of the second comparative example. Fig. 4A is a plan view of a microstrip antenna 30 according to a second embodiment of the present invention, and Fig. 4B is a cross-sectional view of the microstrip antenna 30 taken along the line BB. Fig. 5A is a plan view of a microstrip antenna 50 according to a third embodiment of the present invention, and Fig. 5B is a cross-sectional view of the microstrip antenna 50 taken along the line CC. FIG. 6 is a plan view of the microstrip antenna 50 without the patch conductor 60 shown.

The following describes several embodiments of the present invention. Each embodiment is an example, and partial substitution or combination of the configurations shown in different embodiments is possible. In each embodiment, differences from those described in previous embodiments will be explained. In particular, similar effects resulting from similar configurations will not be mentioned in each embodiment.

First Embodiment
Fig. 1(A) is a plan view of a microstrip antenna 10 according to a first embodiment of the present invention. Fig. 1(B) is a cross-sectional view of the microstrip antenna 10 taken along line A-A. The microstrip antenna 10 includes a ground conductor 11, a patch conductor 20, an insulating layer 12, and a distributed constant filter 13. The patch conductor 20 has a first portion 21, a second portion 22, a third portion 23, and a plurality of stubs 24. The stubs 24 are an example of a "first conductor pattern" in the present invention. The insulating layer 12 is an example of a "first insulating layer" in the present invention.

The patch conductor 20 is positioned opposite the ground conductor 11. This positions the entire surface of the patch conductor 20 opposite the ground conductor 11. The patch conductor 20 is fixed at a distance from the ground conductor 11, for example, by a spacer (not shown). The insulating layer 12 is an air layer formed between the ground conductor 11 and the patch conductor 20. The first portion 21 of the patch conductor 20 is formed to extend in the x-axis direction between the second portion 22 of the patch conductor 20 and the third portion 23 of the patch conductor 20. In the y-axis direction, which is perpendicular to the x-axis direction, the first portion 21 is shorter than the second portion 22 and the third portion 23. The distributed constant filter 13 is composed of a stub 24 and is connected to the first portion 21. The distributed constant filter 13 passes the fundamental wave propagating through the first portion 21 in the extension direction of the first portion 21 and attenuates certain harmonics propagating through the first portion 21 in the extension direction of the first portion 21. Harmonics are an example of "unwanted waves" in this specification.

In addition, the insulating layer 12 may be formed from an insulating material instead of an air layer.

Furthermore, the distributed constant filter 13 may attenuate second-order harmonics or higher-order harmonics.

In a modified example, the distributed constant filter 13 may attenuate, for example, both the second and third harmonics.

In a modified example, the distributed constant filter 13 may also attenuate spurious components other than harmonics.

The patch conductor 20 has a rectangular shape, with notches cut from the center of opposing sides toward the center of the rectangle, i.e., a roughly H-shaped shape. The first portion 21 has an elongated shape and extends in the x-axis direction. The second portion 22 and the third portion 23 have a rectangular shape, with two sides parallel to the x-axis direction and two sides parallel to the y-axis direction.

The length L1 of the first portion 21 in the x-axis direction is _λg1 /2. The length L2 of the second portion 22 in the x-axis direction is _λg2 /2. The length L3 of the third portion 23 in the x-axis direction is _λg3 /2. The length L4 of the second portion 22 and the third portion 23 in the y-axis direction is _λg4 /2 or more. Here, _λg1 , _λg2 , and _λg3 are effective wavelengths when the fundamental wave propagates through the first portion 21, the second portion 22, and the third portion 23 in the x-axis direction, respectively. _λg4 is the effective wavelength when the fundamental wave propagates through the second portion 22 or the third portion 23 in the y-axis direction.

Note that the dimensions of the first portion 21, second portion 22, and third portion 23 may differ slightly from the above values.

The second portion 22 and the third portion 23 have a slit 25 and a slit 26, respectively. The slit 26 is formed near the feed point 15.

Note that slits 25 and 26 are not required and do not have to be formed.

The patch conductor 20 is fed by the feed pin 14. The feed point 15 is located, for example, in the third portion 23 of the patch conductor 20.

