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| United States Patent |
4,442,434
|
|
Baekgaard
|
April 10, 1984
|
Antenna circuit of the negative impedance type
Abstract
An aerial circuit comprises a first antenna in connection with an
impedance, which is a "negative impedance", whereby a broad frequency
range is achieved. The "negative impedance" is based on a second antenna
similar to the first antenna, whereby the resulting aerial impedance is
zero for all frequencies. A corresponding ferrite or loop aerial system is
advantageously usable as a stationary system instead of an antenna of the
long wire type, and the signal to noise ratio is better than with
conventional ferrite antennas.
| Inventors:
|
Baekgaard; Knud E. (Struer, DK)
|
| Assignee:
|
Bang & Olufsen A/S (Struer, DK)
|
| Appl. No.:
|
243623 |
| Filed:
|
March 13, 1981 |
Foreign Application Priority Data
| Current U.S. Class: |
343/701; 343/788; 455/274; 455/291 |
| Intern'l Class: |
H01Q 001/26; H01Q 007/06 |
| Field of Search: |
343/701,748,788,861
455/274,291
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Lieberman; Eli
Attorney, Agent or Firm: Antonelli, Terry & Wands
Claims
What is claimed is:
1. An aerial circuit comprising a main antenna and a negative impedance
means for producing a broad effective frequency range of the aerial
circuit the negative impedance means comprising an amplifier and a
frequency dependent impedance imitating an impedance of the main antenna
and seeking to eliminate the same through the amplifier, characterized in
that the frequency dependent impedance is constituted by an auxiliary
antenna of a similar type as the main antenna.
2. An aerial circuit according to claim 1, characterized in that the
auxiliary antenna is of the same type as the main antenna.
3. An aerial circuit according to claim 2, characterized in that the main
antenna is a ferrite type with a core of ferrite or a corresponding
material for reception of at least one of magnetic or electromagnetic wave
signals, the auxiliary antenna is of the same type, and in that the
frequency dependent impedance is a negative coil.
4. An aerial circuit according to claim 3, characterized in being mounted
as a stationary system with respective coils of the two antennas
inductively insulated from each other.
5. An aerial circuit according to claim 2, characterized in that the main
antenna is a frame type antenna for receiving at least one magnetic or
electromagnetic wave signals, the auxiliary antenna is of the same type,
and in that the frequency dependent impedance is a negative coil.
6. An aerial circuit comprising a first antenna means and a second antenna
means independent of and electrically connected to said first antenna
means, said second antenna means including a frequency dependent impedance
means for imitating an impedance of the first antenna means and an
amplifier means electrically connected to a common point between the first
and second antenna means, the frequency dependent impedance is constructed
as a negative coil means, the first antenna means includes a ferrite rod
means having a core mounted thereon, the second antenna means includes a
separate ferrite rod means having a further coil mounted thereon, wherein
an impedance of the coils and amplification of the amplifier means
satisfies the following conditions:
A.ltoreq.(L.sub.1 /L.sub.2)+1,
wherein:
L.sub.1 equals impedance of the coil of the first antenna means,
L.sub.2 equals impedance of the coil of the second antenna means, and
A equals amplification of the amplifier means.
7. An aerial circuit according to claim 6, wherein the amplifier means
includes a single field effect transistor means.
8. An aerial circuit according to claim 6, wherein said first and second
antenna means are inductively isolated.
9. An aerial circuit comprising an antenna and a negative impedance means
for producing a broad effective frequency range of the aerial circuit, the
negative impedance means comprising an amplifier and a frequency dependent
impedance imitating an impedance of the antenna and seeking to eliminate
the same through the amplifier, characterized in that the frequency
dependent impedance is constituted by an auxiliary antenna of the same
type as the main antenna, the aerial circuit being mounted as a stationary
system with the two antennas being inductively isolated from each other.
Description
This invention relates to an aerial circuit.
