JPH0529994A - Diversity antenna system - Google Patents

Abstract

PURPOSE:To obtain an excellent diversity effect even when the distance of antennas is small with respect to the diversity antenna system of antenna changeover type. CONSTITUTION:The system is provided with a means 5, in which elements 10,11 having a resistance being almost twice the real part of the antenna input impedance or below or short-circuits are connected between feeding points 3,3 and 4,4 for each of antennas 1,2. The means 5 is provided with a means controlling the connection of the said element 10 or 11 or the short-circuit to the feeding points 3,3 or 4,4 of the antenna 2 or 1 other than the selected antenna 1 or 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an antenna switching type diversity antenna device, and more particularly to a diversity antenna device capable of obtaining a good diversity effect even when the antenna interval is small.

[0002]

2. Description of the Related Art FIG. 10 is a diagram showing an example of a conventional 2-port antenna switching reception diversity antenna device, in which 48 is an antenna element # 1 and 49 is an antenna element #.
2, 50 and 51 are feeding points of the respective antennas, 52 is a changeover switch, 53 is a receiver, 54 is an envelope detector, 55 is an envelope comparator, and 56 is an input terminal of a standard antenna switching level. This diversity antenna device is used when receiving radio waves during fading that occurs in land mobile communication, etc., and always uses the reception characteristics (power, S
/ N, error rate, etc.) are changed over to the switch 52.

The switching method is as follows. The envelope of the received signal of the antenna is detected by the envelope detector 54 and compared with the reference antenna switching level input from the input terminal 56 by the comparator 55. If the level is below the reference level, switching is performed. The switch 52 is switched. If it is still below the reference level even after switching, the operation of switching again after a certain time is repeated.

[0004]

When adopting the diversity method, the most important point required for the antenna characteristics is that the inter-antenna correlation coefficient is low. However, in the conventional case, as shown in FIG. 10, when the receiver is connected to one antenna feeding point, the other antenna feeding point is opened. In such a configuration, when the antenna spacing is very narrow, such as about 0.1 wavelength, the inter-antenna correlation coefficient becomes as high as about 0.8, and there is a problem that the diversity effect is hardly obtained. The reason will be described below.

Generally, the antenna correlation coefficient depends on the antenna spacing and the antenna radiation pattern shape. When the radiation patterns at each feeding point are exactly the same, the antenna spacing x and the correlation coefficient ρ
The relationship is as in "Equation 1". (Reference “WC Lee:
"Mobile Communications Engineering", pp. 273-
336, McGraw-Hil 1982 ").

[0006]

[Equation 1]

This relationship shows that the antenna interval must be about 0.4 wavelength in order to obtain a sufficiently low correlation coefficient as shown by the dotted line indicated by numeral 16 in FIG. This is about 13 cm in the 900 MHz band mobile communication system, which is difficult to install in a portable wireless device. In addition, the correlation coefficient becomes 0.8 when the antenna interval is 0.1 wavelength. There is a method of deforming the radiation pattern to reduce the correlation coefficient when the antenna interval is narrow. As shown in FIG. 2, the feeding point currents of the antennas # 1 and # 2 are I ₁ and I ₂ , the voltages are V ₁ and V ₂ , the self impedances are Z ₁₁ and Z ₂₂ , and the mutual Z.
If the impedances are Z ₁₂ and Z ₂₁ , then "Equation 2" holds.

[0008]

[Equation 2]

Here, in the conventional case, when the receiver is connected to the antenna # 1, the antenna # 2 is opened. That is, I ₂ = 0.

[Equation 3]

[0010] In this way, the input impedance of the antenna # 1, will not the same as self-impedance Z ₁₁ becomes the same, the impedance in free space. That is, it can be seen that the current distribution in the vicinity of the feeding point does not change between the single case and the case where the antenna comes close to the diversity structure, and the radiation pattern hardly changes. As an example, consider a case where two dipole antennas are horizontally arranged as shown in FIG. The result of calculating the radiation pattern using the method of moments is shown by numeral 17 in FIG. 4 by limiting the incoming wave to the horizontal plane and dividing each dipole antenna into seven. Here, numeral 16 indicates the case in free space, and the antenna interval is 0.1 wavelength.

