U.S. patent application number 09/335881 was filed with the patent office on 2001-12-27 for method for forming helical antenna.
Invention is credited to OGURA, KEIICHI.
Application Number | 20010054780 09/335881 |
Document ID | / |
Family ID | 18454873 |
Filed Date | 2001-12-27 |
United States Patent
Application |
20010054780 |
Kind Code |
A1 |
OGURA, KEIICHI |
December 27, 2001 |
METHOD FOR FORMING HELICAL ANTENNA
Abstract
A helical antenna having an antenna element covered with an
insulating layer is formed by monolithically form the insulating
layer so that the antenna element is integrated with the insulating
layer. To be more precise, first, a cylindrical shell, which
received a core pin in its center hole, is inserted into the
antenna element along the longitudinal axis of the antenna element.
The antenna element is put in a cavity of a molding die. A molding
resin which was heated and molten at a temperature higher than the
melting point of the cylindrical shell is injected into the cavity
to form an insulating cover over the cylindrical shell and the
antenna element. The surface of the cylindrical shell starts
softening upon contacting with the molten resin, and sticks around
the helically coiled antenna line, whereby the pitch of the antenna
element can be maintained substantially constant even if an
injection pressure is applied during the molding.
Inventors: |
OGURA, KEIICHI;
(KANAGAWA-KEN, JP) |
Correspondence
Address: |
ARMSTRONG,WESTERMAN, HATTORI,
MCLELAND & NAUGHTON, LLP
1725 K STREET, NW, SUITE 1000
WASHINGTON
DC
20006
US
|
Family ID: |
18454873 |
Appl. No.: |
09/335881 |
Filed: |
June 18, 1999 |
Current U.S.
Class: |
264/254 ;
264/255; 264/263; 264/266; 264/267; 264/278 |
Current CPC
Class: |
B29C 45/1671 20130101;
B29C 45/14467 20130101; H01Q 11/08 20130101; B29L 2031/3456
20130101; H01Q 1/244 20130101; H01Q 1/36 20130101 |
Class at
Publication: |
264/254 ;
264/255; 264/263; 264/266; 264/267; 264/278 |
International
Class: |
B29C 045/14; B29C
033/12 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 16, 1998 |
JP |
10-357583 |
Claims
What is claimed is:
1. A method for forming a helical antenna having a helical antenna
element covered with an insulating layer, the antenna element and
the insulating layer being monolithically moulded, and the antenna
element having a base which is supported by a metal fitting, the
method comprising the steps of: (a) inserting the antenna element
into an cylindrical cavity of an first moulding die, the inner
diameter of the cylindrical cavity being equal to or slightly
greater than the outer diameter of the antenna element; (b)
injecting an first insulating resin into the cylindrical cavity to
produce a primary moulded product in which the antenna element is
monolithically integrated into the first insulating resin; (c)
putting the primary moulded product in a cavity of a second
moulding die so that a prescribed gap is formed between the outer
surface of the primary moulded product and the inner surface of the
second moulding die; and (d) injecting a second insulating resin
into the cavity of the second moulding die so that the cylindrical
surface of the primary moulded product is monolithically covered
with an insulating layer made of the second insulting resin,
whereby the antenna element is completely covered with the
monolithically formed insulating layer.
2. A method for forming a helical antenna having a helical antenna
element covered with an insulating layer, the antenna element and
the insulating layer being monolithically moulded, and the antenna
element having a base which is supported by a metal fitting, the
method comprising the steps of: (a) inserting a core pin of a
moulding die into a cylindrical shell made of an insulating resin;
(b) inserting the cylindrical shell, in which the core pin of the
moulding die was inserted, into the antenna element along the
longitudinal axis of the antenna element; (c) retaining the antenna
element in a cavity defined by the moulding die so that a
prescribed gap is formed between the antenna element and the inner
surface of the moulding die; (d) injecting a molten resin into the
cavity to form an insulating cover which monolithically integrates
the antenna element and the cylindrical shell; (e) removing the
core pin from the cylindrical shell to produce a cylindrical
antenna; (f) covering the aperture of the cylindrical antenna with
an insulating cap, whereby the helical antenna having the antenna
element completely surrounded by the insulating cover is
completed.
