JPS6144418B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an on-vehicle antenna, and an object of the present invention is to improve antenna characteristics. When installing a C-tuned radio receiver using a varactor diode in a car, problems such as tracking occur due to the car antenna's capacitive nature, as shown in the equivalent circuit in Figure 1, and the antenna circuit is untuned. There must be. Note that R ₁ and C ₁ are the radiation impedance of the antenna, and C ₂ is the capacitance of the cable. In addition, as a result, practical sensitivity (S/N) will inevitably deteriorate compared to μ-tuned radio receivers. The present invention eliminates this drawback. First, S/N is determined by the signal level supplied to the radio receiver, the square root of the signal source impedance seen by the radio receiver, and the noise figure (NF) of the radio receiver. Therefore, if NF is constant, how to improve S/N

[Table] should be made larger. On the other hand, in Figure 1, R ₁ has a sufficiently low impedance compared to C ₁ , so if the input impedance of the radio receiver is sufficiently high, the signal level supplied to the radio receiver will depend on the division ratio of capacitances C ₁ and C ₂ . It will be decided. Also, the signal source impedance seen from the radio receiver is C ₁ +C ₂ , and this relationship is as shown in FIG. Now, if an inductance L _{1 is inserted into a capacitance C 1 as} shown in Figure 3, the signal level division ratio will become smaller near the resonance frequency due to series resonance.
As the signal level supplied to the radio receiver increases, the signal source impedance seen from the radio receiver also decreases, improving the S/N ratio. FIG. 4 shows this state. If this resonant frequency is set to a frequency lower than the reception band, the signal level supplied to the radio receiver at frequencies above the resonant frequency will be the division ratio of L ₁ and C ₂ , and will deteriorate in proportion to the square of the frequency. Therefore, it is advantageous to set the resonance frequency to a higher frequency than the reception band. In addition, when considering the VHF band, in parallel to L ₁
It is sufficient to provide a capacity for passing VHF band signals. By the way, although the above embodiment is useful for high reception frequencies, it is less effective for low reception frequencies. Figure 5 shows an embodiment that takes this point into consideration, where an inductance L ₂ is provided in parallel to the capacitance C ₂ , and as shown in Figure 6, the signal level division ratio is reduced by parallel resonance near the resonance frequency. , so that the signal level supplied to the radio receiver is increased. In addition, in FIG. 5, _L1 is an inductance connected in series to the radiation impedance of the antenna. In this case, the signal source impedance seen by the radio receiver also increases, and the S/N seems to deteriorate.

As is clear from the table, the S/N is proportional to the square root of the signal source impedance of the radio receiver, so the S/N improvement due to a smaller division ratio is greater, leading to an overall S/N improvement. . Note that if this resonant frequency is set to a frequency higher than the receiving band, the signal level supplied to the radio receiver at frequencies below the resonant frequency will be divided into _C1 and _L2 , and will deteriorate in inverse proportion to the square of the frequency. It is advantageous to set the resonance frequency to a frequency lower than the reception band. Also, _L2 may be located inside the radio receiver. Furthermore, when considering the capacitance C ₃ between the antenna and the body, L ₁ is series resonance with respect to the sum of C ₁ and C ₃ , and L ₂
can be thought of as parallel resonance with C ₁ + C ₂ + C ₃ . As is clear from the above embodiments, according to the present invention, signals can be received with a good S/N ratio, and only a small number of partial points are added for this purpose.

[Brief explanation of the drawing]

Figure 1 is an equivalent circuit diagram of a conventional antenna, Figure 2 is its characteristic diagram, Figure 3 is an equivalent circuit diagram of an antenna according to an embodiment of the present invention, Figure 4 is the same characteristic diagram, and Figure 5 is another diagram. FIG. 6 is an equivalent circuit diagram of the embodiment, and FIG. 6 is a characteristic diagram thereof. R ₁ , C ₁ ...radiation impedance of antenna,
C ₂ ... Cable capacity, L ₁ ... Inductance.

Claims (1)

[Claims] 1. A first inductance is provided between the antenna body and the feed line, a resonance point is selected at a frequency above the reception band, and a second inductance is provided in parallel with the cable capacity and radio receiver input capacity. Provide an inductance,
An in-vehicle antenna characterized by having resonance between the same capacitance and impedance below the reception band. 2. The vehicle-mounted antenna according to claim 1, wherein the first inductance is provided with a VHF band signal passing capacity in parallel.