[0001] The present invention relates to a compensation amplifier for an automobile antenna
utilizing, as an antenna element, a heating wire incorporated in an automobile rear
window, for removing fog formed thereon, which compensates for a loss in the power
of signals transmitted between the antenna element and a radio receiver circuit.
[0002] In recent times, the heating wire for removing fog formed on the rear window of an
automobile has come to be used also as an antenna element for a radio receiver. An
example of this type of antenna is disclosed in Japanese Patent Disclosure (Kokai)
No. 52-64,257 which corresponds to U.S. Patent No. 4,086,594 and U.K. Patent No. 1,520,030.
[0003] Fig. 1 shows this prior art. Heating wire H is incorporated in the rear window of
an automobile. A blocking circuit made up of radio signal blocking coil 9 and choke
coil 7 for suppressing interference are interposed between heating wire H and automobile
battery B. The radio signal, such as a radio broadcasting signal, is picked up between
heating wire H and blocking coil 9, and is amplified by preamplifier 13. The output
signal from preamplifier 13 is supplied to a radio receiver circuit (not shown) via
feeder line 2. Filtering capacitor 8, for removing noises from battery B, is connected
between choke coil 7 and a ground potential terminal. Choke coil 7, blocking coil
9, filtering capacitor 8, and preamplifier 13 together form compensation amplifier
1. Heater on/off switch 3 is connected between choke coil 7 and battery B.
[0004] In this prior art, the inductance of blocking coil 9, is set at about 2 mH. According,
the high frequency radio signal received by heating wire H, which is used as an antenna
element, does not flow to the ground potential terminal but is instead transmitted
tot he radio receiver circuit via preamplifier 13 and feeder line 2. Blocking coil
9 is wound on a pot core which does not have air gaps.
[0005] Since the direct current flowing through heating wire H is about 10A, the heating
wire is required to be relatively thick in order to permit this large direct current
to flow therethrough. Accordingly, the size of blocking coil 9 must be large enough
to ensure that the necessary inductance can be obtained, which results in the undesirable
enlargement of the overall size of compensation amplifier 1.
[0006] Further, in the above prior art, the pot core is used in order to render the overall
size of the automobile antenna amplifier as small as possible. However, use of a pot
core is expensive, and a surface of the pot core must be grinded to a mirror finish
in order to eliminate air gaps. The necessity to do this increases the manufacturing
cost of the automobile antenna.
[0007] The object of the present invention is to provide a compensation amplifier for an
automobile antenna utilizing a heating wire as an antenna element, which is small
in size and has a low manufacturing cost.
[0008] According to this invention, a compensation amplifier for an automobile antenna is
provided, which comprises a band-pass filter coil interposed between a power source
of a vehicle and a heating element serving also as an antenna element, the band-pass
filter coil forming a band-pass filter with a floating capacitor connected between
the heating element and a ground potential terminal, and a compensation circuit connected
to the band-pass filter, for amplifying a signal output therefrom.
[0009] This invention can be more fully understood from the following detailed description
when taken in conjunction with the accompanying drawings, in which:
Fig. 1 is a circuit diagram of a prior compensation amplifier for an automobile antenna;
Fig. 2 is a circuit diagram of an embodiment of a compensation amplifier for an automobile
antenna according to this invention;
Fig. 3 is an equivalent circuit diagram of a circuit for compensating an FM signal
shown in Fig. 2;
Fig. 4 is an equivalent circuit diagram of a circuit for compensating an AM signal
shown in Fig. 2; and
Figs. 5A and 5B show examples of a cancelling coil and an AM band-pass filter coil
shown in Fig. 2.
[0010] Fig. 2 is a circuit diagram of an embodiment of the compensation amplifier for an
automobile antenna according to this invention. This embodiment has coils interposed
between heating wire H and DC (direct current) battery B, like the prior art. However,
these coils are not used for blocking the high frequency signal; in other words, these
coils are not used as choke coils. Instead, these coils are used as a part of a band-pass
filter, and this is the difference between this invention and the prior art.