3. The method according to claim 2, wherein the insulating resin
forming the insulating cover is heated and molten at a temperature
higher than at least the melting point of the insulating resin of
the cylindrical shell, and the molten resin at said temperature is
injected into the cavity.
4. The method according to claims 2 or 3, further comprising the
steps of: retaining the cylindrical antenna, from which the core
pin has been removed, in a second moulding cavity; and injecting a
second moulding resin to monolithically form an insulating cap at
the aperture of the cylindrical antenna.
Description
[0001] The present invention claims the benefit of the filing date
of Japanese Patent Application Serial No. H10-357583, the contents
of which are incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to a method for forming a helical
antenna having an antenna element which is covered with, for
example, an insulating layer, and more particularly, to a method
for forming a helical antenna, in which the antenna element is
integrated into the insulating layer by monolithically moulding the
insulating layer around the antenna.
[0004] 2. Description of the Related Art
[0005] In recent years, helical antennas having helical antenna
elements (or aerial elements) are widely used in portable
communication devices, such as cellular phones, because of their
wide-band characteristics and the advantage of requiring less
space.
[0006] An antenna element is generally attached to a portable
communication device so that it projects from the case.
Accordingly, the antenna element is covered with an insulating
layer for purposes of preventing the antenna element from deforming
due to external forces, and preventing the resonant frequency
changing.
[0007] FIGS. 9 and 10 illustrate a conventional method for forming
a helical antenna having an antenna element which is covered with a
monolithically moulded insulating layer.
[0008] The conventional antenna element 101 is formed by helically
coiling a conductive material. One end of the antenna element 101
is fixed to a metal fitting 102. The antenna element 101 is
electrically connected to the aerial coupled circuits and the
transmitting/receiving circuit of a portable communication device
(not shown) via the metal fitting 102 and the feeder 103 connected
to the metal fitting 102.
[0009] The coil pitch P of the antenna element 101 is determined
based on the resonant frequency of the helical antenna. The coil
pitch is almost the same over the entire length of the antenna
along the longitudinal axis.
[0010] In order to cover the antenna element 101 with the
monolithically moulded insulating layer 104, a core pin 105, which
functions as a moulding die, is inserted from the free end of the
antenna element along its longitudinal axis, as shown in FIG.
9.
[0011] The antenna element 101, in which the core pin 105 was
inserted, is accommodated in the cavity 107 which is defined by the
top and bottom dies 106a and 106b, as illustrated in FIG. 10. The
antenna element 101 is supported by the core pin 105 in the cavity
107 so that a prescribed gap is generated between the inner surface
of the cavity 107 and the antenna element 101.
[0012] Then, a molten resin, which is the material of the
insulating layer 104, is injected into the cavity 107 from the gate
G to fill the gap between the antenna element 101 and the moulding
dies 106a and 106b.
[0013] When the moulding resin is cured, the antenna element 101 is
integrated into the insulating cover 104, whereby a helical antenna
with an antennal element covered with the insulating layer is
completed.
[0014] However, the conventional method has a problem that the
injection pressure of the moulding resin, which forms the
insulating layer, causes the coil pitch P of the antenna element
101 to change, and consequently, the electric parameters of the
antenna deviate from the designed values.
[0015] For example, in FIG. 9, the pitches P1, P2, and P3 measured
at three different positions (i.e., near the base, in the middle,
and near the tip) along the coil axis before the moulding of the
insulating cover are 2.49 mm, 2.43 mm, and 2.464 mm, which are
close to each other with the offsets within the acceptable error.
However, with the injection gate located near the metal fitting
102, as illustrated in FIG. 10, the pitches P1', P2', and P3' of
the antenna element 101 after the moulding become 3.18 mm, 2.68 mm,
and 1.44 mm, and the end portion of the antenna element 101 is
undesirably compressed. This pitch divergence inevitably occurs
even if the gate position is adjusted. Thus, the resonant frequency
of the helical antenna greatly deviates from the designed value
depending on the moulding conditions, which results in a low
product yield in mass production.
[0016] In order to overcome this problem, Japanese Patent
Application Laid-open No. 8-894017 discloses a helical antenna,
which can prevent the pitch divergence of the antenna element 111
by forming a helical guide groove 113 in the insulating cap 112 in
accordance with the coil pitch P of the antenna element 111, as
shown in FIG. 11, and which can protect the antenna element 111
from external forces by bonding an insulating cover 114, which was
formed in advance, around the antenna element 111 by adhesive, or
by fitting the antenna element 111 into the insulating cover 114,
as shown in FIG. 12.
