Antenna device

This invention provides an antenna device of the type including a main antenna composed of a retractable flexible antenna, wherein the main antenna is slidably received in a tubular antenna case, a coil spring composed of a conductor is connected at one end to a base of the main antenna and, at an opposite end, to the antenna case, and holding means disposed on the base of the main antenna for holding the main antenna against displacement. With this arrangement, when the main antenna is retracted in the antenna case, the coil spring is contracted. The contracted coil spring serves as a short antenna coil so that the necessary antenna characteristics can be maintained even when the main antenna is received inside the antenna case. The antenna device can be transported while it is received in a pocket or the like of the user, with the main antenna kept in a retracted position, and with its receiver function kept in an activated condition.

TECHNICAL FIELD 
This invention relates to an antenna device. 
BACKGROUND ART 
Antennas incorporated for radio wave radiation in handy-type compact radio 
communication equipment, movable units of cordless telephone sets, etc. 
include a telescopic rod antenna which is used from the point of view of 
portability, transportation and storage, and a flexible antenna which is 
used from the point of view of protection of the antenna body against 
damage. 
FIGS. 3 and 4 show a radiotelephone equipped with a conventional flexible 
antenna which can be received in a body of the radiotelephone. In this 
figure, numeral 31 is a flexible antenna having an internal spiral 
conductor 32, numeral 34 is a stopper provided at the base of the flexible 
antenna, and numeral 35 is a bearing provided within a cabinet 38 for 
slidably holding the flexible antenna 31. Thus, the flexible antenna 31 is 
slidably supported by the bearing 35 and is protected by the stopper 34 
against removal from the cabinet 38. Numeral 36 is an electrode strip 
provided on the base of the flexible antenna 31. The electrode strip 36 
and the internal spiral conductor 32 are held in conduction with each 
other. Numeral 37 is an electrode strip secured to the bearing 35. When 
the flexible antenna 31 is pulled out as shown in FIG. 3, the electrode 
strip 36 and the electrode strip 37 contact with each other, thereby 
electrically connecting the spiral conductor 32 and transmitter and 
receiver circuits (not shown). 
In the unused condition, the flexible antenna 31 is received within the 
cabinet 38, as shown in FIG. 4. In use, the user pulls out the flexible 
antenna 31 from the cabinet 38 while gripping the front end of the 
flexible antenna 31. When the flexible antenna 31 is fully extended, the 
electrode strip 36 on the flexible antenna side is brought into contact 
with the electrode strip 37 secured to the bearing 35, whereupon the 
transmitter and receiver circuits and the spiral conductor 32 are 
electrically connected with each other. 
With this construction, transmission and reception of a message is not 
possible until the flexible antenna 31 is fully pulled out from the 
cabinet, as shown in FIG. 3. 
In some instances, however, it is desired that a receiver function is kept 
alive even when the antenna is received in the cabinet. This is because it 
is very convenient for the user if the radiotelephone has a construction 
enabling such a manner of use that the radiotelephone is held compactly in 
a pocket or the like of the user while keeping the receiver function in an 
activated condition, and when a call signal is received, the user draws 
the radio-telephone from the pocket and pulls out the antenna from the 
cabinet for commencing transmission of a message. To realize this manner 
of use, however, a receive-only sub-antenna which is operable in the 
received condition must be provided within the cabinet in addition to the 
main antenna. Furthermore, in order to connect the transmitter and 
receiver circuits selectively with the main antenna and the sub-antenna 
depending on the position of the main antenna, a switching means must be 
provided. Thus, the above-mentioned arrangement increases the number of 
structural components of the radiotelephone. 
Furthermore, when the antenna of the construction shown in FIG. 3 is 
depressed during transmission of a message, connection between the antenna 
and the transmitter circuit is interrupted for a moment. In this instance, 
the impedance and the resonance frequency of the antenna side as viewed 
from the transmitter circuit change greatly. If such a great change in 
impedance and resonance frequency occurs during the transmission of a 
message, the transmitter circuit may be damaged. 
