JPH0562481B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a microstrip antenna, and particularly to a microstrip antenna that achieves a wide band in a circularly polarized wave system.

(Prior technology) Micro strip antenna
Antenna (hereinafter abbreviated as MSU) is a transmitting/receiving antenna that utilizes the radiation loss of a planar circuit.Its structure is thin and lightweight, and its antenna pattern has a low profile. In particular, antennas are attracting attention as antennas mounted on aircraft, antennas for receiving television broadcasts via satellites, etc.

However, in principle, MSA has an extremely narrow frequency band as an antenna, and widening the frequency band is an important issue in order to use it as an antenna for each of the above-mentioned systems.

Various proposals have been made to widen the band, but they can be roughly divided into those that aim to widen the band of the antenna element itself, and those that aim to widen the band by arranging multiple elements and combining their signals.

Of the two, the method using a multi-element array can relatively easily achieve a wide band, but has the drawbacks of complicated control and large size, making it unsuitable for mounting on an aircraft and requiring the antenna element itself to have a wide band. be.

Generally, the frequency bandwidth BW of MSA is calculated by the following formula: BW = (S-1)/Q√ ... (1) However, BW; antenna frequency band S; standing wave ratio Q at the antenna feeding point; Q of the antenna element itself. The Q factor of the antenna element itself can be increased to increase the bandwidth BW.

It is also known that the Q of the MSA is proportional to the specific inductivity (εr) of the substrate and inversely proportional to the thickness of the substrate, that is, the distance between the antenna pattern and the ground plate.

Based on this theory, the conventional method for widening the band of MSA is to insert a paper honeycomb board between the antenna pattern and the ground plate to reduce the weight, lower the dielectric constant of the substrate, and increase its thickness. A method has been proposed in which the antenna frequency band is increased while the Q is decreased.

Figure 4a shows a specific example of the structure of a circularly polarized MSA using a conventional paper honeycomb substrate, and Figure 4b shows the results of evaluating its characteristics, especially impedance matching, in terms of return loss (dB). It is.

Immediately, the usage conditions for this antenna are that the reception frequency band is 1.545 (GHz) to 1.548 (GHz) and the transmission frequency is 1.545 (GHz) to 1.548 (GHz).
It is designed as an antenna for use in a frequency band of about 100 (MHz) centered around 1.647 (GHz) to 1.650 (GHz), that is, 1.6 (GHz), and the dimensions of each part are as shown in the figure.

The return loss (RL) of an antenna is the ratio of the reflected power to the power supplied to the radiating element, and the relationship between this and the commonly used standing wave ratio S (SWR) is expressed by the following equation, as is well known. It can be done.

RL=-10log(S-1/S+1) ² [dB] ...(2) As is clear from the figure, in the conventional microstrip antenna using a honeycomb substrate, the center frequency o = 1.6 (GHz). Return loss -33dB (S≒
1.05) but within the desired band (1.545~1.650G
Hz) near both ends, the difference was large, approximately -10 dB (S≈2), and uniform and sufficient characteristics could not be obtained within the desired band.

Additionally, if the thickness of the paper honeycomb substrate is increased, stable characteristics cannot be obtained due to the influence of higher-order modes. In the antenna, the axial ratio characteristic, which is an important factor in circularly polarized antennas, deteriorates.

Therefore, not only is there a limit to the substrate thickness in the method using paper honeycomb materials, but also the wavelength shortening ratio on the microstrip is naturally limited due to the lower dielectric constant of the substrate material. As the antenna pattern becomes smaller, the antenna pattern becomes larger at the same frequency, and the antenna volume increases due to the thicker substrate, which also comes with the secondary drawbacks.

(Objective of the Invention) The present invention has been made in view of the above-mentioned circumstances, and has improved the frequency bandwidth without impairing the original characteristics of the MSA, which are thin, small, lightweight, and low profile.
The purpose is to provide MSA.