[0017] However, the helical antenna 110 disclosed in 8-894017
requires an assembling step or a bonding step for attaching the
insulating cover to the helical antenna. In addition, the bonded or
fitted insulating cover cannot sufficiently protect the antennal
element. If the communication device is dropped, the insulating
cover 114 is likely to break or to be disengaged from the antenna
element 111, and as a result, the antenna element is exposed.
SUMMARY OF THE INVENTION
[0018] The present invention was conceived in order to overcome
these problems in the prior art, and it is an object of the
invention to provide a method for forming a helical antenna with an
antenna element covered with a monolithically formed insulating
layer, which can reliably protect the antenna element from external
forces, and can maintain the electric properties of the antenna
element constant.
[0019] It is another object of the invention to provide a method
for forming a helical antenna which does not require an extra
assembling step for attaching an insulating cover to the antenna
element, and which can prevent breakage or disengagement of the
insulating cover, as well as undesirable exposure of the antenna
element.
[0020] In order to achieve these objects, in one aspect of the
invention, a helical antenna having a helical antenna element
covered with an insulating layer is formed so that the antenna
element and the insulating layer are monolithically moulded, and
that the base of the antenna element is supported by a metal
fitting. This method comprises the following steps:
[0021] (a) inserting the antenna element into an cylindrical cavity
of an first moulding die, the inner diameter of the cylindrical
cavity being equal to or slightly greater than the outer diameter
of the antenna element;
[0022] (b) injecting an first insulating resin into the cylindrical
cavity to produce a primary moulded product in which the antenna
element is monolithically integrated into the first insulating
resin;
[0023] (c) putting the primary moulded product in a cavity of a
second moulding die so that a prescribed gap is formed between the
outer surface of the primary moulded product and the inner surface
of the second moulding die; and
[0024] (d) injecting a second insulating resin into the cavity of
the second moulding die so that the cylindrical surface of the
primary moulded product is monolithically covered with an
insulating layer made of the second insulting resin, whereby the
antenna element is completely covered with the monolithically
formed insulating layer.
[0025] Because the inner diameter of the cylindrical cavity is
equal to or slightly greater than the outer diameter of the antenna
element, the antenna element is inserted into the cavity with its
outer surface is in contact with the inner surface of the
cylindrical cavity. When the first insulating resin is injected in
the cylindrical cavity, the injection pressure causes the antenna
element to expand outward in the radial direction, which further
causes a friction force between the antenna element and the inner
face of the moulding die. This friction force prevents the antenna
element from moving along its longitudinal axis (or the coil axis).
When the first insulating resin is set, the antenna element having
a constant pitch is completed as the primary moulded product, with
little deviation.
[0026] This primary product is retained in the second moulding die,
and the second moulding resin is injected in the gap between the
second moulding die and the primary product in order to
monolithically form the insulating layer over the outer surface of
the primary product. Thus, the antenna element is covered with the
insulating layer in such a manner that the antenna element and the
insulating layer are integrated into a single body of the helical
antenna.
[0027] Since the pitch of the antenna element is fixed by the cured
first moulding resin, it does not change even if the injection
pressure of the second moulding resin is applied to the primary
product. Accordingly, the final product, that is, the helical
antenna can have the designed electric properties. In addition, the
antenna element is completely covered with the insulating layer in
the monolithic manner, it is durable against an impact, and
undesirable exposure of the antenna element can be prevented.
[0028] In another aspect of the invention, a helical antenna having
a helical antenna element covered with an insulating layer is
formed by the following steps:
[0029] (a) inserting a core pin of a moulding die into a
cylindrical shell made of an insulating resin;
[0030] (b) inserting the cylindrical shell which received the core
pin of the moulding die in the center hole, into the antenna
element along the longitudinal axis of the antenna element;
[0031] (c) retaining the antenna element in a cavity defined by the
moulding die so that a prescribed gap is formed between the antenna
element and the inner surface of the moulding die;
[0032] (d) injecting a molten resin into the cavity to form an
insulating cover which monolithically integrates the antenna
element and the cylindrical shell;
[0033] (e) removing the core pin from the cylindrical shell, and
obtaining a cylindrical antenna;
[0034] (f) covering the aperture of the cylindrical antenna with an
insulating cap, whereby the helical antenna having the antenna
element completely surrounded by the insulating cover is completed.
