U.S. patent number 3,918,063 [Application Number 05/479,101] was granted by the patent office on 1975-11-04 for antenna having combined elements for vhf and uhf signals.
This patent grant is currently assigned to Sony Corporation. Invention is credited to Kosuke Akiba, Toshitada Doi, Koya Nakamichi.
United States Patent |
3,918,063 |
Nakamichi , et al. |
November 4, 1975 |
Antenna having combined elements for VHF and UHF signals
Abstract
A composite antenna apparatus having a combination of a first
element for a VHF signal formed in a modified loaded loop antenna
and a second element for a UHF signal formed in a modified arraying
antenna, both of which are disposed on their respective supports
rotatable independently and compactly arranged.
Inventors: |
Nakamichi; Koya (Chofu,
JA), Doi; Toshitada (Yokohama, JA), Akiba;
Kosuke (Tama, JA) |
Assignee: |
Sony Corporation (Tokyo,
JA)
|
Family
ID: |
13345061 |
Appl.
No.: |
05/479,101 |
Filed: |
June 13, 1974 |
Foreign Application Priority Data
|
|
|
|
|
Jun 15, 1973 [JA] |
|
|
48-67443 |
|
Current U.S.
Class: |
343/722; 343/726;
343/744; 343/758 |
Current CPC
Class: |
H01Q
5/00 (20130101) |
Current International
Class: |
H01Q
5/00 (20060101); H01Q 5/02 (20060101); H01Q
003/02 (); H01Q 021/30 () |
Field of
Search: |
;343/725,726,727,728,744,722,758 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lieberman; Eli
Attorney, Agent or Firm: Eslinger, Esq.; Lewis H.
Sinderbrand, Esq.; Alvin
Claims
We claim as our invention:
1. Antenna apparatus having combined elements for receiving VHF and
UHF signals, comprising:
a VHF antenna member disposed in a loop formed of plural, spaced
apart conductive plates, a first pair of adjacent plates being
connected to feeding terminals, a second pair of adjacent plates
opposite said first pair of plates being interconnected by a load
impedance, and at least third and fourth pairs of adjacent plates
being interconnected, respectively, by at least first and second
respective UHF signal trap means, each of said trap means
comprising a parallel resonant circuit for presenting maximum
impedance to received UHF signals;
a UHF antenna member spaced from said VHF antenna member and
extending within a circular area having a diameter less than the
diameter of said VHF antenna loop, said UHF antenna member
including a wave-guide conducting plate, a radiator conducting
plate and a reflector conducting plate, and having optimum
directivity in a direction substantially normal to the plane of
said VHF antenna loop;
first rotatable support means mechanically coupled to said VHF
antenna member for rotating same in the plane of said loop; and
second rotatable support means coaxial with said first rotatable
support means and mechanically coupled to said UHF antenna member
for rotating same in a plane parallel to said loop.
2. The antenna apparatus of claim 1 wherein said first rotatable
support means comprises a ring-shaped body to which said spaced
apart conductive plates are coupled; said first and second
rotatable support means being rotatable independently of each
other.
3. The antenna apparatus of claim 2 wherein said second rotatable
support means comprises a spherical-shaped body to which said
wave-guide, radiator and reflector conducting plates are
coupled.
4. The antenna apparatus of claim 2 wherein said VHF antenna member
further includes at least one conductive rod coupled to one of said
conductive plates by a further UHF signal trap comprising a
parallel resonant circuit for presenting maximum impedance to
received UHF signals.
5. The antenna apparatus of claim 4 wherein two conductive rods are
coupled to two of said conductive plates, each of said conductive
rods comprising a main antenna portion and a sub-antenna portion
slidably mounted on said main antenna portion by a slidable
insulating member, an end portion of said sub-antenna portion being
wound about said insulating member to form a parallel resonant trap
circuit for presenting a higher impedance to VHF signals above a
predetermined frequency.
6. The antenna apparatus of claim 5 wherein said VHF antenna member
is formed of at least eight conductive plates having at least four
UHF signal trap means coupled thereto.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to an antenna apparatus having
both elements for a VHF signal and a UHF signal, and more
particularly to a compact composite antenna apparatus comprising a
combination of a first element for the VHF signal and a second
element for the UHF signal arranged compactly without mutual
interference.
