Antenna circuit

An antenna circuit includes a ferrite core around which a main coil and a negative feedback coil are wound, and a field effect transistor (FET) with its source grounded, and with a voltage induced in the main coil being applied across the gate and the source of the FET to thereby form an aperiodic circuit and the drain of the FET being connected through the negative feedback coil to a load.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to antenna circuits, and more particularly 
is directed to an improved antenna circuit of the kind including a 
so-called bar-type antenna. 
2. Description of the Prior Art 
An antenna circuit is known in the prior art for use in an AM receiver and 
employs a so-called bar-type antenna constituted by a coil wound around a 
ferrite core. In such antenna circuit, a field effect transistor (FET) has 
its source connected to ground, and the coil of the bar-type antenna is 
connected between the gate of the FET and ground in parallel with a 
variable condenser. The drain of the FET is connected through a load to a 
voltage supply source and also to a mixer circuit which further receives a 
local oscillating signal to provide, as its output, an intermediate 
frequency signal. However, in such tuned antenna circuit, the inductance 
of the bar-type antenna has to be adjusted by varying the position of the 
coil relative to the ferrite core prior to being fixed to the latter. 
Therefore, the core cannot be directly wound around the ferrite core, but 
rather has to be wound on a bobbin which is at least initially movable 
along the core. Further, in order to increase the band width over which 
tuning can be effected, the Q factor has to be lowered which reduces the 
sensitivity of the antenna circuit. Further, in the described antenna 
circuit according to the prior art, phase rotation occurs with a change in 
the tuning frequency. Moreover, since the described antenna circuit 
employs a variable condenser for effecting tuning, any difference which is 
likely to occur between the characteristics of the variable condenser of 
the antenna circuit and the variable condenser of the local oscillator 
circuit results in a tracking error which lowers the sensitivity. In a 
synthesizer type receiver which uses a variable capacitor instead of the 
variable condenser in the local oscillator circuit, such use of a variable 
capacitor tends to increase the tracking error. 
In order to avoid the above mentioned defects of the first described 
antenna circuit according to the prior art, it has been proposed to 
provide an antenna circuit of an aperiodic type which omits the previously 
described variable condenser. However, in practice, due to the stray 
capacity of the bar-type antenna and the circuit wiring, the input 
capacity of the FET and the inductance of the bar-type antenna, resonance 
occurs in the antenna circuit. In other words, the antenna circuit has a 
frequency characteristic with a peak at its resonance frequency. By reason 
of the foregoing, the sensitivity of the antenna circuit varies 
considerably in accordance with the receiving frequency. If signals, such 
as, a broadcast wave and the like exist in proximity to the resonance 
frequency, the multi-signal disturbance characteristic of the circuit is 
deteriorated. Further, since the gain of the antenna circuit is high at 
the peak portion of its frequency characteristic, there is a tendency for 
a parasitic oscillation to occur due to positive feedback. 
For the foregoing reasons, in the known antenna circuit of the aperiodic 
type, it is necessary that the resonance frequency be provided outside the 
receiving band and that a resistor be provided to damp the peak of the 
frequency characteristic. However, the foregoing measures lead to other 
problems. For example, if the resonance frequency is moved outside the 
receiving band at the high frequency end of the latter, the inductance of 
the bar-type antenna has to be decreased and this, in turn, lowers the 
voltage induced in the coil of the bar-type antenna so that the 
sensitivity thereof is deteriorated. On the other hand, if the resonance 
frequency is shifted to the lower side of the receiving band, the bar-type 
antenna becomes capacitive within the receiving band so that the induced 
voltage is divided by the input capacity of the FET, which again 
deteriorates the sensitivity. Further, the resistor added to the circuit 
for damping the peak of the frequency characteristic is a source of noise 
so that the amount of damping that can be achieved by the added resistor 
is limited and, as a result thereof, the frequency characteristic cannot 
be sufficiently flattened. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of this invention to provide an antenna 
circuit that avoids the above-described problems associated with the prior 
art. 
More specifically, it is an object of this invention to provide an antenna 
circuit with a flattened frequency characteristic. 
Another object of this invention is to provide an antenna circuit with 
substantially uniform sensitivity throughout the receiving band. 
Still another object of this invention is to provide an antenna circuit, as 
aforesaid, with an improved multi-signal disturbance characteristic. 
Still another object of this invention is to provide an antenna circuit, as 
aforesaid, in which parasitic oscillation is suppressed by a negative 
feedback, thereby to stabilize the reception state. 
A further object of this invention is to provide an antenna circuit having 
a bar-type antenna comprised of a coil wound on a ferrite core, and in 
which the number of windings of the coil can be freely increased for 
increasing the induced voltage therein and thereby improving sensitivity 
of the antenna. 