In this case, the antenna element and the insulating layer are
again monolithically moulded, and the base of the antenna element
is supported by a metal fitting.
[0035] When the molten resin is injected into the cavity, it flows
along the outer surface of the cylindrical shell inserted in the
antenna element. Upon contacting with the molten resin, the
cylindrical shell thermally expands and retains the inner face of
the antenna element, whereby the pitch of the antenna element can
be kept constant even if the injection pressure of the molten resin
is applied to the antenna element.
[0036] When the moulding resin which filled the gap between the
inner wall of the moulding die and the antenna element is cures,
the core pin is pulled out. The resultant cylindrical antenna
comprises the cylindrical shell, the antenna element, and the
insulating cover which are all integrated into a single unit.
[0037] The opening of the cylindrical antenna is covered with an
insulting cap. Since the pitch of the antenna element is kept
constant, the designed electrical properties can be achieved with
little deviation. This monolithically formed helical antenna is
durable against external forces or impacts, and can prevent the
antenna element from being exposed.
[0038] Preferably, the insulating resin forming the insulating
cover is heated and molten at a temperature higher than at least
the melting point of the insulating resin of the cylindrical shell,
and the molten resin is injected into the cavity.
[0039] Because the temperature of the molten resin injected into
the cavity is higher than the melting point of the cylindrical
shell made of another type of insulating resin, the cylindrical
shell gets softened upon contacting with the injected resin, and
the helically coiled antenna element digs into the softened shell
surface. Thus, the antenna element is retained by the cylindrical
shell with its initial pitch kept constant.
[0040] When the moulding resin is set, the core pint is removed.
Then, a helical antenna, in which the insulating cover, the antenna
element, and the cylindrical shell which was softened and cured
again are monolithically integrated into a single unit is
completed.
[0041] Preferably, an insulating cap is monolithically formed at
the opening of the cylindrical antenna by retaining the cylindrical
antenna, from which the core pin has been removed, in a second
moulding cavity, and by injecting a second moulding resin to form
the insulating cap.
[0042] Because the insulating cap is integrated into the
cylindrical antenna by injection-moulding, it does not come off
even if external forces are applied. In addition, no additional
steps for bonding or fixing the cap to the cylindrical antenna.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] The above and other objects, features, and advantages of the
invention will be apparent from the following detailed description
of the preferred embodiments with reference to the attached
drawings, wherein:
[0044] FIG. 1 is a cross-sectional view showing the first moulding
step of the method for forming a helical antenna, in which the
antenna element is inserted into the first moulding die, according
to the first embodiment of the invention;
[0045] FIG. 2 is a side view of the primary moulded product formed
by the first moulding die;
[0046] FIG. 3 is a cross-sectional view showing the second moulding
step of the method for forming a helical antenna, in which the
second insulating resin is injected and cured in the cavity,
according to the first embodiment of the invention;
[0047] FIG. 4 is a side view of the antenna element, into which the
cylindrical shell is to be inserted, according to the second
embodiment of the invention;
[0048] FIG. 5 is a cross-sectional view showing the moulding step
of the method for forming a helical antenna, in which the moulding
resin is injected into the moulding die, according to the second
embodiment of the invention;
[0049] FIG. 6 is a cross-sectional view of the cylindrical antenna
formed according to the second embodiment of the invention;
[0050] FIG. 7 is a cross-sectional view showing the second moulding
step of the method for forming a helical antenna according to the
second embodiment of the invention, in which the cylindrical
antenna is inserted in the second moulding die, and the second
insulating resin is injected and cured in the cavity of the second
moulding die;
[0051] FIG. 8 is a cross-sectional side view of the helical antenna
formed by the method of the second embodiment;
[0052] FIG. 9 shows the preliminary step for forming a helical
antenna in a side view according to a conventional method;
[0053] FIG. 10 is a cross-sectional view showing the moulding step
of the conventional method, which is performed after the
preliminary step shown in FIG. 9, in which the moulding resin is
injected in the moulding die to form an insulating cover;
[0054] FIG. 11 is a side view showing another prior art method for
forming a helical antenna, in which an insulating cap is attached
to the antenna element; and
[0055] FIG. 12 is a cross-sectional side view of the helical
antenna formed by the prior art method shown in FIG. 11.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0056] The first embodiment of method for forming an helical
antenna according to the invention will be described with reference
to FIGS. 1 through 3.