Conventional antennas for radio receivers are connected in a resonant
circuit, which has an extremely small impedance in a frequency range about
the resonant frequency, whereby the received weak radio waves can produce
a reasonably strong aerial signal for amplification in a receiver. If the
receiver is provided with a variable tuning circuit, a selective
separation of signals of respective different frequencies corresponding to
different transmitter stations is possible. The aerial circuit is designed
so as to have a broad effective frequency range, whereby it is able to
receive a wide spectrum of signals from many different transmitters.
However, the gain of the antenna is considerably reduced for frequencies
near the limits of the frequency range, and it is common practice,
therefore, that the receiver has a selector switch for adaption of the
aerial circuit to different wave length ranges, e.g. long waves, medium
waves and one or more short wave ranges, with the selector connecting the
antenna with different impedances for a suitable change of the resonant
frequency and range of the aerial system.
Principally it would be attractive to make the aerial circuit effective
over a very broad range without selection between the different sub
ranges, and, in fact, a proposal to this end is known from the British
Patent Specification No. 1,231,022, which discloses a dipole antenna
connected with an impedance of the "negative impedance" type, i.e. an
electrical compensation circuit designed so as to imitate the frequency
response of the antenna itself. By means of an associated amplifier this
imitated impedance is added to the antenna impedance with an opposite
sign, such that the resulting impedance of the aerial circuit is ideally
zero at all frequencies. In practice, however, this ideal result is hardly
achievable, because inevitably some deviations will exist between the
frequency characteristics of the antenna impedance and the compensation
impedance, and even slight deviations will be significant enough to
produce a non-ideal result.
It is the primary purpose of the invention to provide an aerial circuit of
the "negative impedance" type, which may be designed to achieve a
practically ideal elimination of the resulting aerial impedance and thus a
good gain of the antenna for all frequencies over a very broad frequency
range.
According to the invention the antenna imitating impedance is constituted
by an auxiliary antenna of the same type as the primary antenna,
preferably entirely similar thereto. Thus, the "negative impedance" is
based on an antenna and not on a synthetic imitation circuit, whereby any
possible irregularity in the frequency characteristic of the primary
antenna will automatically be duplicated in the auxiliary antenna, as the
two antennas will respond entirely or widely in the same manner to any
frequency, and thereby a practically absolute elimination of the resulting
aerial impedance is achievable irrespective of the irregularities.
The invention furthermore recognizes the usability of the "negative
impedance" for widening the effective frequency range of an aerial system
is not limited to dipole antennas, as experiments have shown that a
corresponding principle may very advantageously be applied to ferrite
antennas or similar antennas responding to the magnetic aspect of the
electromagnetic radio waves, while the dipole antennas respond to the
electrical aspect or field thereof. With a ferrite antenna it is not
possible to use a "negative impedance" of the same type as suggested for
use with the dipole antennas, but a "negative coil" is perfectly usable,
and it will be particularly effective for the desired complete elimination
of the resulting aerial impedance when it is based on an antenna coil and
an associated magnetic core fully corresponding to the primary ferrite
antenna. Ideally two completely similar antennas should be mounted
adjacent each other without mutual inductive coupling, with the "negative"
impedance coupling between them being so effected that it is hardly
possible to distinguish between a primary and an auxiliary antenna, since
the total system will show two equal antennas and an intermediate,
electrically active coupling circuit.
Conventionally, the most effective type of antenna for radio waves over a
wide frequency range, at least in the AM-range, is the so-called "long
wire antenna", i.e. a long wire insulated from the ground. This type of
antenna, however, shows three main disadvantages, viz.: it can be
difficult to mount; It is not suited for portable receivers (though well
suited as a stationary antenna therefor); and it willingly receives not
only radio waves, but also electrical noise signals from many sources of
noise, e.g. sparks in car motors or electromotors in the neighborhood of
the antenna.