[0011] Band-pass filter coil La1 for an AM (amplitude-modulation) signal, cancelling coil
Lc which cancels out DC magnetization caused by AM band-pass filter coil La1, and
band-pass filter coil Lf1 for an FM (frequency modulation) signal are connected between
heating wire H and battery B. FM band-pass filter coil Lf1 and AM band-pass filter
coil La1 are wound on a toroidal core with air gaps. An inductance of AM band-pass
filter coil La1 is set at 1 mH or less. Feed-through capacitor C for removing noises
from battery B is interposed between cancelling coil Lc and battery B. Heater on/off
switch 3 is connected between feed-through capacitor C and battery B. Floating capacitor
Cs is connected between heating wire H and a ground potential terminal. Coupling capacitor
Co is connected between cancelling coil Lc and AM band-pass filter coil La1.
[0012] The FM signal is supplied to an FM signal preamplifier including bipolar transistor
TR through an FM signal compensation circuit including coupling capacitor Cc, coil
Lf2, capacitor Cf, and resistor Rf. Band-pass filter coil Lf1 and floating capacitor
Cs are used as a part of an input band-pass filter for an FM signal compensation circuit.
[0013] On the other hand, an AM signal is supplied to an AM signal preamplifier including
field-effect transistor FET through an AM signal compensation circuit including additional
capacitor Cb, coil La2, capacitor Ca, and resistors Ra1 and Ra2. Band-pass filter
coil La1, floating capacitor Cs, and additional capacitor Cb are used as a part of
an input band-pass filter for an AM signal compensation circuit. Additional capacitor
Cb tunes a frequency of an AM signal.
[0014] The operation of the above embodiment will be described.
[0015] Fig. 3 shows an equivalent circuit diagram of the FM signal compensation circuit.
In Fig. 3, Ra is an antenna resistor. This FM signal compensation circuit is a double
tuning circuit and includes a primary tuning circuit and a secondary tuning circuit.
The primary tuning circuit is made up of FM band-pass filter coil Lf1 of an air-core
type and floating capacitor Cs. The secondary tuning circuit is made up of coil Lf2
and capacitor Cf.
[0016] In the prior art, since the radio signal flows to the ground potential terminal through
floating capacitor Cs, floating capacitor Cs caused losses of the FM signal. However,
in the present invention, since floating capacitor Cs is used as a part of the band-pass
filter for tuning the FM signal, losses are not generated from floating capacitor
Cs. Further, the gain of FM band can be increased by several dB's (Decibels) when
compared with the prior art, due to the band-pass filter.
[0017] Fig. 4 shows an equivalent circuit diagram of the AM signal compensation circuit.
The AM signal compensation circuit is a double tuning circuit and includes a primary
tuning circuit and a secondary tuning circuit. The primary tuning circuit is a high-pass
filter and, includes AM band-pass filter coil La1 which is of an air-core type, floating
capacitor Cs, and tuning capacitor Cb. The secondary tuning circuit is a low-pass
filter and includes coil La2 and capacitor Ca.
[0018] As seen from the above, since the floating capacitor Cs is used as a part of the
band-pass filter for the AM band, losses are not generated from floating capacitor
Cs. Further, the gain of the AM band can be increased by several dB's when compared
with the prior art.
[0019] Figs. 5A and 5B show examples of windings of AM band-pass filter coil La1 and cancelling
coil Lc. In the example of Fig. 5A, AM band-pass filter coil La1 and cancelling coil
Lc are wound on ferrite core F as a bifilar winding. In the example of Fig. 5B, they
are wound separately.
[0020] DC current from battery B flows through AM band-pass filter coil La1, causing DC
magnetization and saturation in ferrite core F. Thus, the inductance of the coil is
lowered below the desired level. In order to prevent this, cancelling coil Lc is wound
such that the direction and the magnitude of the magnetic field of cancelling coil
Lc are opposite to those of band-pass filter coil La1.