2. Description of the Prior Art
In the field of television broadcasting, there have generally been
two different systems in a view point of a signal used for
transmitting informations, one of which uses a VHF (very high
frequency) signal and the other of which uses a UHF (ultra-high
frequency) signal, for the transmission. Accordingly, it is
necessary for receiving both types of transmitted television
signals to provide two different kinds of antennae for receiving
the VHF signal and for receiving the UHF signal, respectively.
Generally, it is annoying for people who want to enjoy watching
television programs on their television receivers to have two
separate antennae for receiving the VHF signal and the UHF signal.
Further, these antennae separately provided require relatively
large space. To avoid these troubles, the appearance of a compact
composite antenna having both elements for the VHF signal and the
UHF signal has been desired.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a novel
composite antenna apparatus having both elements for a VHF signal
and a UHF signal arranged compactly.
Another object of the present invention is to provide a compact
composite antenna apparatus suitable for receiving both VHF and UHF
television signals.
The other objects, features and advantages of the present invention
will become apparent from the following description taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a front view showing an embodiment of the antenna
apparatus according to the present invention;
FIG. 2 is a partially cross-sectional view on the line I--I in FIG.
1;
FIG. 3 is a plane view of the antenna apparatus shown in FIG.
1;
FIG. 4 is a schematic diagram showing an electric construction of a
VHF antenna according to the invention;
FIG. 5 is a cross-sectional view on the line II--II in FIG. 1;
FIG. 6 is a cross-sectional view on the line III--III in FIG.
3;
FIG. 7 is a front view showing a main part of a rod antenna for
receiving two different signals according to the invention;
FIG. 8 is an equivalent trap circuit of the rod antenna shown in
FIG. 7;
FIGS. 9A and 9B are schematic diagrams used for explaining the used
state of the antenna shown in FIG. 7;
FIG. 10 is a graph showing the characteristics of the equivalent
trap circuit;
FIG. 11 is a characteristic graph showing the gain and front to
back ratio of a UHF antenna;
FIG. 12 is a SMith chart showing the characteristics of the UHF
antenna;
FIGS. 13A and 13B are graphs showing the gain of a VHF antenna
provided with a rod antenna;
FIGS. 14 and 15 are, respectively, graphs showing directional
patterns of the VHF antenna;
FIG. 16 is a graph showing the gain and front to back ratio of the
UHF antenna; and
FIG. 17 is a graph showing the directivity of the UHF antenna.
DESCRIPTION OF THE PREFERRED EMBODIMENT
An antenna apparatus according to the present invention will be
hereinbelow described with reference to the drawings. In order to
better understand the invention, a general description will be now
given on the antenna apparatus.
In FIG. 1, reference letter T shows an antenna apparatus according
to the present invention as a whole. The antenna apparatus T
comprises an antenna element 10 for a VHF signal which is arranged
on a substantially horizontal plane and an antenna element 20 for a
UHF signal which is arranged relatively close to the antenna
element 10 and at a position above the antenna element 10, and also
support 30 for holding the antenna elements 10 and 20 at
predetermined positions, respectively. In making the arrangement of
the antenna elements 10 and 20, which are different in frequency
band to be received, compact with superior function, it is
necessary to eliminate mutual interference between the antenna
elements 10 and 20 for using them independently. That is, as well
knowm, in the case where both the antenna elements 10 and 20 are
used independently, if the antenna element 20 for the UHF signal
were disposed ahead with respect to the coming direction of
broadcasting electric signals and the antenna element 10 for the
VHF signal were disposed in the same horizontal plane as that of
the antenna element 20 and behind the same, or the antenna elements
10 and 20 were arranged in the vertical direction with a relatively
large distance therebetween, influences or mutual interference
between the antenna elements 10 and 20, especially the influence of
the VHF antenna element 10 on the UHF antenna element 20 would pose
no serious problem.
However, in such a case the antenna elements 10 and 20 would occupy
a large space and hence they would not be preferred as a room
antenna apparatus. Accordingly, it is necessary for a practical
antenna apparatus provided with both UHF and VHF antenna elements,
that the deterioration of antenna characteristics due to bad
influences or mutual interference between the UHF and VHF antenna
elements must be avoided at first, and that the antenna apparatus
is small in size or compact.