When power is supplied, current flows in the patch conductor 20 in the x-axis direction. The current flowing in the first portion 21 has the opposite phase to the current flowing in the second portion 22 and the third portion 23. As a result, the first portion 21 does not contribute to the main lobe, but causes side lobes. Therefore, the first portion 21 does not contribute to radiation and acts as a transmission line. The second portion 22 and the third portion 23 act as radiating elements.

In the microstrip antenna 10, the length in the y-axis direction of the central portion of the patch conductor in the x-axis direction (the portion corresponding to the first portion 21) is shorter than in a rectangular microstrip antenna without a notch in the patch conductor. This means that this central portion has a higher characteristic impedance and is less likely to allow current to flow. This results in a lower sidelobe level.

In addition, the microstrip antenna 10 has a second portion 22 and a third portion 23 that function as radiating elements, but does not require a power divider like that found in array antennas. Therefore, compared to array antennas that have rectangular microstrip antennas as antenna elements, the microstrip antenna 10 does not experience the losses that occur in a power divider and achieves high radiation efficiency.

The stub 24 is formed to branch off from the first portion 21 extending in the x-axis direction. The stub 24 has an open end and a length that is 1/4 of the effective wavelength of the harmonic. In other words, the stub 24 is a 1/4-wavelength open stub for the harmonic. As can be seen from this, the distributed constant filter 13 is connected in series to the first portion 21, which acts as a transmission line, and is a band-stop filter that passes the fundamental wave and blocks the harmonics. The distributed constant filter 13 is also a resonator that resonates at the harmonic frequency.

The distributed constant filter 13 may be a low-pass filter or band-pass filter configured with a distributed constant line. Furthermore, when attenuating spurious signals at frequencies lower than the fundamental wave, the distributed constant filter 13 may be a high-pass filter configured with a distributed constant line.

2 is a graph illustrating the frequency characteristics of the return loss of the microstrip antenna 10 and the microstrip antenna of the first comparative example. The microstrip antenna of the first comparative example is configured similarly to the microstrip antenna 10 except that it does not have the distributed constant filter 13. The graph shown in FIG. 2 is a calculation result obtained by electromagnetic field simulation. Furthermore, _f0 shown in FIG. 2 is the frequency of the fundamental wave. As shown in FIG. 2, the microstrip antenna 10 has a smaller return loss at the second harmonic frequency _2f0 than the microstrip antenna of the first comparative example.

Figure 3 is a graph illustrating the second-harmonic attenuation and radiation efficiency of microstrip antenna 10, the microstrip antenna of the first comparative example, and the microstrip antenna of the second comparative example. As shown in Non-Patent Document 1, the microstrip antenna of the second comparative example has a circular patch with a slit along the fundamental mode current line. As shown in Figure 3, microstrip antenna 10 has a larger second-harmonic attenuation than the microstrip antennas of the first and second comparative examples, and a higher radiation efficiency than the microstrip antenna of the second comparative example. In this way, microstrip antenna 10 can achieve both a large second-harmonic attenuation and high radiation efficiency.

According to the first embodiment, a distributed constant filter 13 that attenuates unwanted waves such as harmonics is connected to the first portion 21 of the patch conductor 20, which does not contribute to radiation and acts as a transmission line. As a result, radiation from the antenna is not affected by the distributed constant filter 13, and unwanted waves are attenuated throughout the entire patch conductor 20, which acts as a resonator. This prevents a decrease in radiation efficiency and improves the filtering function of the antenna. This makes it possible to achieve a microstrip antenna that achieves both high radiation efficiency and large filter attenuation.

Second Embodiment
Fig. 4(A) is a plan view of a microstrip antenna 30 according to a second embodiment of the present invention. Fig. 4(B) is a cross-sectional view taken along the line B-B of the microstrip antenna 30. The microstrip antenna 30 differs from the microstrip antenna 10 according to the first embodiment in the following respects. That is, the microstrip antenna 30 includes an insulating layer 32 instead of the insulating layer 12, and a patch conductor 40 instead of the patch conductor 20.