[0057] FIG. 1 illustrate an antenna element 3 which is supported by
a metal fitting 2 at its base portion. The antenna element 3 and
the metal fitting 2 are accommodated in a first moulding die 4
which consists of a top portion 4a and a bottom portion 4b. The
antenna element 3 is secured to the metal fitting 2 made of a
conductive material by winding the coil end 3a around the metal
fitting 2, whereby the antenna element 3 is retained by the metal
fitting with one end open. The antenna element 3 is also made of a
conductive material, and, for example, a piano wire having a
diameter of 0.5 mm is used. The antenna element 3 is coiled in a
helical manner under the conditions of, for example, the outer
diameter of 4.5 mm, and the coil pitch P of 2.46 mm with 10 turns,
so that a prescribed resonant frequency can be obtained after the
moulding.
[0058] The metal fitting 2 is a cylinder having a center hole, in
which a feeder 5 made of nickel or titan alloy is firmly inserted
so as to establish an electrical connection with the metal fitting
2. The feeder 5 functions as an electric supply line, and the
antenna element 3 is electrically connected via the metal fitting 2
and the feeder 5 to the aerial coupled circuits and the
transmission/receiving circuits (both of which are not shown) of
the communication device. Since the metal fitting 2 is securely
caught between the top and bottom dies 4a and 4b, the antenna
element 3 is correctly positioned inside the moulding die 4.
[0059] The top and bottom dies 4a and 4b define a cavity 6 which is
cylindrical hollow having a diameter equal to or slightly greater
than the outer diameter of the coil of the antenna element 3. In
this embodiment, the diameter of the cavity 6 is set to 4.53 mm,
which is 0.03 mm greater than the outer diameter of the antenna
element 3. This condition allows the antenna element 3 to be
inserted without bending or rubbing against the inner face of the
moulding die 4, while the inserted antenna element 3 easily comes
into contact with the inner face of the moulding die 4 in the
subsequent primary moulding process, which will be explained in
detail below.
[0060] One end 6a of the cylindrical cavity 6 which receives the
free end of the antenna element 3 is a hemispherical concave. When
the first moulding resin is injected and cured in this concave, an
insulating cap 7 can be monolithically formed.
[0061] The first moulding resin to be injected in the first
moulding die 4 is thermoplactic insulating composite resin, such as
polyester elastomer. The first moulding resin is heated prior to
the injection, and the molten resin is injected from the gate G
into the cavity 6. Due to the injection pressure of the first
moulding resin which flows in the cylindrical cavity 6, a pressure
in the longitudinal direction along the coil axis, which tries to
shift the antenna element 3 along the coil axis, and a pressure in
the radial direction, which tries to expand the coil diameter
outward, are simultaneously applied to the antenna element 3. The
pressure in the radial direction generates a friction force between
the coil of the antenna element 3 and the inner face of the
moulding die 4. This friction force overcomes the pressure in the
longitudinal direction, whereby the antenna element can be
maintained in the correct position in the cavity 6 without shifting
along the coil axis.
[0062] FIG. 2 illustrates a primary moulded product 8 taken out of
the first moulding die 4. The antenna element 3 is embedded in the
column-like primary product 8 along the cylindrical surface 8a. The
hemispherical insulating cap 7 is also monolithically formed on the
tip of the primary product 8. Since the pitch of the antenna
element 3 is not affected by the injection pressure, a high-quality
antenna can be produced with little deviation from the designed
pitch.
[0063] This primary product 8 is then put in the cavity 9 of the
second moulding die 9 in order to monolithically form an insulating
layer 11 to cover the antenna element 3 of the primary product 8.
In the cavity 10, the insulating cap 7 and the feeder 5 are
retained between the top and bottom dies 9a and 9b so that a gap is
formed between the cylindrical surface 8a of the column-like
primary product 8 and the inner wall of the moulding die 9. The
width of the gap corresponds to the thickness of the insulating
layer 11.