The above problem is solved by the construction described
hereinafter. Firstly, the structures of the antenna elements 10 and
20 for the VHF and UHF signals will now be described.
The VHF antenna element 10 is divided into a plurality of, for
example eight conductive elements 10a to 10h as shown in FIG. 4.
Selected ends of the conductive elements 10a to 10h are connected
by trap circuits 1A to 1D for the UHF signal and a load 7,
respectively, to form a loaded loop antenna as a whole. The trap
circuits 1A to 1D are formed to have such characteristics that they
are low in impedance for a received frequency of the VHF signal,
but high in impedance for that of the UHF signal, or such
characteristics that they resonate in the received frequency band
of the UHF signal. In practical embodiment, each of the trap
circuits 1A to 1D is formed to be a parallel resonant circuit
consisting of a capacitor 2 and a coil 3. If free ends of the
conductive elements 10a and 10h are selected as feeding terminals
4a and 4b as shown in FIG. 4, the trap circuits 1A to 1D are
connected between the ends of the divided conductive elements shown
in FIG. 4. In the embodiment shown in FIG. 4 rod antennae 5A and 5B
are attached to the VHF antenna element 10, so that trap circuits
6A and 6B are connected between the rod antennaes 5A, 5B and the
conductive elements 10b, 10g, respectively, for trapping the UHF
signal. For this reason, one ends of the conductive elements 10c
and 10f adjacent to the conductive elements 10b and 10g are
opened.
As mentioned above, the VHF antenna element or member 10 is a loop
antenna which consists of eight-divided conductive elements 10a to
10g. Its various characteristics and antenna characteristics will
be described later together with those of the UHF antenna element
or member 20.
The VHF antenna member 10 is fixedly arranged on the horizontal
plane as shown in the figure and the UHF antenna member 20 is
disposed on the plane above the VHF antenna member 10. As may be
seen in FIG. 3, the UHF antenna member 20 consists of three
conductive elements. The embodiment shown in FIG. 3 is intended for
expanding a frequency band to be received. To this end, the UHF
antenna member 20, which consists of a wave-guide 20A, a radiator
20B and reflector 20C arranged in this order with respect to the
incoming direction of the broadcasting wave shown by an arrow a in
FIG. 3, is not a well known Yagi antenna consisting of rod-shaped
conductive elements, but consists of conductive plate elements of
desired shape. That is, the radiator 20B comprises a pair of
conductive sector plates 9a and 9b which are attached separately
and from the ends of inner faces of which feeding terminals are led
out. Ahead of the radiator 20B, there is disposed the waveguide 20A
made of a V-shaped conductive plate apart from the radiator 20B
with a predetermined distance, while behind the radiator 20B there
is disposed the reflector 20C formed of an arc-shaped conductive
plate with a predetermined distance from the radiator 20B. All the
conductive plates may be formed of aluminum. As shown in FIG. 3,
the UHF antenna member 20 is so selected in its maximum outer
diameter that it is located within the inner diameter of the loop
of the VHF antenna member 10 and that the outer configurations of
the radiator 20B and the reflector 20C are selected to be parts of
the outermost circle thereof.
The VHF and UHF antenna members 10 and 20, constructed as above,
are assembled by means of the support 30 with the mutual distance
mentioned above. As shown in FIGS. 1 and 2, the support 30 includes
a cone-shaped rotary body 11 for supporting the VHF antenna member
10 and a rotary shaft 12 disposed within the rotary body 11 for
supporting the UHF antenna member 20. The rotary body 11 and the
rotary shaft 12 are assembled such that they can be rotated
independently. To this end, a disc-shaped base 13 made of, for
example, rubber or the like is provided under the rotary body 11,
and a rotatable cylinder 14 is provided on the base 13 rotatable
with respect to the base 13. The rotary shaft 12 is fixedly
inserted into the rotatable cylinder 14, so that the rotary shaft
12 is rotated externally through the cylinder 14. The rotary shaft
12 has a suitable length projected upwardly beyond the rotary body
11 and has attached thereto a holder 40 which holds the UHF antenna
member 20 together with the rotary body 11.