The thickness h1 of the insulating layer 32 facing the first portion 21 and stub 24 of the patch conductor 40 is thinner than the thickness h2 of the insulating layer 32 facing the second portion 22 and third portion 23 of the patch conductor 40. In other words, the distance between the first portion 21 and stub 24 of the patch conductor 40 and the ground conductor 11 is shorter than the distance between the second portion 22 and third portion 23 of the patch conductor 40 and the ground conductor 11.

The patch conductor 40 is manufactured, for example, by bending a conductive plate using sheet metal processing or the like.

The thickness h1 of the insulating layer 32 is selected to obtain a large filter attenuation. More specifically, the thickness h1 of the insulating layer 32 should be as thin as possible while still satisfying design constraints. The thickness h2 of the insulating layer 32 is selected to obtain a high radiation efficiency. For example, when the operating frequency is in the 5 GHz band, the appropriate thickness h1 of the insulating layer 32 is approximately 0.4 mm, and the appropriate thickness h2 of the insulating layer 32 is approximately 1.5 mm.

In the second embodiment, the insulating layer 32 is set thin where the distributed constant filter 13 is formed. This increases the unloaded Q of the distributed constant filter 13 as a resonator. As a result, the distributed constant filter 13 has steeper resonance characteristics, i.e., steeper filter characteristics. This results in greater filter attenuation.

In addition, the thickness of the insulating layer 32 is set separately at the locations where the second and third portions 22 and 23, which act as radiating elements, are formed and at the location where the distributed constant filter 13 is formed, so radiation efficiency does not decrease.

The second embodiment is particularly effective in operating frequency bands where the difference in the appropriate thickness of the insulating layer 32 becomes significant, specifically, in operating frequency bands below 5 GHz where the difference in the appropriate thickness of the insulating layer 32 becomes approximately 1 mm or more.

Third Embodiment
Fig. 5(A) is a plan view of a microstrip antenna 50 according to a third embodiment of the present invention. Fig. 5(B) is a CC cross-sectional view of the microstrip antenna 50. Fig. 6 is a plan view of the microstrip antenna 50 in which the patch conductor 60 is not shown.

The microstrip antenna 50 differs from the microstrip antenna 30 according to the second embodiment in the following respects. Specifically, the microstrip antenna 50 includes a patch conductor 60 instead of the patch conductor 40, an insulating layer 52 instead of the insulating layer 32, and a distributed constant filter 53 instead of the distributed constant filter 13. Furthermore, the microstrip antenna 50 includes an insulating substrate 56 and a conductor pattern 58. The conductor pattern 58 is an example of the "second conductor pattern" of the present invention.

The patch conductor 60 has a first portion 61 instead of the first portion 21. The first portion 61 has a width that varies depending on the position in the x-axis direction. The ground conductor 11 is formed on one side of the insulator substrate 56. The conductor pattern 58 is formed on the other side of the insulator substrate 56. The conductor pattern 58 extends in the x-axis direction and has a width that varies depending on the position in the x-axis direction. The conductor pattern 58 is formed, for example, by patterning copper foil on a printed circuit board using etching techniques. The first portion 61 of the patch conductor 60 is disposed on the conductor pattern 58 and is connected to the conductor pattern 58 by soldering or the like. The distributed constant filter 53 is configured as a stepped impedance low-pass filter by the first portion 61 of the patch conductor 60 and the conductor pattern 58, which are connected to each other. The distributed constant filter 53 passes the fundamental wave and attenuates high-band spurious signals.

Note that although the distributed constant filter 53 is constructed of a microstrip line, the distributed constant filter 53 may also be constructed of a coplanar waveguide or other distributed constant line.

The insulating layer 52 facing the first portion 61 of the patch conductor 60 and the portion formed by the conductor pattern 58 is composed of an insulating substrate 56. The insulating layer 52 facing the second portion 22 and third portion 23 of the patch conductor 60 is composed of an insulating substrate 56 and an air layer 57. The air layer 57 is an example of a "second insulating layer" in the present invention. The thickness h4 of the air layer 57 is equal to or greater than the thickness h3 of the insulating substrate 56.