[0064] FIG. 3 is a cross-sectional view showing that the second
moulding resin is injected in the gap between the primary product 8
and the second moulding die 9 from the two gates G formed in the
second moulding die 9. The second moulding resin is the same
material as the first moulding resin, that is, a thermoplastic
insulating composite resin, such as polyester elastomer. By
injecting the molten second moulding resin into the cavity 10 from
the gates G, the antenna element 3 is covered with an insulating
layer with a uniform thickness. This thickness equals the step size
from the cylindrical surface 8a of the column-like antenna to the
bottom face of the insulating cap 7. Accordingly, as a result of
the second moulding (or the injection), the insulating layer 11 and
the insulating cap 7 are integrated with each other so as to have a
smooth and continuous surface.
[0065] Thus, the helical antenna 1, in which the antenna element 3
is incorporated into the monolithically formed insulating cap 7 and
the insulating layer 11, is completed. Since the helical antenna 1
of this embodiment is a column-like rigid antenna, the antenna
element 3 can be safely protected from undesirable external forces,
and it can prevent breakage of the insulating layer 11 or the
insulating cap 7.
[0066] Because the pitch of the antenna element 3 can be maintain
constant throughout the moulding process, the electric properties
of the resultant antenna, including the resonant frequency, can be
maintained stable.
[0067] Although, in the first embodiment, the insulating layer 11
is formed in the second moulding step, the insulating layer 11 may
be monolithically formed first so as to incorporate the antenna
element.
[0068] Next, the second embodiment of the method for forming a
helical antenna according to the invention will be described with
reference to FIGS. 4 through 8. The same elements as those in the
first embodiment will be denoted by the same numerical references,
and the explanation for them will be omitted.
[0069] In the second embodiment, a cylindrical shell 14, in which a
core pin 13 of a moulding die 12 is inserted in advance, is used,
as shown in FIG. 4. The outer diameter of the cylindrical shell 14
is slightly smaller than the inner diameter of the coil of the
antenna element 3.
[0070] The cylindrical shell 14 which received the core pin 13 is
inserted in the antenna element 13 from the free end of the
helically coiled antenna element 3 along the coil axis until the
tip of the core pin 13 comes into contact with the metal fitting 2.
This state is illustrated in FIG. 5.
[0071] The cylindrical shell 14 is made of an insulating material
whose melting point is lower than the injection temperature of the
molten resin for forming the insulating layer 15. In this
embodiment, while the moulding resin for forming the insulating
layer 15 is a thermoplastic insulating composite resin, such as
polyester elastomer, with a melting point of 200.degree. C., the
cylindrical shell 14 is made of vinyl chloride having a melting
point of 160.degree. C. which is lower than the injection
temperature.
[0072] The antenna element 3 supported by the metal fitting 2, and
the cylindrical shell 14 with the core pin 13 inserted in the
antenna element 3 are put in the cavity 16 of the moulding die 12.
In the cavity 16, the core pint 13 and the feeder 5 are retained
between the top and bottom dies 12a and 12b so that a gap is
generated between the antenna element and the inner wall of the
moulding die 12, as shown in FIG. 5. The width of the gap
corresponds to the thickness of the insulating layer 15.
[0073] In this state, the moulding resin, which was heated up to a
temperature higher than the melting point of the cylindrical sheel,
is injected in the cavity 16 from the gate G of the moulding die 12
in order to form the insulating layer 15.
[0074] The injected resin flows along the outer surface of the
cylindrical shell 14 and covers the coil of the antenna element 3.
Since the melting point of the cylindrical shell 14 is lower than
the temperature of the injected resin, the surface of the
cylindrical shell starts softening upon contact with the injected
resin. The melting surface of the cylindrical shell 14 sticks to
the coil of the antenna element 3, and keeps the pitch P of the
antenna element 3 constant in the axial direction during the
injection of the moulding resin.
[0075] Tables 1 and 2 show the pitch P of the antenna element 3
before and after the injection-moulding. Table 1 lists the pitches
P1, P2 and P3 (see FIG. 4) at the base, the center, and the tip of
each of the arbitrarily selected five antenna elements 3 before the
moulding. Table 2 lists the pitches P4, P5 and P6 (see FIG. 5) at
the same positions of arbitrarily selected five antenna elements 3
after the moulding.