A still further object of this invention is to provide an antenna circuit, 
as aforesaid, with an improved signal-to-noise (S/N) ratio. 
Yet another object of this invention is to provide an antenna circuit, as 
aforesaid, in which the gain can be freely determined by varying the 
amount of negative feedback so that the freedom in designing the circuit 
can be increased. 
In accordance with an aspect of this invention, an antenna circuit 
comprises a ferrite core, a field effect transistor having a grounded 
source, a gate and a drain, a main coil and a negative feedback coil wound 
around the core, and a load, with the main coil being connected to the 
field effect transistor so that a voltage induced in the main coil is 
applied across the gate and source of the field effect transistor to 
thereby form an aperiodic circuit, and with the opposite ends of the 
negative feedback coil being connected to the drain and load, 
respectively. 
In a preferred embodiment of the invention, the load is constituted by a 
transformer, for example, an interstage transformer, which has a 
self-resonance providing a frequency characteristic with a first peak, 
while the negative feedback coil has a negative feedback frequency 
characteristic with a second peak, and the first and second peaks are 
arranged at frequencies displaced from each other to provide a 
substantially flat frequency characteristic curve for the circuit as a 
whole. 
Further, in accordance with this invention, an antenna circuit, as 
aforesaid, is desirably provided with a resistor connected to the source 
of the field effect transistor for avoiding electrostatic breakdown of the 
latter. 
The above, and other objects, features and advantages of this invention, 
will become apparent from the following detailed description of a 
preferred embodiment which is to be read in conjunction with the 
accompanying drawings wherein the same reference numerals are employed for 
identifying corresponding elements and components.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In order that the present invention and its advantages may be fully 
understood, an antenna circuit 10 used in an AM receiver according to the 
prior art and the problems associated therewith will first be described in 
detail with reference to FIG. 1. In this respect, it will be seen that 
antenna circuit 10 generally comprises a so-called bar-type antenna 1 
having a ferrite core and a coil wound therearound, a variable condenser 
2, and a field effect transistor (FET) 3. FET 3 has its source connected 
to ground, while the gate is connected to one end of the coil of bar-type 
antenna 1 which has its other end connected to ground. Condenser 2 is 
connected in parallel with the coil of antenna 1, and the drain of FET 3 
is connected through a load (shown as a resistor) to a voltage supply 
source +V.sub.DD, and also to an input of a mixer circuit 4. Mixer circuit 
4 also receives a local oscillation signal from a local oscillator circuit 
5 so as to provide an intermediate frequency signal IF. 
As earlier noted, in the tuned-type antenna circuit 10, the inductance of 
bar-type antenna 1 has to be adjusted by displacement of the coil along 
the ferrite core. Therefore, the coil cannot be directly wound upon the 
ferrite core, but rather has to be wound on a bobbin which is, in turn, 
movable along the ferrite core of antenna 1. Further, in antenna circuit 
10, the Q factor has to be decreased in order to widen the band width for 
tuning, and that tends to decrease the sensitivity of the antenna circuit. 
Furthermore, phase rotation occurs upon changing of the tuning frequency. 
Moreover, by reason of the presence of variable condenser 2 for effecting 
tuning, the inevitable difference between the characteristics of variable 
condenser 2 of antenna circuit 10 and a variable condenser included in 
local oscillator circuit 5 causes tracking error with the result that the 
sensitivity of antenna circuit 10 is lowered. If a synthesizer type 
receiver is used so that a variable capacitor can be substituted for the 
variable condenser of circuit 5, such variable capacitor tends only to 
increase the tracking error. 
Referring now to FIG. 2, it will be seen that, in an antenna circuit 20 of 
aperiodic type proposed in the prior art for avoiding the problems 
associate with antenna circuit 10, variable condenser 2 of circuit 10 is 
omitted and the remaining elements of antenna circuit 20 which correspond 
to those described above with reference to FIG. 1 are identified by the 
same reference numerals and will not be further described. However, in 
practice, due to the stray capacity of bar-type antenna 1 and the wiring 
therefor, the input capacity C.sub.iss of FET 3 and the inductance of 
bar-type antenna 1, antenna circuit 20 is subject to resonance. Thus, as 
indicated by the curve 11 appearing in full lines on FIG. 3, the frequency 
characteristic of antenna circuit 20 has a substantial peak at its 
resonance frequency f.sub.o. By reason of the peak in the frequency 
characteristic of antenna circuit 20 appearing at the resonance frequency 
f.sub.o, the sensitivity of the antenna circuit is rapidly increased at or 
near the resonance frequency so that the sensitivity varies substantially 
with changes in the receiving frequency. Further, if signals, such as, a 
broadcast wave and the like, exist in proximity to the resonance frequency 
f.sub.o, there is deterioration of the multi-signal disturbance 
characteristic. Moreover, since the gain of the antenna circuit 20 is high 
at the peak of its frequency characteristic, there is a tendency for 
parasitic oscillation to occur due to positive feedback. 