In addition, the insulating layer 52 may have a layer made of an insulating material instead of the air layer 57.

When the frequency band used is a high frequency band such as the microwave band, the conductor pattern of the distributed constant filter becomes very fine, making it difficult to form the conductor pattern integrally with the patch conductor. According to the third embodiment, the conductor pattern 58 is formed on an insulating substrate 56. This allows the conductor pattern 58 to be formed very finely using etching techniques. Therefore, the distributed constant filter 53 can be easily manufactured even when the frequency band used is a high frequency band.

In addition, where the second portion 22 and the third portion 23, which act as radiating elements, are formed, the insulating layer 52 is composed of an insulating substrate 56 and an insulating layer, such as an air layer, that is equal to or thicker than the insulating substrate 56. Therefore, even if the conductor pattern of the distributed constant filter 53 is formed on the insulating substrate 56, the radiation efficiency of the second portion 22 and the third portion 23, which act as radiating elements, can be set high.

This makes it possible to realize a microstrip antenna with excellent filtering functionality, even when the frequency band used is a high frequency band.

In addition, the patch conductor 60 is fixed to the conductor pattern 58 by soldering or the like. Therefore, a holding structure for the patch conductor 60, such as a spacer, is not required.

10, 30, 50... Microstrip antenna 11... Ground conductor 12, 32, 52... Insulating layer 13, 53... Distributed constant filter 14... Feed pin 15... Feed point 20, 40, 60... Patch conductor 21, 61... First portion 22... Second portion 23... Third portion 24... Stub 25, 26... Slit 56... Insulating substrate 57... Air layer 58... Conductor pattern

Claims (8)

a ground conductor;
a patch conductor facing the ground conductor and having a first portion, a second portion, and a third portion;
a first insulating layer formed between the ground conductor and the patch conductor;
a distributed constant filter connected to the first portion and formed by a first conductor pattern,
the first portion is configured to extend between the second portion and the third portion;
the first portion is shorter than the second portion and the third portion in a direction perpendicular to the extension direction of the first portion;
a length of the first portion in the extension direction of the first portion is substantially equal to λg ₁ /2, λg ₁ being an effective wavelength of the fundamental wave when the fundamental wave propagates through the first portion in the extension direction of the first portion;
A microstrip antenna, wherein the distributed constant filter passes the fundamental wave propagating through the first portion in the extension direction of the first portion and attenuates unwanted waves propagating through the first portion in the extension direction of the first portion. The microstrip antenna of claim 1, wherein the distributed constant filter is composed of an open stub having a length that is 1/4 the effective wavelength of the undesired wave. A microstrip antenna as described in claim 1 or 2, wherein the distributed constant filter includes a resonator that resonates at the frequency of the unwanted waves. A microstrip antenna as described in any one of claims 1 to 3, wherein the first insulating layer facing the first conductor pattern is thinner than the first insulating layer facing the second portion and the third portion. an insulating substrate;
a second conductor pattern formed on the insulating substrate and including at least a portion of the first conductor pattern;
the ground conductor is formed on one surface of the insulating substrate, and the second conductor pattern is formed on the other surface of the insulating substrate;
the first portion is disposed on the second conductor pattern;
5. The microstrip antenna according to claim 1, wherein the first insulating layer facing the second portion and the third portion is composed of the insulating substrate and a second insulating layer having a thickness equal to or greater than that of the insulating substrate. The microstrip antenna of claim 5, wherein the second insulating layer is an air layer. the first portion extends in a first direction;
the second portion and the third portion have a rectangular shape, and the rectangle has two sides parallel to the first direction and two sides parallel to a second direction perpendicular to the first direction;
The length of the second portion in the first direction is substantially λg ₂ /2, and λg ₂ is the effective wavelength of the fundamental wave when the fundamental wave propagates through the second portion in the first direction,
The length of the third portion in the first direction is substantially λg ₃ /2, and λg ₃ 7. The microstrip antenna according to claim 1, wherein λ is an effective wavelength of the fundamental wave when the fundamental wave propagates through the third portion in the first direction. 8. A microstrip antenna as claimed in any preceding claim, adapted for use at a single frequency.