The currents flowing between the transmitter circuit and the main antenna 
and also between the transmitter circuit and the sub-antenna are 
high-frequency currents in a radio frequency band. Contacts of the 
switching means must be disposed at portions through which the 
high-frequency currents flow. In order to minimize a high-frequency 
current loss, an expensive switching means is needed, which will increase 
the manufacturing cost of the radiotelephone. 
DISCLOSURE OF THE INVENTION 
This invention was made in view of the foregoing problems, and is to 
provide an antenna device which is equipped with a retractable flexible 
antenna as a main antenna, is capable of maintaining a receiver function 
even when the flexible antenna is retracted, and can be manufactured at a 
low cost and is highly reliable in operation. 
An antenna device according to this invention comprises a main antenna 
slidably received in a tubular antenna case, a coil spring composed of a 
conductor and secured at one end to a base portion of the main antenna 
and, at an opposite end, to the antenna case, and holding means disposed 
on the base portion and engageable in pressure contact with an inner wall 
of the antenna case for holding the main antenna in position against 
displacement. 
With this construction, when the main antenna is retracted into the antenna 
case, the coil spring is contracted. The coil spring formed from a 
conductor has electric qualities of a coil. The inductance of the coil 
increases when the coil is contracted. Thus, the coil spring serves as a 
short antenna coil by means of which the necessary characteristic of the 
antenna can be maintained even when the main antenna is held in its 
retracted position. Consequently, the user is able to carry the 
radiotelephone while holding it in a pocket or the like, with the main 
antenna received in a body of the radiotelephone, and with a receiver 
function kept alive. The receive-only built-in antenna and the switching 
means described above are no longer needed, so that a high reliable 
antenna device can be obtained at a low cost.

BEST MODE FOR CARRYING OUT THE INVENTION 
An embodiment of this invention will be described below with reference to 
the accompanying drawings. 
FIG. 1 is a cross-sectional view of an antenna mounting portion of a 
cordless telephone having a retractive flexible antenna. In FIG. 1, 
numeral 1 is a flexible antenna composed of a spiral conductor 2 covered 
with a sheath 3. Numeral 4 is a slide member provided at a base of the 
flexible antenna 1. Numeral 5 is an antenna case constructed into a 
tubular form in which the flexible antenna 1 is slidably received and 
movable longitudinally in the direction indicated by the arrow A. 
FIG. 2 shows structural details of the slide member 4. Numeral 6 is a 
spring, 7 is a friction plate, and 8 is an iron ball. The iron ball 8 is 
disposed on one side of the slide member 4 while the friction plate 7 is 
disposed on the opposite side of the slide member 4. The spring 6 is 
received in a through-hole formed in the slide member 4 and urges the iron 
ball 8 and the friction plate 7 away from one another. The friction plate 
7 is, therefore, forced against an inner wall of the antenna case 5 to 
produce a friction force therebetween. 
Numeral 9 is one of plural holes formed in an inner wall of the antenna 
case 5. The holes 9 are arranged along the longitudinal direction of the 
antenna case 5. The iron ball 8 is urged toward the inner wall of the 
antenna case 5 so that the iron ball 8 can be fitted in each of the holes 
9. 
The spiral conductor 2 has a base portion formed straight. The straight 
base portion extends through the slide member 4 and has a front end 
constituting an electrode portion 10. 
Numeral 11 is a coil spring for urging the flexible antenna 1 inwardly, 
that is, in such a direction as to pull the flexible antenna 1 into the 
antenna case 5. One end of the coil spring 11 is secured by a screw 14 to 
the electrode portion 10 so that the coil spring 11 and the spiral 
conductor 2 are electrically connected together. Numeral 13 is a fastening 
metal disposed at an end of the antenna case 5. The opposite end of the 
coil spring 11 is secured by a screw 15 to the fastening metal 13 so that 
the coil spring 11 and the fastening metal 13 are electrically connected 
together. 
The above-mentioned part, namely a part composed of the flexible antenna 1, 
the slide member 4, the antenna case 5, the coil spring 11 and the 
fastening metal 13 is called an antenna unit 16. 