[0076] As is clear from Tables 1 and 2, after the moulding, the
pitch P4 at the base of the antenna element 3 becomes slightly
longer than the pitch P5 at the tip of the antenna element 3
because the injection gate G is formed near the base of the antenna
element. However, this slight deviation is within the acceptable
error, and is much better than the pitch deviation in the
conventional method, in which the pitches P1', P2' and P3' after
the injection become 3.18 mm, 2.68 mm, and 1.44 mm, respectively.
In contrast, with the method of the present invention, the pitch of
the antenna element can be maintained substantially constant even
after the moulding for integrating the antenna element 3 and the
insulating layer 15 into a single unit.
1 TABLE 1 pitch (mm) of the antenna element sample No. P1 at base
P2 at center P3 at tip 1 2.49 2.42 2.46 2 2.49 2.42 2.46 3 2.49
2.44 2.47 4 2.49 2.43 2.47 5 2.49 2.44 2.46 Average 2.49 2.43
2.464
[0077]
2 TABLE 2 pitch (mm) of the antenna element sample No. P4 at base
P5 at center P6 at tip 11 2.64 2.46 2.45 12 2.63 2.45 2.44 13 2.68
2.45 2.44 14 2.65 2.46 2.45 15 2.67 2.45 2.44 Average 2.654 2.454
2.444
[0078] The injected moulding resin fills the gap between the
antenna element 3 and the inner wall of the moulding die 12. When
this molten resin hardened, the insulating layer 15 in formed on
the antenna element, while integrating the antenna element 3 and
the softened surface of the cylindrical shell into a single unit
(shown in FIG. 5). Then, the core pin 13 is removed, and a
cylindrical antenna 17 with the antenna element 3 covered with the
monolithic insulating layer 15 is obtained.
[0079] This cylindrical antenna 17 functions as a helical antenna
as it is. However, it is preferable to cover the opening 18a (shown
in FIG. 6) of the center hole 18 of the cylindrical antenna 17 with
an insulating cap 19.
[0080] In this embodiment, the cylindrical antenna 17 is put in the
second moulding die 21 to monolithically form the insulating cap 19
at the end portion of the cylindrical antenna 17, thereby covering
the opening 18a.
[0081] Prior to putting the cylindrical antenna 17 in the second
moulding die 21, a core stick 20 is inserted in the center hole 18,
as shown in FIG. 6. The core stick 20 has substantially the same
shape as the core pin 13. The reason for inserting the core stick
20 is to prevent the second moulding resin from flowing into the
center hole 18, because if the second moulding resin having filled
the center hole 18 is cured, the outer surface of the helical
antenna 30 may become uneven due to the strain.
[0082] If the cylindrical antenna 17 with the core stick 20 is put
in the cavity 22 of the second moulding die 21, which is defined by
the top and bottom moulding dies 21a and 21b, a hemispherical gap
22a (shown in FIG. 7) is generated next to the tip of the
cylindrical antenna 17. The second moulding resin, which was heated
and molten, is injected into the hemispherical gap 22a form the
gate G. The second moulding resin is preferably the same material
as the first moulding resin used to form the insulating layer 15.
Accordingly, the second moulding resin is polyester elastomer in
this embodiment.
[0083] FIG. 8 illustrates the helical antenna 30 removed from the
second moulding die 21 after the second moulding resin (or the
insulating cap 19) was cured. As the result of the hardening of the
second moulding resin, the insulating cap 19 covering the aperture
18a is integrally coupled with the end portion of the insulating
layer 15.
[0084] The helical antenna 30 formed according to the second
embodiment also has the advantage that the insulating layer 15
hardly breaks or the insulating cap 19 hardly comes off from the
antenna 30 even if unexpected external forces are applied to the
helical antenna 30.
[0085] Furthermore, displacement or deformation of the antenna
element 3 due to the injection pressure can be prevented, and the
pitch of the antenna element 3 can be maintained at the designed
value.