To the outer periphery of the rotary body 11, which can be freely
rotated, near its upper end, the conductive elements 10a to 10h
forming the VHF antenna member 10 are attached with the
predetermined distance. The manner of the attachment is, for
example, as follows. As shown in FIG. 5, projections 28a and 28b
are provided at both ends of the conductive element 10a made of,
for example, aluminum conductive plate, while apertures 29a and 29b
are bored through the rotary body 11 at the position, where the
element 10a is attached, for passing therethrough the projections
28a and 28b, respectively. After the projections 28a and 28b are
passed through the apertures 29a and 29b from the outside of the
rotary body 11 to its inside, they are bent on an inner surface 31
of the body 11 to be attached to the body 11. The other elements
10b to 10h are attached to the body 11 similarly. Thus, all the
elements 10a to 10h of the VHF antenna member 10 are arranged on
the substantially same horizontal plane. The trap circuits 1A to
1D, 6A, 6B and the load 7 are connected to the conductive elements
10a to 10h on the inner surface 31, respectively. Line-shaped
conductors 32 provided on the inner surface 31 serve to improve the
impedance characteristic of the VHF antenna member 10, and
accordingly they are provided between the conductive elements 10a
to 10h, respectively.
The feeding portion of the VHF antenna member 10 is electrically
connected through a connector 33 to a feeder wire 34 and the
conductive elements 10f and 10g, as shown in FIG. 1. It is, of
course, possible to use adhesive upon the attachment of the
elements to ensure the attachment.
The UHF antenna member 20, which may be attached to the rotary
shaft 12, is attached to a holder 40 with a plane shape
substantially same as that of the UHF antenna member 20 as shown by
one-dot chain line in FIG. 3. This attachment will be described, by
way of example. As shown in FIG. 6, ends of a conductor 35 are bent
to be fixed to the holder 40. The holder 40 is provided with a
cylindrical engaging member 41 (refer to FIG. 2) which may engage
with the rotary shaft 12. Thus, a distance h.sub.1 between the
antenna members 10 and 20 is determined.
A feeding wire 36 of the antenna member 20 passes through the
inside of the rotary shaft 12 and then led out to the outside
through a bore (not shown) formed in the rotatable cylinder 14.
Reference letter S (refer to FIGS. 1 and 2) indicates a cover which
may cover the UHF antenna 20. Thus, the UHF antenna 20 is formed of
a cup-shape as a whole.
The rod antennae 5A and 5B shown in FIG. 2 are provided for
improving the gain upon the reception of the VHF signal, especially
in its low frequency band, but in this embodiment they are formed
to be of practical use for the case where a frequency band of the
VHF signal to be received is relatively wide (for example, 50MHz to
220MHz). In other words, the rod antennae 5A and 5B are made to be
a so-called two-frequency type antenna with which one of antennae
can receive the VHF signals of different bands.
FIG. 7 shows the essential construction of one of the rod antennae
5A and 5B, for example, 5A. In the figure, reference numeral 50A
indicates a main antenna member which may be rotatably mounted on
the outer periphery of the rotary body 11 shown in FIGS. 1 and 2.
In order to receive a signal with a relatively low frequency band
of about 50MHz to 90MHz, a sub-antenna member 50B is slidably
mounted on the main antenna member 50A to form a low frequency band
receiving antenna together with the antenna member 50A. The
sub-antenna member 50B is formed of a wire conductor 51 with a
diameter of 2.00mm which consists of an iron wire that is chromium
plated, by way of example. On end portion of the wire conductor 51
is wound on an outer periphery of an insulator 52 made of synthetic
resin and mounted on the main antenna member 50A, as shown in FIG.
7. To this end, a spiral groove 52a is formed on the outer
periphery of the insulator 52 with a desired pitch, and the wire
conductor 51 is wound on the spiral groove 52 a. Thus, the
sub-antenna member 50B is slidably connected to the main antenna
member 50A. In FIG. 7, reference numeral 53 generally shows the
above connecting member.