In view of the above-mentioned problems associated with antenna circuit 20 
of the aperiodic type, it is necessary to select the resonance frequency 
f.sub.o to be outside the receiving band. Further, as shown, a resistor 6 
is connected to ground in parallel with the coil of bar-type antenna 1 for 
damping the peak of the frequency characteristic, as indicated by the 
curve shown in broken lines at 12 on FIG. 3. By way of example, if the 
receiving band is a middle waveband from 531 kHz to 1611 kHz, the 
resonance frequency f.sub.o is selected to be in the range from 1.8 MHz to 
2.0 MHz, and the resistance value of resistor 6 is selected to be 560 
k.OMEGA., thereby to damp the peak of the frequency characteristic. 
However, if this arrangement is employed with the resonance frequency 
f.sub.o being selected to be outside the receiving band, for example, at 
the higher side thereof, the inductance of bar-type antenna 1 is decreased 
and, as a result thereof, the induced voltage of the bar-type antenna is 
lowered so as to deteriorate the sensitivity of the antenna circuit. On 
the other hand, if the resonance frequency f.sub.o is moved to the lower 
side of the receiving band, bar-type antenna becomes capacitive in the 
receiving band so that the voltage induced in the coil of antenna 1 is 
divided by the input capacity C.sub.iss of FET 3 for again deteriorating 
the sensitivity of the circuit. Furthermore, damping resistor 6 is a 
source of noise and, if the amount of damping is kept low in order to 
minimize the noise, the desired flatness of the frequency characteristic 
cannot be achieved. 
Referring now to FIG. 4, in which the parts corresponding to those 
previously described are identified by the same reference numerals, it 
will be seen that, in an antenna circuit 30 according to an embodiment of 
the present invention, the bar-type antenna 1' includes a ferrite core 1a 
(FIG. 7A) on which a main coil L.sub.1 and a negative feedback coil 
L.sub.2 are both wound. A hot side end t.sub.2 of main coil L.sub.1 is 
connected to the gate of FET 3, while the cold side end t.sub.1 of main 
coil L.sub.1 and the source of FET 3 are connected to ground. In the 
illustrated embodiment, the hot side end t.sub.2 of main coil L.sub.1 is 
the end at which the winding of that coil on core 1a is started. 
As shown on FIG. 4, one end t.sub.3 of negative feedback coil L.sub.2 is 
connected to the drain of FET 3, and the winding of negative feedback coil 
L.sub.2 on ferrite core 1a begins at the end t.sub.3 of that coil. The 
other end t.sub.4 of coil L.sub.2 is connected through a load to a voltage 
supply source terminal 8. In the embodiment shown on FIG. 4, the load 
through which negative feedback coil L.sub.2 is connected to the voltage 
supply source is constituted by an input coil L.sub.3 of an interstage 
transformer 7 which has its output coil L.sub.4 connected to mixer circuit 
4. 
Further, in the embodiment of the invention shown on FIG. 4, the local 
oscillator circuit 5 includes a phase locked loop (not shown) so as to 
provide a channel selector of the synthesizer type. A channel-selection 
control device 17, which may be constituted by a single-chip type, 4-bit 
micro-computer produced by Nippon Electric Company, Ltd., under the 
designation .mu.PD-7503, is connected with local oscillator circuit 5 and 
is operated in response to actuation of a keyboard 18. 
In the antenna circuit 30 in accordance with this invention, the voltage 
induced in main coil L.sub.1 is amplified by FET 3 and converted to a 
drain current of the FET which flows in negative feedback coil L.sub.2 and 
in input coil L.sub.3 of transformer 7, whereby the voltage of the 
received signal is supplied to mixer circuit 4 and converted, in the 
latter, to an intermediate frequency signal IF. Since the drain current of 
FET 3 flows in coil L.sub.2, a negative feedback is applied to the input 
side of FET 3, and the amount of such negative feedback can be freely 
determined by suitably selecting the coupling coefficient or winding ratio 
of coils L.sub.1 and L.sub.2. Further, since the negative feedback is 
applied in respect to the current flowing in coil L.sub.2, no phase 
displacement will occur. 
