Helical antenna for portable radio communication equipment

A small size helical antenna for radio communication equipment such as a portable transmitter/receiver, a pocket telephone, or a mobile telephone of small power type. The helical antenna is comprised of first and second parallel conductors wound in a coil shape. The second conductor is folded in parallel to the remaining part of the second conductor at some length in accordance with the transmitting/receiving frequency from the top end. The folded part and the upper part of the first conductor form a radiator of the dipole antenna structure and the parallel part of the first and the second conductors form a parallel feeder. The third conductor having the same length as the folded part may be provided on the lower part of the second conductor. According to the above-described structure, no return current flows from the helical antenna to the casing of the radio communication equipment on which the helical antenna is connected, thereby the directivity becomes maximum in a horizontal plane and an effect caused by holding the casing by a human hand is decreased.

BACKGROUND OF THE INVENTION 
1) Field of the Invention 
The present invention relates to a helical antenna for portable radio 
communication equipment. More specifically, the present invention relates 
to a small helical antenna for a portable transmitter/receiver or a pocket 
telephone (mobile telephone) of a small power type used for an in-plant 
communication system or a tele-terminal. 
2) Description of the Related Art 
Recently, according to developments in radio communication equipment, a 
number of communication systems have adopted a radio communication system 
instead of using a wired system. As a result, there are no useable 
frequencies left in the low frequency band, so that gradually higher 
frequencies are being assigned for new radio communication systems, for 
example, frequency bands of 400 MHz to 800 MHz are assigned. It is now 
being planned to use a 1500 MHz band for a relational radio communication 
system as described above, as explained hereinafter. 
In this way, as the frequency used for a radio communication system gets 
higher, the length of the antenna required gets shorter and the size gets 
smaller. However, as the size of the antenna gets smaller, it becomes more 
difficult to obtain a desirable antenna directivity. 
Conventionally, a whip antenna that has a small-diameter and a vertical 
rod, and a helical antenna that has a coil shape and is mounted 
perpendicular to a flat metal-plate reflector, are used especially in 
mobile communications, portable radio and television receivers, 
field-strength meters, and the like. A dimensional relation between the 
whip antenna or the helical antenna and the casing thereof is different in 
accordance with the transmitting/receiving frequency required for the 
antenna. Usually, a casing of radio communication equipment having the 
whip or helical antenna is not designed in accordance with the optimum 
radiation therefrom but is designed in accordance with the performance and 
the output power of the equipment. 
Accordingly, in the conventional antenna, as the transmitting/receiving 
frequency required for the antenna gets higher, the antenna does not 
provide the desired directivity. Further, in conventional radio 
communication equipment having an antenna, a return current from the 
antenna flows in the casing of the radio communication equipment, so the 
directivity of the antenna changes when the casing is held by a human 
hand. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a helical antenna for a 
portable transmitter/receiver or a pocket telephone (mobile telephone) of 
a small power type used for an in-plant communication system or a 
tele-terminal, whose directivity can be maximum in a horizontal plane, and 
having little effect from a human body when the casing is held by a human 
hand. 
According to an aspect of the present invention, there is provided a small 
size helical antenna for radio communication equipment such as a portable 
transmitter/receiver, a pocket telephone, or a mobile telephone of a small 
power type, the helical antenna comprising: a first conductor being 
continuously wound helically from the top end to the bottom end that will 
be connected to a casing of the equipment; and a second conductor being 
wound in parallel over the helically wound first conductor with a 
predetermined spacing; wherein the second conductor has the same length as 
the first conductor, a predetermined length from the top end thereof is 
folded in parallel to the wound body of the second conductor to form a 
folded part, the unfolded part thereof is wound over the first conductor 
with its bottom end aligned with the bottom end of the first conductor to 
form a parallel feeder, and the folded part and the predetermined length 
from the top end of the first conductor comprise a radiator. 