Ferrite antennas distinguish themselves by not showing these disadvantages,
while their disadvantage is that their efficiency is quite low. Since they
are compact and of small size they are easy to mount practically
everywhere, and they are well suited for use inside portable radio sets,
this being the predominating field of use of these antennas. It has been
found that the magnetic component of the radio waves, as compared to the
electrical component, is far less infestable with random noise from
phenomena in the near surroundings, and, even though the antenna gain is
much weaker than in a "long wire antenna", the signal may be subjected to
a very high degree of amplification without the received noise being
disturbing. In other words, the signal to noise ratio of a ferrite antenna
is far better than for a long wire antenna, and the efficiency of a
built-in ferrite antenna, therefore, is generally satisfactory. It only
remains that the built-in antenna is bound to follow the receiver, i.e.
the reception will be poor whenever the receiver is brought into
surroundings, e.g. inside a building of reinforced concrete, where the
radio waves are extra weak and often unreceivable.
In such surroundings, therefore, it will normally be desirable or necessary
that an outside antenna be mounted in a more favorable position, and
normally portable radios are correspondingly provided with a socket for an
antenna plug for connection to such a stationary antenna, whereby, of
course, the drawback should be accepted that the portable radio by such
connection will be movable to a limited degree only.
Under these circumstances, it would be highly desirable that the stationary
exterior antenna could be of the ferrite type, because such an antenna,
due to its small size, will be much easier to mount than a long wire
antenna. In practice, however, such use of a ferrite antenna has been
excluded by the fact that the effective frequency range of conventional
ferrite antennas has been too small for ordinary use, and, for various
reasons, it would be unattractive to arrange for a remote controlled
switching between different antenna coils on the basic magnetic core of
the antenna for adaption to different frequency ranges.
However, with the use of the above mentioned "negative coil" or "negative
inductance" it will be possible to achieve such a wide ranged elimination
of the resulting impedance of the ferrite antenna that the antenna will be
usable over the required broad frequency range without any need of remote
selection between sub ranges thereof. Consequently it will then be
possible in practice to replace an exterior long wire antenna by a ferrite
antenna which is indeed much easier to mount. It will even be possible to
make use of the simple type of antenna from which the ferrite antenna has
been developed, viz. the so-called frame antenna, which is an effective
and simple antenna, which just requires more space than the ferrite
antenna; however, for the stationary mounting, the extra space
requirements of a frame antenna will not normally be of any importance.
As well known, ferrite and frame antennas are directional, which is both
advantageous and disadvantageous according to the circumstances. In a
stationary installation, however, it is possible to mount the antenna or
set of antennas so as to suppress noise signals from certain directions,
and besides it is possible to use two antennas or antenna sets with
preferred directions or electrically coupled so as to be non-directional
in total.
For achieving a broad frequency range and a good antenna gain, it may be
desirable to use a high number of windings on the antenna coil or coils,
and many windings condition a high so-called parallel capacity of the
coil, which is undesired in conventional aerial circuits. However, with
the use of a "negative coil", according to the invention, even the
parallel capacity will be compensated for or eliminated, and the
invention, therefore, makes it possible to increase the number of windings
compared with the known art, whereby the efficiency of the aerial system
may be still further increased.
With the use of a parallel or auxiliary ferrite or frame antenna in the
negative impedance circuit the following additional advantage is obtained:
The received wave signal will occur in both antennas, and, because the
signals are well oriented they may be simply added to each other, while
the unavoidable self generated noise in the two antennas or the coils
thereof will be of random orientation, whereby the corresponding sum of
the noise signals from the two antennas will not amount to a doubling of
the signal, but only to an increase by the factor .sqroot.2, i.e. the
signal to noise ratio will be .sqroot.2 times better than in conventional
antenna circuits, this being a significant advantage of the invention.
FIG. 1 is a schematic diagram of a first embodiment of an aerial circuit
constructed in accordance with the present invention; and
FIG. 2 is a schematic diagram of another embodiment of an aerial circuit
constructed in accordance with the present invention.