By connecting the sub-antenna 50B with the main antenna 50A by
means of the insulator 52, the electric equivalent circuit of the
connecting member 53 can be shown as a trap circuit 54 in FIG. 8.
That is, a capacitor 54a is formed between the main antenna 50A and
the wire conductor 51 wound thereon, and a coil 54b is formed by
the wound wire conductor 51. Accordingly, if the respective
constants of the equivalent trap circuit 54 are selected to make
the trap circuit 54 resonate to a VHF signal with a relatively high
frequency band which is received by the main antenna 50A, the trap
circuit 54 shows a high impedance for the VHF signal of a high
frequency band, but shows a low impedance for the VHF signal of a
low frequency band.
For this reason, in the case where the rod antenna 5A is used under
the condition that the connecting member 53 is moved to a top end
55 of the main antenna 50A, the main antenna 50A and the
sub-antenna 50B are electrically connected as shown in FIG. 9A for
a low frequency band of the VHF signal and the rod antenna 5A acts
as an antenna with a total length L. However, for a high frequency
band of the VHF signal, since the impedance of the equivalent trap
circuit 54 is high, the sub-antenna 50B is electrically isolated
from the main antenna 50A as shown in FIG. 9B and hence the
effective length as an antenna becomes a length L' of the main
antenna 50A.
From the above description, it may be apparent that since the rod
antenna 5A is superior in trapping effect, it can be used as the
so-called two-frequency type antenna to improve the directivity or
directional characteristic of the VHF antenna 10 described
later.
FIG. 10 is a graph showing the characteristics of the equivalent
trap circuit 54 in which the abscissa respresents the frequency F
in MHz and the ordinate the reactance Re in K.OMEGA.. In the graph
of FIG. 10, a dotted line curve 60 shows the trapping chracteristic
of an ordinary trap circuit, while a solid line curve 61 shows that
of the trap circuit 54. From this graph, it is obvious that the
trap circuit 54 shown in FIG. 8 has a greatly improved ideal
trapping characteristic. In the trap circuit with the trapping
characteristic shown by the dotted line curve 60, the capacitor
(corresponding to the capacitor 54a) is selected of 2pF and the
coil (corresponding to the coil 54b) is made of a formal wire with
the diameter of 0.6 mm and wound by 13 turns to form a resonant
circuit. The resonant frequency of the trap circuit 54 is
determined by the number of turns of the coil 54b, the winding
pitch of the coil 54b, the coil 54b itself or its diameter and so
on.
As described above, the antenna apparatus T is formed of the VHF
and UHF antenna members 10 and 20 and the support 30 which are
shaped desirably, as mentioned above. The respective
characteristics of the antenna apparatus T will be now described in
order.
The characteristic of the UHF antenna 20 will be now described.
With the present invention, since the antenna elements or
conductive elements are not made of rod-shaped conductors as in the
prior art, but made of wide conductive plates with the
predetermined width and shape, the frequency band to be received is
widened to provide a so-called wide band antenna, which is
ascertained by experiments. That is, the compared results on the
gain and front to back ratio of the directivity between an ordinary
Yagi antenna consisting of three elements and the UHF antenna 20 of
the present invention are shown in the graph of FIG. 11 in which
the abscissa represents the frequency F in MHz, the positive
ordinate the gain G in dB, and the negative ordinate the front to
back ratio Ra, respectively. In the graph of FIG. 11, curves 62 and
64 show the characteristics of the ordinary Yagi antenna, while
curves 63 and 65 show those of the UHF antenna 20. It will be
obvious from the curves that the solid line curve 63 which shows
the gain of the UHF antenna 20 is wider in frequency band than the
solid line curve 62 and that the front to back ratio of the UHF
antenna 20 is relatively high in a wide frequency band. As a
result, the UHF antenna 20 of the invention eliminates the narrow
band characteristic of the prior art antenna and proposes an
antenna of a wide band. Further, with the prior UHF antenna, it is
found that as antenna elements increase in number, the frequency
characteristic of its impedance is greatly deteriorated. The UHF
antenna 20 of the invention can improve such a deterioration also.