According to the helical antenna of the present invention, the folded part 
of the second conductor and the predetermined length from the top end of 
the first conductor comprise a radiator of a dipole antenna structure and 
transmitting and receiving is carried out by using this radiator. As a 
result, no return current flows from the helical antenna to the casing of 
the radio communication equipment on which the helical antenna is 
connected, so that the directivity becomes maximum in a horizontal plane 
and the effect caused by holding the casing with a human hand is 
decreased. Further, even if the casing is made of insulated resin, the 
return current does not flow to a radio communication circuit (printed 
circuit board) thereby preventing unstable operation of the circuit. 
Furthermore, since the first and the second conductors are wound in a coil 
shape, the height of the antenna becomes short, and a disturbance of the 
radiation pattern is small because the radiation part is apart from the 
casing held by a human hand. 
Further, according to the existence of the third conductor whose free end 
is facing the free end of the folded part, an unbalanced current does not 
flow to the lower part from the folded point of the parallel feeder.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before describing the preferred embodiments, an explanation will be given 
of the conventional antenna, with reference to FIGS. 1 to 4C. 
FIG. 1 is a front view of a portable radio communication equipment 100 
having a whip antenna 101 on the casing 103, and FIG. 2 is a front view of 
an other portable radio communication equipment 200 having a helical 
antenna 102 on the casing 103. The whip antenna 101 has a small-diameter 
and a vertical rod and the helical antenna 102 has a coil shape, both 
mounted perpendicular to the casing 103. 
A dimensional relation between the whip antenna 101 and the casing 103 is 
different in accordance with the transmitting/receiving frequency required 
for the whip antenna 101 as shown in FIG. 3A to 3C. The whip antenna 101 
in FIG. 3A having a height of 1.25 m is suitable for 
transmitting/receiving a frequency of 60 MHz, the whip antenna 101 in FIG. 
3B having a height of 0.5 m is suitable for transmitting/receiving a 
frequency of 150 MHz, and the whip antenna 101 in FIG. 3C having a height 
of 7.5 cm is suitable for transmitting/receiving a frequency of 800 MHz, 
although the height of the casing 103 is always 0.2 m. As shown in FIGS. 
3A to 3C, the casing 103 of the radio communication equipment having the 
whip antenna 101 is not designed in accordance with the optimum radiation 
therefrom but is designed in accordance with the performance and the 
output power of the equipment. 
However, in the prior art, when the transmitting/receiving frequency 
required for the whip antenna gets higher, the directivity of the whip 
antenna does not agree with the desired directivity as shown in FIGS. 4A 
to 4C. FIG. 4A is a directional characteristic pattern in a vertical plane 
of the whip antenna 101 shown in FIG. 3A (60 MHz), FIG. 4B is the same 
pattern of the whip antenna 101 shown in FIG. 3B (150 MHz), and FIG. 4C is 
the same pattern of the whip antenna 101 shown in FIG. 3C (800 MHz). 
Further, in the conventional radio communication equipment having the whip 
antenna 101, a return current from the antenna 101 flows in the casing 103 
of the radio communication equipment, so that the directivity of the 
antenna changes when the manner of holding the casing 103 by a human hand 
is changed. The dash line in FIG. 4C is the directional characteristic 
pattern in a vertical plane of the whip antenna 101 when the manner of 
holding the casing 103 by a human hand is changed. 
These defects also exist in radio communication equipment having the 
helical antenna. Accordingly, in the prior art, the problem of directivity 
of the antenna for portable radio communication equipments still exists. 
FIG. 5A is a side view of a helical antenna 10 before being wound 
helically, showing a structure thereof according to the first embodiment 
of the present invention, and FIG. 5B is a perspective side view of the 
helical antenna 10 shown in FIG. 5A. 
In FIGS. 5A and 5B, reference numeral 1 denotes a first conductor, 1A 
denotes an upper part of the first conductor, 2 denotes a second 
conductor, 2A denotes a folding point, 2B denotes a folded part of the 
second conductor, 12 denotes a radiator of a dipole structure made of the 
upper part 1A and the folded part 2B, 13 denotes a joining point of the 
radiator 12, 14 denotes a parallel feeder, BT1 and BT2 denote a bottom end 
of the first and the second conductor 1 and 2, S1 denotes a space between 
the first and the second conductors 1 and 2, S2 denotes a space between 
the second conductor 2 and the folded part 2B, and TP1 and TP2 denote a 
top end of the first and the second conductors 1 and 2. 