3 TABLE 3 antenna element 3 sample No. resonant freq. MHz VSWR 1
881.7 1.05 2 881.4 1.05 3 881.1 1.05 4 880.6 1.04 5 881.3 1.05 6
880.8 1.03 7 881.5 1.03 8 881.4 1.03 9 880.8 1.03 10 880.6 1.04 MAX
881.7 1.05 MIN 880.6 1.03 AVERAGE 881.12 1.04 .sigma. 0.376
0.0089
[0086] Tables 3 and 4 exhibit the electric properties of the
antenna before and after the moulding of the insulating layer 15
according to the invention. More specifically, Table 3 shows the
electric properties of arbitrarily selected ten antenna elements 3
before the insulating layer 15 is formed, and Table 4 shows the
electric properties of arbitrarily selected ten helical antennas
30, in each of which the insulating layer 15 and the insulating cap
19 are monolithically formed.
4 TABLE 4 helical antenna 30 sample No. resonant freq. MHz VSWR 21
831.5 1.07 22 830.4 1.07 23 830.1 1.08 24 830.4 1.06 25 831.1 1.08
26 831.7 1.08 27 831.6 1.08 28 830.8 1.08 29 831.0 1.08 30 830.6
1.07 MAX 831.7 1.08 MIN 830.1 1.06 AVERAGE 830.92 1.075 .sigma.
0.526 0.006
[0087] As is clear from Tables 3 and 4, any of the samples of
monolithically formed helical antenna 30 have the resonant
frequencies near 830 MHz which is the designed value. The standard
deviations (a) of both the resonant frequency and the voltage to
standing wave ratio (VSWR) of the final product (i.e, the helical
antenna 30) are not very much offset from the standard deviations
generated in forming the antenna element 3. Thus, the electric
properties of the antenna can be maintained even after the moulding
process with little variations.
[0088] As compared with the first embodiment, the helical antenna
30 formed by the second embodiment can be made slimmer because the
insulating layer 15 is formed first on the antenna element and the
cylindrical shell. In the first embodiment, the insulating layer 11
is monolithically formed on the primary product 8 after the primary
product was moulded. In this case, the insulating layer 11 needs to
have a certain thickness to smooth the uneven surface of the
primary product 8. In contrast, in the second embodiment, a
relatively thin insulating layer 15 can be formed over the antenna
element 3.
[0089] Although, in the second embodiment, the insulating cap 19 is
formed monolithically using the second moulding die 21, the
insulating cap 19 may be formed separately, and may be attached to
the opening 18a by adhesive or other connection means.
[0090] The moulding resin for forming the insulating layer 15 is
heated up to a temperature higher than the melting point of the
cylindrical shell 14, and the molten resin at that temperature is
injected into the cavity 16. However, the invention is not limited
to this method.
[0091] The moulding resin heated up to a temperature a slightly
lower than the melting point of the cylindrical shell 14. In this
case, the cylindrical shell 14 thermally expand upon contacting
with the injected molten resin, and presses against the antenna
element 3 in the radial direction from the inside, thereby
preventing the deformation of the antenna element 3 in the axial
direction due to the injection pressure.
[0092] In either embodiment of the helical antenna 1 or 30, the
antenna element is connected to the feeder 5 via the metal fitting
2. However, a rod antenna element may be connected to the base or
the tip of the helical antenna element 3 in series.
[0093] As has been explained, even after the moulding resin is
injected to form the insulating layer over the antenna element, the
electrical properties of the antenna element can be maintained
without being affected by the injection pressure. Consequently, the
product yield of the helical antenna can be increased.
[0094] The method of the present invention does not require the
assembling step of an insulating cover because the insulating layer
is monolithically formed over the antenna element. The
monolithically formed insulating layer does not easily breaks or
comes off, and can reliably protect the antenna element.
[0095] According to the second embodiment, the insulating layer 15
is formed only once over the antenna element, and a slim helical
antenna with a reduced diameter can be achieved.
[0096] By heating the moulding resin up to a temperature higher
than the melting point of the cylindrical shell inserted in the
antenna element, the softened cylindrical shell retains the antenna
element at the correct position, whereby deformation of the antenna
element due to the injection pressure can be prevented.
[0097] The monolithically formed insulating cap can also reliably
protect the antenna element because it does not come off from the
antenna element even on undesirable impact.
[0098] Although the invention has been described based on the
preferred embodiments, the invention is not limited to these
embodiments. It should be appreciated that there are many
modifications and substitutions without departing from the spirit
and scope of the invention, which is defined by the appended
claims.
* * * * *