Note: Descriptions are shown in the official language in which they were submitted.
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A HEADPHONE
The invention relates to a headphone having a lead that
functions not only to supply audio signals to the headphone earpieces,
but also as an antenna for receiving radio frequency (RF) signals for a
receiver.
The use of a headphone lead as an antenna is not new.
Previously this has taken the simplest form for FM broadcast receivers
in which the headphone simply functions as a length of wire attached
to an RF input receivex. A disadvantage of such an arrangement is
that it is inefficient, since the relatively long length of wire (compared
with the dimensions of the receiver housing) makes for an inefficient
antenna dipole.
It is an object of the invention to overcome the above problems.
According to a first aspect of the present invention there is
provided a headphone having at least two wires, at least one of which
is an audio wire, said at least one audio wire being connected to a jack
fox input into a receiverfaudio unit, and said wires also being
connected to a common ground line, said ground line extends from the
jack such that it is connected to the casing of said receiver audio unit
when the.. jack is connected thereto so as to extend the length of, the
audio wire.
CONFIRMATION COPY
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In a portable FM broadcast receiver, the signal field strength is
so high, and the anticipated performance minimal enough, that the
antenna efficiency is not critical.
However the headphone arrangement of the antenna typically
comprises two ear phones each having similar configuration of audio
wires, so as to provide stereo sound. In such a case a preferred
embodiment comprises two headphones each connected to at least two
wires, at least one of which is an audio wire; each of said audio wires
being connected to a jack for input into a receiver/audio unit and at
least another of said wires is connected to a common transmission
ground line, said ground line extending from the jack such when
connected to an audio unit the length of the audio wire is extended.
Such a headphone arrangement is adapted to be plugged into an
adapter unit, which comprises a socket fox a jack connector for
connection to a receiver/audio unit, and connected to said jack
connector and extending therefrom first and second feeds each
comprising a wire connected to left and right headphones respectively,
and including a transmission line/screen lead/ground feed extending,
from the connector at a proximal end, partially along the length of the
left and right feed from said connector, and for each headphone, an
earth connection wire extending partially along the length of the audio
feeds to a common point connection, said common point connection
being connected to the distal end of said ground feed via an impedance
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and wherein each of said earth connections is connected to the
respective audio line via a capacitance.
The transmission line is preferably a co-axial cable or it may
S form, along with the other wires, a twisted three core lead.
The invention will now be described with reference to
background principles and examples, and with reference to the
following figures of which:
Figure ~ illustrates the background principles of the invention;
Figure 2 shows schematic and practical embodiments of the
invention;
1S
Figure 3 shows a preferred embodiment of the invention
showing a refinement of the figure 2 embodiment;
Figure 4 shows an alternative embodiment of the invention; and
Figure S illustrates the practical dimensions of leads according
to preferred embodiments of the invention.
Figure 1 a shows a simple representation of a simple dipole
2S antenna 1 housed in a housing 2 of a receiver transmitter 3 and with
radiation resistance R~t. The antenna 1 is fed directly by a self
contained (i.e. no power or ground connections) and electro-
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magnetically fully screened RF source of matched impedance. In this
example the antenna is referred to as a transmitter rather than operating
in a receiving mode and this facilitates explanation.
If an RF source is considered to be sufficiently small then its
housing 2 can be included in a lower part of the dipole as shown in
Figure 1b. The inventor has determined that by extending this
concept, the exact location of the RF source, within the lower half of
the dipole, is immaterial. The RF source can be relocated to the
bottom of the lower part of the dipole using a coaxial feed 4 (or other
extension of the housing) up the centre of the lower part of the dipole
with no change in electro-magnetic radiation behaviour of the antenna
system as shown in Figure lc. The effective electrical length of this
coaxial feeder part of the system is immaterial to the antenna function.
Figure 2a shows an adaptation of a standard headphone system
to an embodiment of the invention based on the principle of the
invention. Figure 2a shows a receiver unit 5 having left 6 and right 7
audio output being fed via a socket 8 to a stereo headphone jack 9
connected to left and right headphones 10 and 11 respectively via a
point where the lead divides from which audio/ground pairs continue
to each ear piece. Each of the headphones also has a ground
connection 12 which is connected to casing 13 of the receiver unit.
By breaking ground connection to each ear phone, and placing
RF coupling capacitors 14 and 15 (having low impedance at RF, but
high impedance at audio) across the audio/ground pair feeds to the ear
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phones they are made into effectively consolidated RF pairs. The
resulting lead arrangement is similar to the end fed dipole system of
figure 1 c with the exception that the upper half of the dipole consists
of two branches rather than one as shown in Figure 2b and 2c. Figure
5 2c shows a simple representation of the circuit of Figure 2b.
