Field of the Invention
[0001] The invention relates to an improved radio antenna for a vehicle and in particular
to a monopole antenna for a motor vehicle.
[0002] The majority of vehicles manufactured today are fitted with a radio antenna for reception
of FM radio broadcasts. The most common type of antenna is the monopole antenna. This
may be placed on the front or rear wing, on the A pillar or centre line of the car
roof. Such antenna may consist of a fixed length rod or be telescopic.
[0003] The length and orientation of a monopole antenna relative to the vehicle bodywork
play a critical part in determining the radio reception qualities in the FM band (87.5
- 108 MHz). The complex impedance of the antenna varies as a function of antenna length
(at a given frequency). In order for the antenna to perform efficiently ie to transfer
most of the signal power available from the antenna to the radio, the antenna impedance
must be matched to the input impedance of the radio to which it is connected. For
monopole antennas in general, near optimum impedance occurs when the physical length
of the antenna is approximately equal to one quarter of the wavelength of the desired
radio signal. This type of antenna provides good sensitivity and is referred to as
a quarter wave monopole, whose length is usually chosen to be a quarter wave at the
centre of the band (98 MHz) which works out to be approximately 0.75 metres.
[0004] However, in a number of modern car designs, especially where the antenna is to be
roof mounted, a quarter wave monopole antenna would be too large for the vehicle.
Not only would the length of the antenna itself be unwieldy, the longer an antenna
is the larger its diameter needs to be for structural support and rigidity. A large
diameter may detract from the appearance of the vehicle and increase the drag coefficient
and wind noise. Thus it is necessary in a number of vehicles to be able to produce
an efficient monopole antenna which is shorter than a quarter wave. This is known
as an electrically short antenna.
[0005] Prior art systems have been developed which use an additional electronic circuit
at the output of the antenna known as an impedance matching network. The purpose of
such a network is to maximise the transfer of power available from the antenna to
the receiver by matching the complex impedance of the antenna to the receiver input
impedance. If a perfect complex conjugate impedance match between the antenna and
radio can be achieved, and the matching network is loss less, then all the power available
from the antenna will be transferred to the receiver. Such a system would ensure that
the performance of an electrically short antenna would match that of the quarter wave
monopole. The matching network typically consists of a number of passive electrical
components such as capacitors and inductors in a suitable circuit configuration connected
to the output of the antenna. Due to the necessity to buy and then package additional
electrical component, any such matching network always adds to the total cost of an
electrically short antenna system.
[0006] A number of vehicle antennas have been proposed which include along the length of
the mast, a coil which acts as an inductor, electrically in series with the antenna
mast. Typical antennas of this type are described in DE 3931807 and US 4462033.
[0007] Glass mounted antennas for mobile telephones sometimes use the glass as a capacitive
coupling between the mast and the telephone. The antenna mast is coupled to a metal
plate which is mounted on the exterior surface of the glass, and a similar metal plate,
mounted on the interior surface of the glass is connected to the telephone.
[0008] There has also been recognition in some prior art antennas that a capacitor can be
provided by using the vehicle body as a plate of the capacitor coupled to ground.
One example of such an antenna is described in CA 950978 which is a non standard antenna
in the form of a horizontal rod mounted spaced from the rear of the vehicle where
the air between the rod and the vehicle body acts as the dielectric. An antenna base
which is capacitively mounted upon the body via a dielectric layer is also described
in W092/10865. In this system the antenna base and coaxial cable are arranged such
that the coaxial cable is capacitively coupled to ground via the dielectric layer.
Summary of the Invention
[0009] According to the invention there is provided a radio antenna for a vehicle, for mounting
upon the body of the vehicle and for electrical connection to a coaxial cable coupled
to the radio system of the vehicle, the antenna comprising a mast for receiving radio
signals, a base for mounting upon the exterior surface of the vehicle and means for
electrically coupling the base to the coaxial cable of the vehicle, the mast being
mounted on the base via an electrically conductive coil spring member, one end of
the mast being mounted upon and electrically connected with one end of the spring
member, the other end of the spring member being mounted upon and electrically connected
with the electrically conductive base, and the base including a mounting surface covered
by an insulating base layer for contacting the body of the vehicle and sealing the
gap between the base and the body such that when the antenna is mounted upon the vehicle
the electrically conductive base does not touch and is not in direct electrical contact
with the body of the vehicle, but there is a capacitive connection between the base
and the vehicle body via the insulating base layer.