That is, with the Smith chart (FIG. 12) which may show an
admittance characteristic, a dotted line curve shows the case where
no element is provided except the radiator 20B. According to this
curve, the standing wave ratio (S.W.R.) is 3 or more than 3 in a
frequency band of about 500MHz, but the S.W.R. becomes close to 1
in a frequency band of about 700MHz. However, if experiments are
performed under the condition that the number of antenna elements
in the UHF antenna 20 are increased to be three for the same
frequency band (about 470 to 770MHz), the results are counter to
the above, that is the S.W.R. becomes lower than 3 (shown by solid
line curves). In other words, as the elements of the U.H.F antenna
of this invention increase in number, the coupling characteristic
is improved to also improve the frequency characteristic. From
this, it will be apparent that the UHF antenna 20 performs the
effect that it acts as a wide band antenna.
The respective characteristics of the UHF antenna 20 are mentioned
as above, but a description will be now given on the VHF antenna
10. When the rod antennae 5A and 5B provided with the equivalent
circuits 54 mentioned above are attached to the VHF antenna 10, the
gain of the VHF antenna 10 itself is improved, which will be
described with reference to the graphs shown in FIGS. 13A and 13B
in which the ordinates represent the gain G in dB and the abscissas
the frequency F, respectively. In the graphs of FIGS. 13A and 13B,
dotted line curves 67 and 68 show the characteristics of a prior
art rod antenna which is not contemplated to form a two-frequency
antenna and hence in which the whole length L (refer to FIG. 9) is
formed by the main antenna, while solid line curves 66 and 69 show
the characteristics of the rod antennae 5A and 5B. It may be
apparent from the comparison therebetween that the gain is
approximately same in the low frequency band of the VHF signal in
the invention and the prior art as shown by the curves 66 and 67 in
FIG. 13A, but in its high frequency band a marked difference
appears. That is, with the prior art rod antenna the gain decreases
as shown by the dotted line curve 68 in FIG. 13B, but with the rod
antennae 5A and 5B of the invention there is no decrease in gain as
shown by the solid line curve 69 in FIG. 13B. This fact means that
the equivalent trap circuit 54 shown in FIG. 8 performs with a
superior effect.
Accordingly, the directivity of the rod antennae 5A and 5B of the
invention in a high frequency band of the VHF signal is, of course,
improved as compared with that of the prior art. The graph of FIG.
14 shows the directivity of the prior art rod antenna which is
already shown in FIG. 13. In the graph of FIG. 14, a solid line
curve 70 indicates the directivity in 170 MHz, a dotted line curve
71 that in 190MHz and a one-dot chain line curve 72 that in 220MHz,
respectively. FIG. 15 is a graph showing the directivity of the rod
antennae 5A and 5B of the invention in which the same reference
numerals indicate the directivities in the same frequency bands as
those of FIG. 14. From the comparison of FIG. 14 with FIG. 15, it
is obvious that the VHF antenna 10 of the invention has greatly
improved directivity.
As described previously, the VHF antenna 10 has as one object to
avoid the deterioration due to the UHF antenna 20 caused by the
close arrangement of both antennae adapted to receive different
frequency bands and also to reduce the size of the antenna
apparatus T. To this end, as described previously, the loop antenna
is divided into eight parts and the trap circuits 1A to 1D, which
may resonate with received UHF signals, are provided. Thus, when
the UHF signal arrives at the antenna, it is not induced in the
conductive elements 10a to 10h because of the trap circuits 1A to
1D with the result that the conductive elements 10a to 10h, by way
of example, have no deleterious affect as reflectors for the UHF
antenna 20 and hence the directivity is not disturbed. By the way,
if the prior art VHF antenna (loop antenna) consisting of eight
divided elements but having no trap circuits is used, the divided
elements act as a reflector for the UHF signal to disturb its
directivity and hence to deteriorate the gain. The VHF antenna 10
of the invention, however, has negligible reflector effect.
During reception of the VHF signal, even if the UHF signal arrives,
it does not act as a jamming wave for the VHF signal because the
respective trap circuits 1A to 1D have high impedance therefor.
Accordingly, even if both the antennae 10 and 20 are arranged close
to each other, there is almost no mutual interference and they
achieve the same effects as if they are disposed far apart, yet the
antenna apparatus T is formed as a compact structure.