Figure 2d shows an alternative embodiment of the invention. As
before the arrangement comprises two ear phones each having a pair of
audio leads 16 a,b and 17 a,b. One of each of the pair of audio Ieads
I6a and 17a is connected to a jack which is shown in connection with
the receiver unit 18. The jack connection in the socket 19 is such that
the leads 16a and 17a are connected to left and right outputs from an
audio unit 20, each via a fi~rther impedance 21, 22. This impedance is
high at RF but Iow at audio frequencies. The leads are also connected
1 S to the receiver unit via capacitances 23 and 24, In this way the
headphones act as an antenna for input into the receiver 2S of the
receiver unit as well as the output from the audio unit.
Other leads to the headphones 16b and 17b, which may be
regarded as ground Ieads are truncated and connected together to a
common point "A". Leads 16a and 17a are also connected to this point
via capacitors 26 and 27. Leads I6b and 17b are further connected, via
inductor 28 (which is of high impedance at RF but low impedance at
audio frequencies) to the end of a coaxial cable 29. The coaxial cable
2S 29 is essentially a sheathing around the jack and is connected or
earthed to receiver unit housing 30.
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The impedance of a dipole is typically higher than the
commonly used SOS Rk' interface to the receiver. An impedance
transform, with a suitable two-element matching network, is preferred
to achieve an enhanced performance. This is implemented
S schematically in Figures 3a and 3b. A practical implementation is
shown in Figure 3c. The arrangement is similar to that of Figure 2c
except, that it includes for each earpiece, a parallel arrangement of a
capacitance 3I, and impedance 32. Dotted line in the Figure 3c
represents a matching circuit.
As far as the length of the wires (used as head set leads) is
concerned it is possible to extend their length to form a practical
headphone system for Digital Audio Broadcast (DAB) application.
The DAB VHF band is from I7SMHz to 239 MHz and is centred on
1 S 20~M~Iz which has a wavelength of 36cm. Whilst this length is ideal
for an individual ear-piece feeder (comprising a top half of a dipole), it
is too short to be of practical use, as the lower portion of a headphone
lead set includes the radio device itself. Pocket radio product headsets
typically have an overall antenna length in excess of lm.
An embodiment of the invention incorporates these practical
requirements and uses an asymmetrical dipole, in which upper half is
approximately ~,/4 but the lower half is closer to around 3~,/4. This is
shown in Figure 4. The resulting load impedance change in this design
2S step is simply absorbed in the choice of value of the matching
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components. By use of such matching as is appropriate, a wide range
of lead lengths can be accommodated.
As mentioned in the above embodiments a co-axial feeder is
S used as an effective extension of the casing. However the invention is
not limited to the use of a co-axial cable as the transmission (feed) line.
A further practical advantage of the invention is that it requires
relatively low cost components. Although the end-fed dipole antenna
classically uses a coaxial feeder, the principle of its operation does not
rely on this. As long as the feeder behaves as a transmission line at the
RF frequencies concerned, (which can be ascertained by measurement
of the loss per unit length of the line), then the field within the feeder
structure is "locally-contained" and the end-fed dipole principle
applies. Thin, flexible twin core coaxial lead is expensive and
I S alternatives may be used.
In a particularly preferred embodiment a twisted three-core lead
is used. 'The investor has determined by measurements of the
transmission line properties of a commercially available headphone
lead wire with twisted three core, that it gives a reasonably low loss
per unit length. The cable has a characteristic impedance approximate
to SOS2. An embodiment employing this cable is shown in Figure S.
Transmission feed line comprises a three-core lead, using one of the
cores as the RF "ground" and the other two to function collectively as
2S carrier of the RF signal. The principle of using a three-core lead as RF
feeder works equally well with signal and ground lines reversed but
the particular orientation chosen is most appropriate for integrating
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with additional functionality of a stereo audio connection. The other
two are the leads connecting the headphone to the jack. Apart from
this, remaining components are identical to those in Figure 3c. The
effectiveness of this arrangement was compared with that of a matched
dipole test antenna and a matched ideal coaxial-based end fed wire
dipole. The headphone antenna system and matched ideal coaxial
based end fed wire dipole gains were very close (within 1-2dB) and
both had a gain of -l OdB with respect to the test antenna in the centre
of the band, dropping to -l3dB the band edges.
The invention has been described by way of examples only and
variation may be made to the embodiments without departing from the
scope of the invention.