Numeral 17 is a printed circuit board having formed thereon a transmitter 
and a receiver circuit, etc. (not shown). Numeral 18 is a terminal pin 
formed integrally with the fastener metal 13. The terminal pin 18 is 
soldered with a conductor foil on a printed circuit board 17 so that it is 
electrically connected with the transmitter and receiver circuits formed 
on the printed circuit board 17. Bosses 19 projecting from the antenna 
case 5 are fitted with holes formed in the printed circuit board 17 so as 
to secure the antenna unit 16 to the printed circuit board 17. 
Hereinafter, a description will be given of the procedure for assembling 
the antenna unit. 
Firstly, the flexible antenna 1 is attached to the slide member 4. Then, 
the electrode portion 10 of the slide member is connected by the screw 14 
to the coil spring 11, and the fastening metal 13 is connected by the 
screw 15 to the coil spring 11. Thereafter, the flexible antenna 1 is 
inserted into the antenna case 5. In this instance, the iron ball 8, the 
spring 6 and the friction plate 7 are also inserted into the antenna case 
5 while they are held in an assembled condition within the hole in the 
slide member 4, as shown in FIG. 2. Subsequently, the coil spring 11 and 
the fastening metal 13 are inserted in sequence into the antenna case 5, 
and the fastening metal 13 is thereafter attached to the antenna case 5. 
Then, after the antenna case 5 with parts received therein is attached to 
the printed circuit board 17, the terminal pin 18 is soldered to a 
conductor foil on the printed circuit board 17. Finally, the printed 
circuit board 17 with the antenna unit 16 attached thereto is inserted 
into a cabinet 12. 
Hereinafter, a description will be given of the operation of the antenna 
unit mentioned above. The flexible antenna 1 is slidable in the direction 
of the arrow A within the antenna case 5, as described above. In response 
to the movement of the flexible antenna 1, the coil spring 11 undergoes 
axial expansion and contraction. The expandable and contractible coil 
spring 11 forms a part of the antenna. When the flexible antenna 1 is 
received in the cabinet 12, the coil spring 11 is contracted. The 
thus-contracted coil spring 11 means that the inductance of the coil 
spring 11 is increased. The coil spring 11 having such an larger 
inductance serves as a short antenna coil. 
POSSIBILITY OF INDUSTRIAL UTILIZATION 
As described above, since the coil spring serves as a short antenna coil, 
the impedance and the resonance frequency of the antenna side as viewed 
from the transmitting circuit do not change greatly even when the length 
of the antenna is shortened by retracting the flexible antenna 1 into the 
cabinet 12. The necessary characteristics of the antenna can, therefore, 
be maintained. As a consequence, radio waves can be caught or received 
even when the flexible antenna 1 is received in the cabinet 12. 
Furthermore, the slidable main antenna, namely the flexible antenna 1 is 
always connected to the transmitter and receiver circuits of the printed 
circuit board 17 via the coil spring 11. This arrangement obviates the 
intervention of a contact terminal and thereby eliminates an imperfect 
contact problem. 
When the receive mode is shifted to the transmission mode, the user pulls 
out the flexible antenna 1 while gripping the front end of the flexible 
antenna 1. The flexible antenna 1, as it is withdrawn from the antenna 
case 5, extends the coil spring 11 whereupon the coil spring 11 due to its 
resiliency creates a force tending to pull the flexible antenna 1 back 
into the antenna case 5. In this instance, the iron ball 8 fits in one of 
the holes 9 to lock the slide member 4 against movement relative to the 
antenna case 5, thereby holding the flexible antenna in an extended 
position. The friction plate 7 which is provided on the opposite side to 
the iron ball 8 is forced against the inside wall of the antenna case 5, 
generating an adequate friction between the friction plate 7 and the 
antenna case 5. 
During movement, the iron ball 8 makes a click each time when it snaps into 
one of the holes 9 in the antenna case 5. With the click stop thus 
provided, the flexible antenna 1 can be displaced with a relatively small 
force.