As is clear from this figure, the case where there is one set of antennas in the free space and the case where the antenna spacing is 0.1 wavelength are not connected to the receiver (as in the conventional antenna switching diversity system). The radiation pattern is almost unchanged when the antenna (antenna with the feeding point opened) is present. From this, the correlation coefficient is expected to increase. FIG. 5 shows the relationship between the correlation coefficient and the antenna spacing in this case with numeral 17. As is clear from this figure, a low correlation coefficient cannot be obtained in the range where the antenna spacing is narrow.

That is, in the conventional antenna switching diversity antenna device, since the antenna feeding point to which the receiver or the transmitter is not connected is open, the correlation coefficient becomes high when the antenna interval is narrow, so that it is sufficient. However, there is a drawback that excellent diversity characteristics cannot be obtained.
On the other hand, there is a problem that a small antenna switching diversity antenna cannot be realized because the antenna must be enlarged in order to obtain a low correlation coefficient.

The present invention has been made in order to solve the above problems, and an object thereof is to realize a small antenna switching diversity antenna device having a low correlation coefficient and a high diversity effect.

[0014]

SUMMARY OF THE INVENTION The present invention provides a system having a plurality of antennas, of which the antenna having the best reception condition is selected and used by connecting to a transmitter or a receiver. For each antenna, between the feeding points, approximately 2 of the real part of the input impedance of the antenna is
A diversity antenna device provided with means capable of connecting an element having a resistance value not more than double or a short circuit, and also provided means for controlling to connect the element or the short circuit to a feeding point of an antenna other than the selected antenna is there.

[0015]

In the diversity antenna of the present invention, the feeding points of the antennas other than the antenna selected as described above are short-circuited or terminated with low resistance. Therefore, when the antenna spacing is narrow, the mutual coupling becomes the antenna input impedance. By acting, the current distribution on the antenna becomes different from that when the antenna exists in free space, and the radiation pattern changes, so that a low correlation coefficient is obtained. That is, according to the present invention, it is possible to realize a small antenna switching diversity antenna device having a low correlation coefficient and a high diversity effect.

[0016]

EXAMPLES The operation and the like of the present invention will be described in detail below based on examples. FIG. 1 is a diagram showing a first embodiment of the present invention, showing a case of two-element antenna reception diversity, in which (a) is a case where a receiver is connected to antenna # 1, (b) Shows the case where the receiver is connected to the antenna # 2. In the figure, numeral marks 1 and 2
Are antennas # 1 and # 2, 3 and 4 are feeding points of each antenna, 5 is a changeover switch, 6 is a receiver, 7 is an envelope detector, 8 is an envelope comparator, and 9 is a reference switching level input terminal. Reference numerals 10, 11 (ZL) represent load resistors # 1, # 2 connected to the antenna not connected to the receiver.

In the figure, switching of antennas is determined by the envelope level comparator 8 comparing the reception level with the reference level. When switching the antenna,
Since the changeover switch 5 has a structure as shown in the figure, when the switch 5 is changed over so that the receiver is connected to the antenna # 1 as shown in FIG. When the resistor ZL is connected and the receiver is connected to the antenna # 2 as shown in FIG.
ZL is connected to 1. If ZL has a resistance equal to or less than twice the real part of the antenna input impedance, or 0Ω (short circuit), the antenna radiation pattern is deformed and the correlation coefficient becomes low. The reason for this will be described below.

FIG. 2 shows a model in the case of two antennas.
In the figure, 12 is the feeding point of antenna # 1 and 13 is the antenna
This is the feeding point of Tena # 2. # 1 and # 2 as shown in FIG.
The antenna feed point current to I₁, I₂, The voltage is V₁, V
₂, Z self-impedance ₁₁, Z_{twenty two}, Mutual Z Impey
Dance Z₁₂, Z_{twenty one}Then, "Equation 4" is established.

[0019]

[Equation 4]

Here, the receiver is connected to the antenna # 1,
Considering the case where the resistor ZL is connected to the antenna # 2, V
₂ = ZL · I _{2 and} becomes like "Equation 5".

[0021]

[Equation 5]

As described above, the impedance of the feeding point changes depending on the values of Z ₁₂ , Z ₂₁ (mutual impedance), and ZL, and the current distribution on the antenna changes from the free space, so that the radiation pattern is deformed. is expected. That is, when the antennas are very close to each other, the mutual impedance becomes large and the radiation pattern is deformed. On the other hand, when the value of ZL is large, the change in the feeding point impedance is small and the pattern deformation is small, so the value of ZL is limited.