On a first ferrite rod is mounted a coil L.sub.1, and on a separate ferrite
rod is mounted another coil L.sub.2, the coils being entirely or
substantially similar and connected in series in an aerial circuit having
terminals T.sub.1 and T.sub.2. Between the terminal T.sub.1 and the common
point P of the two coils is mounted an amplifier F, to which power is
supplied in any suitable manner, e.g. through the connection wires between
the terminals T.sub.1, T.sub.2 and the associated radio receiver (not
shown). It is also possible, as shown in FIG. 2, for the aerial circuit to
be of the frame (loop) type and to be provided without a core of ferrite
or a corresponding material. If or when the inductance of the two coils
L.sub.1, L.sub.2 are identical the amplifier F should show the
amplification factor of two. It receives its input from the coil L.sub.2
and supplies the amplified output to the coil L.sub.1 in such a manner
that the coil L.sub.1 receives a voltage which is equal to, but also
opposed to the voltage simultaneously being generated in the coil L.sub.1,
just as in L.sub.2. The coils L.sub.1, L.sub.2 can be equated with voltage
generators, and the total "impedance" of the series connected coils and
the amplifier, as seen from these voltage generators, will thus be zero
for any frequency, and consequently, the full effect of the aerial circuit
is at disposal for all frequencies.
Even though the resulting impedance of the coil system is zero, there will
nevertheless exist, vectorially, a voltage between the ends of the system,
viz. twice the induced voltage, and this signal is applied to the
selective tuning circuit of the radio receiver through wires which may
well be quite long. The two coils L.sub.1, L.sub.2 are mutually
"negative", but the induced energy in both coils L.sub.1, L.sub.2 will be
added to the available output signal. As mentioned, the vectorially
co-directional signals of the two coils L.sub.1, L.sub.2 will be added to
the double value (gain increase 6 dB), because they are correlated, while
self generated non-correlated noise signals are only added to .sqroot.2 of
their value, i.e. the noise increase will be 3 dB only, such that the
resulting signal to noise ratio will be considerably improved.
In practice the amplifier F may consist of a single field effect transistor
having its gate connected to the point P, its drain connected to the power
supply and its source connected to the midpoint of an auto transformer
connected between the opposite ends of the coil system, preferably an auto
transformer having a so-called toroid ring core.
The impedance of the coil L.sub.1 may be chosen so as to be different from
L.sub.2, and the amplification A in amplifier F should then just satisfy
the condition:
A.ltoreq.(L.sub.1 /L.sub.2)+1.
As mentioned, it will be highly advantageous to use two practically similar
coils L.sub.1, L.sub.2, but principally it would be possible to produce a
"negative coil" by other circuits, e.g. a condenser in connection with a
so-called negative impedance converter.
It will be appreciated that an antenna system according to the invention
may have a very broad effective frequency range, though, of course,
certain limits will exist for practical reasons. It would, of course,
still be possible to arrange for a switch over arrangement should it be
desired to cover a still broader range. Alternatively two sets of antenna
circuits may be used and connected to the receiver through separate wires,
whereby the switch over may be effected from the radio receiver.
As mentioned, two or several antenna circuits may be used together to
produce a non-directional antenna system, and the invention will be
perfectly usable in large systems e.g. commercial or common antenna
systems, in which a "battery" of effective ferrite or frame antennas will
show small space requirements.
The invention is not limited to any particular type or types of antennas,
not even to radio receiver antennas. Thus, a ferrite or frame antenna
system according to the invention will be usable with advantage in
receivers in so-called "teleloop" systems, which are based solely on
magnetic induction between a large loop and the receivers inside the loop
and, in that case, there is usually no need to consider any particularly
broad frequency range, as mostly only audio frequencies are in question,
but already the improved signal to noise ratio of the antenna circuit will
make it advantageous in use. Other fields of application will be tone
heads in tape recorders, air coils in pick up's and coils for short range
wireless data transmission. The principles of the invention may even apply
to some transmitter antennas, in which the system of the invention may
contribute to the attainment of a high antenna current.
* * * * *
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