Accordingly, even if the frequency characteristic of antenna circuit 30 in 
the absence of the negative feedback has a peak occurring at the resonance 
frequency f.sub.o, as represented by the curve 11 shown in dotted lines on 
FIG. 5, the application of the described negative feedback modifies the 
frequency characteristic to one that is substantially flat, as indicated 
by the curve 13 in solid lines on FIG. 5, and which has only a relatively 
small peak or rise 13a at the resonance frequency. By reason of the 
foregoing, the antenna circuit 30 according to this invention has a 
substantially uniform sensitivity over the entire receiving band, and the 
multi-signal disturbance characteristic of the antenna circuit can be 
improved. Further, since the negative feedback provided in antenna circuit 
30 substantially suppresses parasitic oscillation, stable reception is 
ensured. Moreover, even when the resonance frequency f.sub.o of the 
antenna circuit according to this invention is located within the 
receiving band, the substantially flat frequency characteristic that is 
obtained makes it possible to freely select the number of windings in main 
coil L.sub.1. Thus, a large number of windings can be provided in main 
coil L.sub.1 for increasing the induced voltage therein and thereby 
improving the sensitivity of the antenna circuit. 
Generally, FETs having a large mutual conductance gm are characterized by a 
small noise factor NF and a large input capacity C.sub.iss. However, since 
the antenna circuit according to the present invention is not troubled by 
the presence of its resonance frequency within the receiving band, the FET 
3 may be one which has a large mutual conductance gm and a correspondingly 
small noise factor NF for improving the signal-to-noise (S/N) ratio. 
Further, since the damping resistor 6 of the prior art antenna circuit 20 
shown in FIG. 2 is omitted from the antenna circuit 30 embodying the 
present invention, the S/N ratio is further improved by the absence of 
this noise source. Moreover, in the antenna circuit embodying this 
invention, the gain can be freely determined by controlling the amount of 
negative feedback with the result that substantial freedom is afforded in 
designing the antenna circuit. 
If the input coil L.sub.3 of transformer 7 is used as the load in antenna 
circuit 30, the peak 13a appearing on the frequency characteristic curve 
13 of antenna circuit 30 at resonance frequency f.sub.o, and which is made 
very small by the negative feedback, may be amplified by transformer 7 so 
as to appear as a relatively more conspicuous peak, as at 13'a on FIG. 6. 
Therefore, in accordance with this invention, the frequency characteristic 
curve 15 of transformer 7 which has a peak portion 15a due to the stray 
capacity and self-resonance of the transformer is selected to dispose the 
peak portion 15a at the low frequency side of the receiving band. If the 
peak 15a due to self-resonance of transformer 7 is relatively large, the 
amount of negative feedback may be reduced by decreasing the number of 
windings of negative feedback coil L.sub.2 so that the level of the peak 
15a of frequency characteristic curve 15 of transformer 7 is generally 
matched by the level of peak 13'a of the frequency characteristic curve 
13', with the result that the overall frequency characteristic can be 
represented by a substantially flat frequency characteristic curve of 
relatively high level, at least between the frequencies corresponding to 
the peaks 15a and 13'a. When the foregoing relationship is realized, the 
gain of the antenna circuit can be substantially increased. 
Referring again to FIG. 4, it will be seen that, in the illustrated antenna 
circuit 30 according to this invention, a resistor 19, for example, having 
a resistance value of 10 to 20.OMEGA., is connected to the source of FET 3 
for protecting the latter from damage due to electrostatic breakdown. 
Referring now to FIGS. 7A and 7B, it is noted that main coil L.sub.1 is 
desirably formed by directly winding and binding around core 1 a suitable 
number of turns, for example, 120 turns, of a conductor constituted by 10 
wires each having a diameter of 0.07 mm, and which are bundled and twisted 
together and covered with an insulating layer on the outer peripheral 
surface. The negative feedback coil L.sub.2 is formed by similarly 
directly winding 10 turns around ferrite core 1 of a conductor which is 
constituted by bundling and twisting 4 wires similar to the wires employed 
in forming coil L.sub.1. The negative feedback coil L.sub.2 is wound on 
core 1 at a predetermined distance from main coil L.sub.1 which, as shown 
on FIG. 7A, is substantially centered in respect to the middle of core 1 
identified by the line o--o'. When the main coil L.sub.1 is thus centered 
in respect to the length of core 1, the voltage induced in main coil 
L.sub.1 is maximized and thereby provides excellent sensitivity for the 
antenna circuit. 
Although the antenna circuit 30 according to this invention has been 
described above in association with a local oscillator circuit 5 of the 
PLL synthesizer type, it will be understood that the local oscillator 
circuit 5 may employ a variable condenser for effecting the tuning and, in 
that case, the channel-selection control device 17 and the keyboard 18 of 
FIG. 4 can be omitted. Further, a resistor (not shown) can be used as the 
load in place of the input coil or winding L.sub.3 of transformer 7. 
Having described in detail an illustrative embodiment of the invention and 
a few modifications thereof, it is to be understood that the invention is 
not limited to the foregoing, and that various changes and further 
modifications may be effected therein by one skilled in the art without 
departing from the scope or spirit of the invention as defined in the 
appended claims.