(Summary of the Invention) In order to achieve this object, the present invention creates an impedance mismatch between a radiating element (antenna element) formed by a microstrip pattern and a feeding circuit for the element. The configuration is configured to widen the applicable frequency band.

(Example) The present invention will be described in detail below based on the illustrated example.

Figures 1a and 1b are circularly polarized MSA according to the present invention.
FIG. 2 is a plan view and a side view showing the configuration of the FIG.

In the figure, 1 is a Teflon (registered trademark) substrate with a thickness h = 4 mm, and a radiating element 2 with a diameter of 64.72 mm (designed at o = 1.5975 GHz) is mounted on its surface.
A radiator 4 is constructed by forming a circular conductive pattern 3 on the back surface and a conductive pattern 3 as a ground on the entire surface of the back surface.

Similarly, 5 is a Teflon (registered trademark) substrate with a thickness h = 0.8 mm, and one side has a ground pattern 6 on the entire surface, and the other side has a branch line type 3 dB hybrid circuit 7 with a microstrip line pattern. and an impedance matching circuit 8, the feeding circuit 9 and the radiating element 2 of the radiator 4 are connected to respective ground planes 3.
and 6 are overlapped in contact with each other, and one of the two input terminals 10 of the power supply circuit 9 is provided with a terminating resistor 11.
and the other end 12 is used as the input/output end of the antenna, and the two output ends 13 and 1 of this feeding circuit 9 are connected.
4 has respective substrates 1 and 5 and ground patterns 3 and 6.
the radiator 4 through pins 15, 16 passing through the
A microstrip antenna is constructed by connecting the radiating element 2 at a predetermined position formed on the opposite surface of the antenna.

At this time, it is of course necessary to insulate the through pins 15 and 16 from the earth patterns 3 and 6 of each board, and the feeding point to the radiating element 2 is an offset point where the input impedance is 100Ω.
Set to .

The microstrip antenna described above is as shown in FIG. 1b when viewed from the side.

Note that the 3 dB hybrid circuit 7 can be easily obtained based on conventionally known technology, so a detailed explanation will be omitted. For example, as shown in FIG.
The output signal of 50Ω and appearing at the two output terminals 13 and 14 may have a pattern symmetrical about its center axis so that its level is divided into two and has a phase difference of 90° from each other.

What is important in this case is that the frequency band characteristics of the antenna are affected by the difference in the matching state between the impedance Zout of the output terminals 13 and 14 of the 3 dB hybrid circuit 7 and the input impedance Z _R of the radiating element 2 of the antenna radiator 4. This makes an extremely large difference.

That is, when matching the input impedance Z _R of 100Ω of the radiating element 2 as described above and the output impedance of 50Ω of the 3dB hybrid circuit 7, conventionally a strip line of 70.7Ω per line is used as shown in Fig. 1d. Then, after converting the impedance from 50Ω to 100Ω, a 100Ω stripline is added, and the input impedance of the radiating element 2 to be fed on the microstrip line, that is, the feeding point impedance Z _R (=100Ω) is converted, and then the power is fed. It was on.

The reason for this is believed to be that the physical characteristics of the characteristic impedance MSA of the stripline are different from the feeding point impedance of the radiating element 2.

In other words, the characteristic impedance of a microstripline does not theoretically change with frequency, but the feeding point impedance of a radiating element has meaning only at its resonant frequency.

Therefore, when attempting impedance matching in such an antenna, it is thought that the above-mentioned reason was known from experience and a matching circuit as shown in FIG. 1d was used based on this knowledge.

In other words, it is common sense to make the impedance of the radiator input and the feed circuit output as similar as possible at the feeding point of the antenna, to stabilize the transmission characteristics of the stripline impedance conversion circuit, and to achieve ideal matching. As mentioned above, attempts have been made to reduce the Q by reducing other elements such as the dielectric constant (εγ) of the substrate material in order to achieve a wide band antenna.