In order to ascertain that the embodiment of the VHF antenna having
eight divided elements has less affect on the gain for the UHF
signal than an embodiment which is not eight divided (for example,
divided into four or less sections), the inventors of the invention
carried out the following experiments. In this case, a VHF antenna
divided into four elements is typified as an antenna to be compared
with the embodiment of the invention. As shown in the graph of FIG.
16 in which the abscissa represents the frequency F in MHz, the
positive ordinate the gain G in dB and the negative ordinate the
front to back ratio Ra, the gain for the UHF signal decreases for
the case where the four divided element antenna is used as shown by
a solid line curve 73, but for the case where the eight divided
element VHF antenna 10 of the invention is used the gain is
superior in a received frequency band as shown by a solid line
curve 74. In the graph of FIG. 16, a dotted line curve 75 shows the
front to back ratio of the invention and a dotted line curve 76
that of the prior art.
It is described that since the VHF antenna 10 is divided into eight
parts and the trap circuits 1A to 1D are connected thereto, the UHF
signal is hardly induced in the conductive elements 10a to 10h to
thus improve the gain for the UHF signal. However, the above signal
induction cannot be eliminated entirely by provision of the trap
circuits 1A to 1D. As a result, the VHF antenna 10 acts minimally
as a reflector for the UHF antenna 20. For this reason, if the UHF
antenna 20 is located above the VHF antenna 10, the optimum
direction in which the directivity of the UHF antenna 20 can be
orientated is upward because this directivity is opposite to the
reflector. If, in the graph of FIG. 17, its X--X axis is taken as
the horizontal plane of the UHF antenna 20 and the reflector 20B of
the UHF antenna 20 is located at the center point g of the graph,
the directivity in the vertical plane is orientated upward shown by
a solid line curve 77. The graph of FIG. 17 shows the case where a
frequency of the UHF signal is selected as 600MHz. In this case,
the maximum directivity is inclined from the vertical plane by
about 20.degree. to the side of the waveguide (in the graph to the
left side).
Accordingly, with the existence of the VHF antenna 10, the
directivity of the UHF antenna 20 is orientated to the vertical
direction with the result that the maximum direction of the
directional pattern coincides with the wave incoming direction (in
general, the wave comes from the upper side due to diffraction) to
improve the directivity much more.
The reason why the outer diameter of the UHF antenna 20 is selected
smaller than the inner diameter of the VHF antenna 10 and the
former is disposed within the latter is to reduce the size of the
antenna apparatus T and also to minimize the affect of the VHF
antenna 10 on the UHF antenna 20 as much as possible.
In the above embodiment, the VHF antenna 10 is divided into eight
parts or conductive elements, but the same effect can be performed
by dividing the VHF antenna 10 into more than eight parts. Further,
in the above embodiment the length of the divided conductive
elements is selected equal with one another, but it is also
possible to divide the VHF antenna 10 into conductive elements
different in length. It is only necessary to arrange to avoid that
the UHF signal is induced in the VHF antenna 10.
As may be apparent from the foregoing, with the antenna apparatus T
according to the present invention, the VHF antenna 10 is disposed
on the substantially horizontal plane and the UHF antenna 20 is
disposed within the VHF antenna 10 and above the same spaced a
predetermined distance therefrom as shown in FIGS. 1 and 2, so that
the VHF antenna 10 acts to a minimum degree as a reflector of the
UHF antenna 20 and hence the directivity of the UHF antenna 20 is
directed to the vertical direction thereof as shown in FIG. 17. As
a result, the maximum direction of the directivity coincides with
the wave incoming direction to provide the antenna apparatus T with
superior directivity qualities as compared with the prior art.
With the antenna apparatus T of the present invention, the UHF
antenna 20 can be located close to the VHF antenna 10, so that the
antenna apparatus T itself can be reduced in size and thus be
formed as a compact structure. Accordingly, the antenna apparatus T
is preferred as a room antenna for use with a television receiver
which room antenna is often disposed on the cabinet of the
television receiver.
In the foregoing, only one preferred embodiment of the present
invention is shown and described, but it will be obvious that many
modifications and variations could be effected by those skilled in
the art without departing from the spirit and scope of the novel
concepts of the invention.
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