As shown in FIG. 5B, the conductor 1 and 2 are both flat band plates having 
the same width, height, and thickness. A predetermined length of the 
second conductor 2 from the top end TP2 is folded at the folding point 2A 
in parallel to the rest of the second conductor 2 with a space S2 to form 
a folded part 2B. The length of the folded part 2B is determined in 
accordance with the transmitting/receiving frequency required for the 
helical antenna. Then the second conductor 2 is piled on the first 
conductor 1 with its bottom end BT2 accorded to the bottom end BT1 of 
first conductor 1. When the second conductor 2 is wound on the first 
conductor 1, the upper part 1A of the first conductor 1 having the 
predetermined length from the top end TP1 forms an upper radiation part 
12A, and the folded part 2B of the second conductor 2 forms a folded 
radiation part 12B, thereby forming the radiator 12 of a dipole antenna 
structure. The rest of the first and the second conductor 1 and 2 form in 
parallel a feeder 14. 
The first and the second conductors 1 and 2 are wound helically from the 
bottom end BT1 and BT2 by using a jig of some type to form the helical 
antenna 1 of the first embodiment of the present invention as shown in 
FIG. 6. FIG. 6 is a perspective side view of the helical antenna 1 
according to the first embodiment of the present invention after the first 
and the second conductor 1 and 2 are wound helically. Note that the 
thickness of the first and the second conductors 1 and 2 is not shown in 
FIG. 6. 
In the present invention, the space S1 between the first and the second 
conductor 1 and 2, and the space S2 between the second conductor 2 and the 
folded part 2B, are necessary to prevent the conductors from contacting 
each other. Accordingly, the space S1 and the space S2 must be guaranteed 
by using the spacing material. FIGS. 7A to 7C show some examples of the 
spacing material. FIG. 7A is a side view of a helical antenna 10 before 
winding showing a first actual structure of the spacing material. In FIG. 
7A, the space S1 and the space S2 are fully filled with an insulating 
material 21. FIG. 7B is a side view of a helical antenna 10 before winding 
showing a second actual structure of the spacing material. In FIG. 7B, the 
space S1 and the space S2 are partly filled with an insulating material 
22. FIG. 7C is a side view of a helical antenna 20 before winding showing 
a third actual structure of the spacing material. In FIG. 7C, a 
predetermined facing part at the same position of the first and the second 
conductors 1 and 2, and the second conductor 2 and the folded part 2B are 
curved to contact each other and the contact point is insulated by an 
insulating material 23 to prevent the contact of the conductors. 
When the helical antenna 10 as shown in FIG. 6 is formed, the helical 
antenna 10 is entirely covered with rubber protector 30 and the bottom 
ends BT1 and BT2 thereof are electrically connected to the terminals 41 
and 42 of the connector 40 respectively as shown in FIG. 8, and a helical 
antenna 20 for portable radio communication equipment is produced. The 
connector 40 of this embodiment is a female connector having an inner 
screw thread, and is screwed on to the male connector 50 provided on the 
casing 103 of the radio communication equipment. 
According to the above-described structure of the helical antenna 10 of the 
present invention, since the upper radiation part 12A and the folded 
radiation part 12B form the radiator 12 of the dipole antenna structure, 
no return current flows from the helical antenna 10 to the casing 103 of 
the radio communication equipment, thereby the directivity becomes maximum 
in a horizontal plane and the effect of holding the casing 103 by a human 
hand is decreased. Further, even if the casing 103 is made of insulated 
resin, no return current flows to a radio communication circuit (printed 
circuit board) thereby preventing an unstable operation of the circuit. 
Furthermore, since the first and the second conductors 1 and 2 are wound 
in a coil shape, the height of the antenna 10 becomes short, and a 
disturbance of the radiation pattern is reduced because the radiator 12 is 
apart from the casing 103 held by a human hand. 