Therefore, consider a case where two dipole antennas are arranged in parallel in a free space as shown in FIG. Here, 14 is antenna # 1 and 15 is antenna # 1.
2, ZL is a load resistance connected to an antenna not connected to the receiver, and S is an antenna interval.

FIG. 4 shows the resistance ZL when S is 0.1 wavelength.
And the radiation pattern is shown. In the figure, (a) is an amplitude pattern on the XY plane, (b) is a phase pattern on the XY plane, 16 is a free space, 17 is an antenna feeding point open (ZL = ∞Ω), 18 is ZL = 75Ω, 1
9 is a characteristic in the case of a short circuit (ZL = 0Ω). Thus, with the antenna feed point open (conventional), the radiation pattern is almost the same as in the free space, but as the resistance ZL becomes smaller, the radiation pattern becomes more deformed and becomes 75Ω,
It can be seen that the deformation is considerably large at 0Ω.

FIG. 5 shows the relationship between the antenna spacing S and the correlation coefficient. In the figure, numeral 16 is in free space, 17 is open antenna feeding point (ZL = ∞Ω), and 18 is ZL = 75.
Ω, 19 is a short circuit (ZL = 0Ω), 20 is ZL = 10Ω,
21 shows the case of ZL = 5Ω. In this way, as the resistance ZL is reduced, a small correlation coefficient can be obtained even when the antenna spacing is very small.

FIG. 6 shows the relationship between the value of the resistance ZL and the correlation coefficient when the antenna interval is 0.1 wavelength. As shown in the figure, when the resistance ZL is 0, the correlation coefficient is also 0, and
The higher the L, the higher the correlation coefficient. From this figure, the resistance ZL is 150Ω in order to reduce the correlation coefficient to 0.5 or less.
It turns out that it must be: Here, as is clear from the above equation, the radiation pattern is determined by the relationship between the load resistance ZL and the antenna input impedance Zs, and the correlation coefficient is determined. On the other hand, since the real part of the input impedance of this dipole antenna is 75Ω, it can be said that the load resistance ZL may be about twice the input impedance of the antenna to be connected (150Ω = 75Ω × 2) or less.

FIG. 7 shows the relationship between the antenna correlation coefficient and the diversity gain. Here, the diversity gain refers to the 1% increase level of the cumulative probability distribution when the antenna switching diversity method is adopted when the diversity antenna having a certain correlation coefficient is moving in the environment where fading exists. .. Here, when the correlation coefficient is 1, it is 0
dB (standard).

As is clear from this figure, the correlation coefficient of 0.5 is a condition required for the antenna when diversity is adopted in the mobile communication system. Above this value, the effect of diversity decreases. .. From this, assuming that the antenna correlation coefficient of 0.5 or less is an effective diversity antenna condition, the resistance value of ZL for which the correlation coefficient is obtained is about 150Ω or less, that is, almost twice the antenna input impedance or less. Becomes

As described above, in the embodiment shown in FIG. 1, when the switch is switched so as to connect the receiver to the antenna # 1, the antenna # 2 is connected to the load resistance ZL which is less than twice the antenna input impedance. And the antenna #
When the receiver is connected to the antenna 2, the load resistance ZL, which is less than twice the antenna input impedance, is connected to the antenna # 1.
The antenna radiation pattern is deformed and the correlation coefficient is lowered even when the wavelength is very small, that is, below the wavelength. As a result, it is possible to realize a small diversity antenna having high diversity gain.

In this embodiment, the case where the resistor is connected to the feeding point of the unselected antenna is shown, but as described above, a short-circuit line may be used instead of the resistor. Further, although the description is given using the dipole antenna here, if mutual coupling exists between the antennas even in other antennas, the radiation pattern is deformed by the connection resistance ZL to the antennas, and thus a similarly low correlation coefficient is obtained. That is clear. Also, the structure of the diversity antenna can be applied even when one antenna has two feeding points. Further, even when the resistor ZL has an imaginary part, the same effect can be obtained as long as this condition is satisfied in the real part.

FIG. 8 is a diagram showing a second embodiment of the present invention and shows a case where there are three sets of antennas. In the figure, numerals 22, 23, and 24 are antenna # 1, respectively.
# 2 and # 3, 25, 26 and 27 are feeding points of each antenna, and 28, 29 and 30 (ZL) are load resistors # 1,
# 2, # 3, 31 is a changeover switch, 32 is a receiver, 33 is an envelope detector, 34 is an envelope comparator, and 35 is an input terminal of a reference switching level. The changeover switch 31 of 31 is of an interlocking type, and if a certain antenna is connected to the receiver, a load resistance is always connected to the other antenna terminal.