[0010] In such a system the matching network is intrinsically built into the physical structure
of the antenna. No discrete electrical components are used in the design which means
that the total cost of the electrically short antenna is effectively reduced while
improving the radio reception capability.
[0011] The coil spring acts as an inductor to artificially increase the electrical length
of the antenna, while at the same time providing mechanical flexibility which helps
with car wash robustness and avoiding the effects of vandalism. The electrically conductive
base, the insulating base layer and the body of the vehicle together act as a capacitor.
The capacitance is obtained through the capacitive effect between the footprint of
the antenna base and the body work of the vehicle separated by non-conducting material.
The capacitor thus formed couples the base of the inductor to ground. However there
is still a direct connection between the inductor and the coaxial cable. Typically
such non-conducting material may comprise a non-water absorbent rubber grommet which
at the same time as acting as the dielectric for the capacitor also acts as a very
effective water seal.
[0012] Because the antenna base does not need to include any further electrical components
the base can be a substantially solid piece of conductive material. Advantageously
the base is die cast.
[0013] Alternatively the insulating base material may be plastic exhibiting properties of
non-compressibility and imperviousness to moisture whilst at the same time having
a time invariant dielectric constant and being physically stable over the typical
temperature range encountered in automotive applications.
[0014] Preferably however the insulating base layer comprises a central plastic piece and
a slightly thicker non water absorbent rubber section around the circumference of
the plastic thus providing a combination of non-compressibility provided by the plastic
and water seal from the rubber seal which compresses to the thickness of the plastic
part.
[0015] Preferably the spring coil member is of steel whose characteristics have been chosen
to give the required stiffness of spring and spring inductance. More preferably the
steel spring is copper plated to minimise the ohmic losses of the network.
[0016] In one example of an antenna in accordance with the invention, it has been found
that an inductance of approximately 290nH and capacitance of 10pF are suitable for
the matching network. It has been found that a spring having 8mm diameter and 18mm
long supplies the required inductance and the capacitance may be achieved using a
base footprint area of 200mm
2 and a dielectric seal lmm thick whose relative permativity is approximately 2.3.
[0017] Preferably the means for mounting the antenna to the exterior surface of the vehicle
comprises an electrically conductive screw passing through a cylindrical bore in the
exterior surface and being surrounded by an insulating sleeve such that the screw
is in contact with the base but insulated from the exterior panel and the insulating
base layer, the head of the screw being for connecting to the coaxial cable thus supplying
means to couple the base to the coaxial cable without providing a direct connection
between the base and the exterior surface.
[0018] In this way the screw acts as the means for coupling the base to the coaxial cable
but also acts as means for mounting the base on the vehicle.
Brief Description of the Drawings
[0019] An example of the radio antenna in accordance with the invention will now be described
and contrasted with the prior art, with reference to the accompanying drawings, in
which : -
Figure 1 is a schematic circuit configuration of an antenna of the prior art; Figure
2 is a graph of mismatch loss versus frequency of the antenna of the prior art with
and without a matching network;
Figure 3 is a schematic circuit configuration of an example of a radio antenna in
accordance with the invention;
Figure 4 is a schematic section through an antenna in accordance with the invention;
Figure 5 is a plan view of part of the antenna as shown in Figure 4;
Figure 6 is a graph of mismatch loss against frequency of the antenna shown in Figure
4 compared with an antenna of the prior art; and,
Figure 7 is a graph of FM tuner audio signal to noise ratio against radio frequency
input level.
Description of the Preferred Embodiment
[0020] The antenna of the prior art is not illustrated. It comprises a roof mounted electrically
short monopole antenna which is 0.48 metres long having an impedance matching network
illustrated in Figure 1 packaged in the base of the antenna. The matching network
consists of only one component, an inductor coil 1 which is coupled between the antenna
3 and coaxial cable 5. The coaxial cable 5 is coupled to the radio system of the vehicle
and is also connected to earth 7. The graph of figure 2 illustrates the measured data
of mismatch loss versus frequency of this antenna with and without the matching network.