FIG. 9 is a perspective side view of the helical antenna 10' according to 
the second embodiment of the present invention. In this embodiment, the 
structure of the helical antenna 10' is the same as the helical antenna 10 
of the first embodiment as shown in FIG. 5A, except that the first 
conductor 1 and the second conductor 2 are not flat band plates but are 
filament shaped. Accordingly, in FIG. 9, the same parts as used in FIG. 6 
are assigned the same reference numerals and the explanation thereof is 
omitted. 
In the second embodiment, since the first conductor 1 and the second 
conductor 2 are filament shaped, the space S1 between the first and the 
second conductors 1 and 2, and the space S2 between the second conductor 2 
and the folded part 2B, are guaranteed by using spacing members as shown 
in FIGS. 10A and 10B. FIG. 10A is an enlarged view showing a spacer 60 
used at dotted part A in FIG. 9, and FIG. 10B is an enlarged view showing 
a spacer 70 used at a dotted part B in FIG. 9. 
The spacer 60 consists of two C-shaped rings 61 and 62 having openings 64 
and 65 respectively, and a connecting bar 63 for connecting the rings 61 
and 62. The spacer 60 is made of insulation material and the first and the 
second conductors 1 and 2 are inserted into the rings 61 and 62 through 
openings 64 and 65 respectively. The spacer 70 consists of three C-shaped 
rings 71, 72, and 73 having openings 76, 77, and 78 respectively, a 
connecting bar 74 for connecting the rings 71 and 72, and a connecting bar 
75 for connecting the rings 72 and 73. The spacer 70 is made of insulation 
material and the first and the second conductors 1 and 2, and the folded 
part 2B are inserted to the rings 71 to 73 through openings 66 to 78 
respectively. These spacers 60 and 70 are provided at predetermined 
intervals. 
FIG. 11 is a side view of a helical antenna 10" before winding showing a 
structure thereof according to the third embodiment of the present 
invention. In this embodiment, the basic structure of the helical antenna 
10" is the same as the helical antenna 10 of the first embodiment as shown 
in FIG. 5A, except the third conductor 3 is added. Accordingly, in FIG. 
11, the same parts as used in FIG. 5A are assigned of the same reference 
numerals and an explanation thereof is omitted. 
The third conductor 3 has the same length as the folded part 2B and is 
wound in parallel on the second conductor 2 with a predetermined space S3 
with a free end TP3 facing the free end TP2 of the folded part 2B with a 
space S4 therebetween and the bottom end BT3 is electrically connected to 
the second conductor 2. 
Then the first, the second, and the third conductors 1 to 3 are wound 
helically from the bottom end BT1 and BT2 by using a jig of some type to 
form the helical antenna 10" of the third embodiment of the present 
invention. FIG. 12 is a perspective side view of the helical antenna 10" 
according to the third embodiment of the present invention when the 
conductors 1 to 3 are all flat band plates having the same width and 
thickness. Note that the thickness of the first to the third conductors 1 
to 3 are not shown in the FIG. 12 embodiment. 
Due to the existence of the third conductor 3, unbalanced current does not 
flow to the lower part of the parallel feeder 14. 
The spaces S1, S2, and S3 are guaranteed by using the spacing material in 
the same manner as explained hereinbefore. FIGS. 13A to 13C are enlarged 
views of parts C in FIG. 11 showing the same examples of the spacing 
material as explained for FIGS. 7A to 7C. FIG. 13A shows the first actual 
structure of the spacing material, wherein the spaces S1 to S3 are fully 
filled with an insulating material 21. FIG. 13B shows the second actual 
structure of the spacing material, wherein the spaces S1 to S3 are partly 
filled with the insulating material 22. FIG. 13C shows the third actual 
structure of the spacing material wherein the predetermined facing part at 
the same position of the conductors 1 to 2 are curved to contact each 
other and the contact point is insulated by an insulating material 23 to 
prevent the contact of the conductors. 
The conductors 1, 2, and 3 in FIG. 11 are explained as flat band plates, 
but these conductors 1, 2, and 3 can also be filament shaped.