Since this embodiment has such a structure, when the receiver is connected to the antenna # 1, the load resistors are connected to the antennas # 2 and # 3. Similarly, when the antenna interval is narrow, the antenna radiation pattern is deformed and a sufficiently low correlation coefficient is obtained. This is the same when the receiver is connected to antenna # 2 or antenna # 3. That is, the antenna is 3
Even in the case of a set, it is possible to realize a small diversity antenna having a high diversity gain.

FIG. 9 is a diagram showing a third embodiment of the present invention, showing a case where an electric switch (diode switch) is provided in the antenna terminal instead of the mechanical switch. Here, numeral marks 36 and 37 are antennas # 1 and # 2.
38 and 39 are the feeding points of the antennas, 40 and 41 are diode switches, and 42 is the bias voltage supply point.
43 is a changeover switch, 44 is a receiver, 45 is an envelope detector, 46 is an envelope comparator, and 47 is a reference switching level input terminal.

The change-over switch 43 is of interlocking type, and when a certain antenna is connected to the receiver, the bias voltage E is always applied to the diode switch of the other antenna terminal. With such a structure, when the receiver is connected to one antenna, the other antenna is short-circuited, and the same effect as that of the first embodiment can be obtained.

In this embodiment, since the feeding point of the antenna is electrically short-circuited by using the diode, there is an advantage that the changeover switch has less structural restriction and can be miniaturized. In addition, since a diode switch can be installed at the power supply terminal of the antenna and only the line that applies the bias voltage can be connected, no metal object such as a switch will come around the antenna, so good antenna characteristics can be obtained. ..

In the above embodiments, the case where the number of antennas is 2 or 3 is described, but the number of antennas in the present invention is not limited to this, and the present invention can be applied even in the case of multiple elements. It goes without saying that it is applicable.

[0037]

As described above, according to the present invention, a good diversity effect can be obtained even when the antenna interval is small. Therefore, a small antenna switching type diversity suitable for a portable radio device or the like can be obtained. There is an advantage that the antenna can be easily realized.

[Brief description of drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention.

FIG. 2 is a diagram showing a model in the case of two-antenna diversity.

FIG. 3 is a diagram illustrating a diversity antenna using a parallel arranged dipole antenna.

FIG. 4 is a diagram showing a radiation pattern when the antenna interval is 0.1 wavelength.

FIG. 5 is a diagram showing a relationship between an antenna interval and a correlation coefficient.

FIG. 6 is a load resistance Z when the antenna interval is 0.1 wavelength.
It is a figure which shows the relationship between L and a correlation coefficient.

FIG. 7 is a diagram showing a relationship between an antenna correlation coefficient and a diversity gain.

FIG. 8 is a diagram showing a second embodiment of the present invention.

FIG. 9 is a diagram showing a third embodiment of the present invention.

FIG. 10 is a diagram showing an example of a configuration of a conventional antenna switching diversity antenna.

[Explanation of symbols]

1,14,22,36 Antenna # 1 2,15,23,37 Antenna # 2 3,12,25,38 Antenna # 1 feeding point 4,13,26,39 Antenna # 2 feeding point 5,31, 43 Changeover switch 6,32,44 Receiver 7,33,45 Envelope detector 8,34,46 Envelope comparator 9,35,47 Reference switching level input terminal 10,28 Load resistance # 1 11,29 Load resistance # 2 16 Characteristics in free space 17 Characteristics when antenna feed point is open (ZL = ∞Ω) 18 Characteristics when ZL = 75Ω 19 Characteristics when short circuit (ZL = 0Ω) 20 Characteristics when ZL = 10Ω 21 Characteristics when ZL = 5Ω 24 Antenna # 3 27 Feed point of antenna # 3 30 Load resistance # 3 40 Diode switch # 1 41 Diode switch # 2 42 Bias voltage Supply point

Claims (1)

Claim: What is claimed is: 1. A system having a plurality of antennas, the antenna having the best reception condition among these antennas is selected and used by connecting to a transmitter or a receiver. For each antenna, a means for connecting an element or a short circuit having a resistance value that is less than twice the real part of the antenna input impedance is provided between the feeding points, and at the feeding point of the antenna other than the selected antenna. A diversity antenna device comprising means for controlling to connect the above-mentioned element or short circuit.