Mismatch loss is a measurable expressed in decibels (dB) which quantifies the degradation
of power transfer from the antenna into a specified load impedance (75 ohms in this
case) due to differences between antenna and load impedances. Maximum power transfer
occurs when the antenna impedance is equal to the complex conjugate of the load impedance
and this condition is represented by a mismatch loss of zero decibels. As can be seen
from figure 2 the matching network employed in the antenna of the prior art significantly
reduces the mismatch loss of the antenna without the matching network.
[0021] Figure 4 illustrates an example of a radio antenna in accordance with the invention,
while figure 3 illustrates the equivalent electric circuit. The antenna 9 is for mounting
upon an exterior surface 11 of the vehicle for electrical connection to a coaxial
cable 13 coupled to the radio system (not shown) of the vehicle. The antenna 9 comprises
a mast 15 for receiving radio signals and a base 17 for mounting upon the exterior
surface 11 of the vehicle. Means 19 electrically and mechanically couple the base
17 to the coaxial cable 13 of the vehicle. The mast 15 is mounted upon the base 17
via an electrically conductive coil spring 21, one end 23 of the mast being mounted
upon and electrically connected with one end 25 of the spring member. The other end
27 of the coil spring member is mounted upon and electrically connected with the electrically
conductive base 17. The base 17 includes a mounting surface 29 covered by an insulating
base layer 31 for contacting the exterior surface 11 of the vehicle such that when
the antenna is mounted upon the vehicle as shown in figure 4 the electrically conductive
base 17 is not in direct electrical contact with the exterior surface 11 of the vehicle.
[0022] At the end 23 of the mast 15 is a metallic disc 33 of the same diameter as the coil
spring 21. At the other end 27 of the coil spring is an internally threaded metallic
member 35. The metal disc 33 spring 21 and metallic member 35 all sit within annular
sleeve 37 which is flexible and plastic. Around this is non water absorbent rubber
cover 39. The internally threaded member 35 mates with threaded stud 41 which extends
from base 17 and is externally screw threaded. A plastic cover 43 covers the base
17. In this case the base 17 is part spherical with a semi-spherical cut-out 45. Because
this is a simple solid without requiring mounting of any further electrical components
the base 17 is die cast.
[0023] The antenna base 17 is secured to the panel 11 by screw 19 which is electrically
conductive. The screw 19 sits within insulating nut 47 which includes a cylindrical
bore for accommodating the screw 19. The screw 19 passes through bore 49 in panel
11 through the semi-spherical cut-out 45 and screws into 17. The semi-spherical cut-out
45 provides tolerance for mounting the screw into the base. The surface 29 of the
base 17 can be seen clearly in figure 5.
[0024] An auxiliary connection 51 couples the panel 11 to co-axial cable 13. This provides
a simple but effective antenna impedance matching network which is equivalent to that
depicted in figure 3 but is built into the physical structure of the antenna avoiding
the cost of using discrete electrical components. It should be noted that details
of the attachment method of the antenna base to the coaxial cable are not described
in detail here since they are well known and apparent to the skilled addressee of
the specification.
[0025] The required values of inductance and capacitance for the correct operation of the
matching network are obtained through careful design of the physical parameters of
the spring 21:
Length 1
Number of turns N.
Diameter d
and the physical parameters of the antenna base 17; Footprint surface area (A)
Distance to body work (S);
and the permativity or dielectric constant of the rubber insulating material( εr )
Suitable inductance and capacitance values for the matching network have been calculated
which are approximately 290 nH and 10pF respectively. To achieve the required inductance
the spring 21 is approximately 80mm long and 8mm diameter. The required capacitance
was achieved using a base footprint area of 200mm2 and a non water absorbent rubber seal lmm thick. The relative permativity of the
rubber seal is estimated to be approximately 2.3.