[0021] As mentioned above, according to this invention, band-pass filter coil La1 is used
as a part of a band-pass filter and not as a choke coil. Accordingly, it does not
matter that high frequency signal received by the heating wire, which is used as an
antenna element, flows into the ground potential terminal. This feature significantly
differentiates the present invention from the prior art. In the prior art, the inductance
of coil 9 is set large so that the radio signal does not flow into the ground. When
the coil is used as the coil alone, it works as a choke coil and blocks the signal
to flow. When the coil is used as a tuning circuit with the capacitor, it permits
to flow the signal of a tuned frequency range.
[0022] As can be understood from the above description, according to the present invention,
it is not necessary to set the inductance value of the band-pass filter as high as
in the prior art. As a result, a toroidal core rather than a pot core can be used
in this invention. By using the toroidal core, the device can be manufactured at a
lower cost than the prior art.
[0023] Further, since both AM and FM coils can be small in size, the overall size of the
entire device can be also small.
[0024] In the above embodiment, the feed-through capacitor C inhibits the power supply noise
caused by battery B from entering into the AM or FM compensation circuit. Additional
capacitor Cb, together with band-pass filter coil La1, not only comprises a band-pass
filter for an AM signal but also functions as a by-pass capacitor for an FM signal.
Thus, a Stable tuning characteristic for an FM signal is obtainable without getting
any influence from the AM compensation circuit.
[0025] The core used in the above embodiment for the windings of band-pass filter coil La1
and cancelling coil Lc is not limited to the torodidal core, and the pot core can
be also used. With the use of the pot core, the size of the entire device can be further
reduced.
[0026] As mentioned above, according to this invention, the overall size of the compensation
amplifier for the automobile antenna can be reduced, and this is advantageous in view
of the lower cost of manufacturing the same.
[0027] This invention is not limited to the above embodiment but can be changed or modified
within the scope and spirit thereof.
1. A compensation amplifier for an automobile antenna utilizing, as an antenna element,
a heating element for removing fog from an automobile rear window, said compensation
amplifier characterized by comprising:
a band-pass filter coil (La1, Lf1) interposed between a power source (B) of
a vehicle and said heating element (H), the band-pass filter coil (La1, Lf1) forming
a band-pass filter circuit with a floating capacitor (Cs) connected between said heating
element (H) and a ground potential terminal; and
compensation circuit means connected to said band-pass filter circuit and for
amplifying a signal output from said band-pass filter circuit.
2. A compensation amplifier according to claim 1, characterized by further comprising
a cancelling coil (Lc) for cancelling out a direct current magnetization which is
caused by a direct current flowing through said band-pass filter coil (La1).
3. A compensation amplifier according to claim 2, characterized in that said band-pass
filter coil (La1) and said cancelling coil (Lc) are wound on a ferrite core (F) as
a bifilar winding.
4. A compensation amplifier according to claim 2, characterized in that said band-pass
filter coil (La1) and said cancelling coil (Lc) are wound on a ferrite core (F) separately.
5. A compensation amplifier according to claim 3 or 4, characterized in that said
ferrite core (F) is a toroidal core with an air gap.
6. A compensation amplifier according to claim 2, characterized in that said band-pass
filter coil (La1) and said cancelling coil (Lc) are connected to each other via a
coupling capacitor (Co).
7. A compensation amplifier according to claim 1, characterized in that said band-pass
filter circuit is a circuit for passing a frequency-modulation radio signal and characterized
in that said band-pass filter circuit and said compensation circuit means form a double
tuning circuit in which said band-pass filter circuit is a primary tuning circuit
and said compensation circuit means is a secondary tuning circuit made up of a coil
(Lf2) and a capacitor (cf).
8. A compensation amplifier according to claim 1, characterized in that said band-pass
filter circuit is a circuit for passing an amplitude-modulation radio signal and characterized
in that said band-pass filter circuit and said compensation circuit means form a double
tuning circuit in which said band-pass filter circuit and a tuning capacitor (Cb)
form a high-pass filter as a primary tuning circuit and said compensation circuit
means is a low-pass filter made up of a coil (La2) and a capacitor (Ca) as a secondary
tuning circuit.