The present invention focuses on this point, and intentionally shifts the impedance of the radiator input and feeder circuit output, slightly lowering the transmission characteristics of the stripline impedance conversion circuit and creating a slight mismatch, thereby achieving a wider band. This made it possible.

That is, the 3dB hybrid circuit 7 of FIG.
is 70.7Ω at the output end to convert from 50Ω to 100Ω
radiating element 2 of direct radiator 4 by simply adding the stripline length λg/4 (λg is the substrate wavelength)
We adopted a method of supplying power to the

This directly matches the impedance (Zin) of the 3 dB hybrid circuit 7 of 50 Ω and the impedance of the radiator (Z _R ) of 100 Ω,
As is well known, Zo=√・_R =√5000≒70.7(Ω)...(3) A strip line impedance conversion circuit with a length of λg/4 whose characteristic impedance is Zo (≒70.7Ω) obtained based on the formula. is inserted between the 3dB hybrid circuit and the radiating element.

If such a feeding circuit is used to construct the microstrip antenna shown in FIG. A slight mismatch occurs, which is different from the normal type shown in FIG. 1d, and wideband antenna characteristics can be obtained as shown by the solid line in FIG.

For reference, the same figure shows the return loss characteristic of a similar microstrip antenna using the conventional feeder circuit shown in FIG. 1(d) by a broken line.

According to the figure, the difference in the applicable bandwidth of both antennas is clear, and the microstrip antenna MSA according to the present invention can obtain an extremely wide applicable frequency band compared to the conventional antenna.

Furthermore, as shown in Fig. 3, the axial ratio characteristic of the antenna according to this embodiment in the boresight direction is 1.54G.
Since it is less than 1 over a wide range of about 100 MHz from Hz to 1.65 GHz, it is clear that it has an axial ratio characteristic superior to that of conventional honeycomb substrate antennas and can be put to practical use as a wideband antenna.

Note that in the above-described embodiment, the presence of the through pins 15 and 16 interposed between the two output ends 13 and 14 of the feed circuit 9 and the feed point of the radiating element 2 has some characteristics. It is to have an impact on the

That is, the penetrating pins 15 and 16 are simply conductive rods of about 5 mm that pass through the substrate of the radiator 4, but since they slightly disturb the impedance, they are intentionally created in the present invention at the feeding point of the radiating element. This should be considered as impedance mismatch.

Although various embodiments of the present invention are conceivable in addition to the embodiments described above, the key point is that the input and output impedances of the antenna's radiator input and the feeder circuit output are made slightly different, so that the applicable frequency can be applied over a wide band. Any means may be used as long as the method can be used as long as the method can be used, so there is no need to explain that it can be applied to all types of antenna devices.

(Effects of the Invention) Since the present invention is configured as explained above, it is possible to achieve a thin and low profile by using an extremely simple method.
This is extremely effective in increasing bandwidth without sacrificing the characteristics of MSA.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of the present invention,
a is a plan view, b is a side view, c and d are 3 dB hybrid circuit pattern diagrams, FIGS. 2 and 3 are diagrams showing actual measurement results of an embodiment of the MSA of the present invention, and FIGS. 4 a and b are diagrams FIG. 2 is an external structural diagram of a conventional MSA and its characteristic diagram. 1 and 5... Teflon (registered trademark name) substrate, 2
... Radiation element, 3 and 6 ... Grounding conductive pattern, 4 ... Radiator, 7 ... 3dB hybrid circuit, 8 ... Matching circuit, 9 ... Power supply circuit, 15 and 16 ... Through pin.

Claims (1)

[Claims] 1. A microstrip antenna characterized in that its frequency characteristics are broadened by slightly shifting the impedance matching between the feeding point of the radiating element of the microstrip antenna and the feeding circuit. antenna. 2. The microstrip antenna according to claim 1, wherein the applicable frequency band of the antenna is arbitrarily set by appropriately changing the impedance mismatch state.