[0026] Figure 6 illustrates the measured mismatch loss of this prototype antenna compared
to that of the prior art in which a significant improvement can be observed. The antenna
described does not have its lowest point of mismatch loss exactly at mid-band position
indicating that minor adjustments to the spring inductance and base capacitance may
be needed. However it shows a 2.55 to 2.0DB improvement over the bottom half of the
FM band (87.5-98 MHz) and 2.0 to 0.75 dB improvement over the top half of the FM band
(98-108 MHz). At the point of minimum mismatch loss shows a 2.3 dB improvement over
the current antenna design. Mismatch loss reduction is not the only benefit offered
in terms of perceived radio reception quality. A further parameter associated with
reception quality is audio signal to noise ratio (S/N) at the output of the radio
and figure 6shows a typical graph of FM tuner audio S/N versus radio frequency RF
input level. The graph has two sections. The first section of the graph has a slope
of 3.5 while the second section has a slope of 1. The point of inflexion is known
as the FM threshold point which occurs when the carrier to noise ratio of the FM signal
reaches about 10, a condition which typically occurs at about 99 dBm or 2.42 uV RF
signal level. Below threshold there is a rapid rise in audio signal to noise ratio
as the radio frequency level increases. The rate of signal to noise improvement is
approximately 3.5 dB for every 1 dB increase in RF signal level. Beyond threshold
the rate of signal to noise improvement reduces to 1 dB for every 1 dB increase in
RF signal level. The graph indicates that under weak signal conditions (below threshold
and typically in the signal range 1-2.5 uV) the correct impedance matching between
satisfaction and can represent the difference between an antenna and radio is of critical
importance to ensure higher audio signal to noise ratios and hence better reception
quality. For every dB of mismatch loss improvement one can expect an increase of 3.5
dB in audio signal to noise.
Comparing the current antenna to the prior art the invention enables the following
audio S/N improvement to be achieved over 1-2.5 uV RF input level range:
8.7-7 dB improvement over bottom half of the FM band 7-2.6 dB improvement over top
half of the FM band. Under such weak signal conditions the signal to noise improvement
can contribute a significant amount to customer unlistenable and acceptable radio
signal. Other benefits of improved impedance matching at stronger signal levels (above
threshold) are that the effects of multipath nulls which manifest themselves as audio
spits are reduced since the radio is capable of recovering more of the signal out
of the nulls.
It will be appreciated by the skilled addressee of this specification that the physical
characteristics of the spring and the antenna base can be chosen to give the required
inductance and capacitance or the antenna impedance matching circuit and also the
required mechanical properties of the antenna.
1. A radio antenna for a vehicle, for mounting upon an exterior surface of the vehicle
and for electrical connection to a coaxial cable coupled to the radio system of the
vehicle, the antenna comprising a mast for receiving radio signals, a base for mounting
upon the exterior surface of the vehicle and means for electrically coupling the base
to the coaxial cable of the vehicle, the mast being mounted on the base via an electrically
conductive coil spring member, one end of the mast being mounted upon and electrically
connected with one end of the spring member, the other end of the spring member being
mounted upon and electrically connected with the electrically conductive base, and
the base including a mounting surface covered by an insulating base layer for contacting
the exterior surface of the vehicle such that when the antenna is mounted upon the
vehicle the electrically conductive base does not touch and is not in direct electrical
contact with the exterior surface of the vehicle.
2. A radio antenna according to claim 1, in which the insulating base layer comprises
a rubber grommet.
3. A radio antenna according to claim 1, in which the insulating base layer is a layer
of substantially non compressible plastics material, which is substantially non water
absorbent and a substantially stable dielectric constant.
4. A radio antenna according to claim 1, in which the insulating base layer comprises
an inner region of plastics and an outer region of rubber, the outer region surrounding
and abutting the plastics inner region.
5. A radio antenna according to any one of the preceding claims, in which the coil spring
member is made of steel.
6. A radio antenna according to claim 5, in which, the steel spring is copper plated.
7. A radio antenna according to any one of the preceding claims in which the means for
mounting the antenna to the exterior surface of the vehicle comprises an electrically
conductive screw for passing through a cylindrical bore in the exterior surface and
being surrounded by an insulating sleeve such that the screw is in contact with the
base but insulated from the exterior panel and the insulating base layer, the head
of the screw being for connecting to the coaxial cable thus supplying means to couple
the base to the coaxial cable without providing a direct electrical connection between
the base and the exterior surface.
8. A radio antenna according to any one of the preceding claims, in which the spring
member and the antenna base insulating base layer and vehicle surface together form
the sole components of the impedance matching circuit of the antenna.
9. A radio antenna arranged substantially as herein described, with reference to and
as illustrated in figures 3 to 6 of the accompanying drawings.