ANTENNA SYSTEM

FIELD OF THE INVENTION

The invention relates to an antenna system and to a transmitter comprising such an antenna system.

BACKGROUND OF THE INVENTION

Antenna systems are essential parts of any transmitting device as a broadcast transmitter, mobile phone, iPod, etc. They provide a communication port with the outside world by either transmitting information or receiving information or both. Normally the antenna systems are optimized for maximum power delivery and maximum efficiency. However, when it is desired to transmit audio content to a FM receiver within a short range e.g. several meters in a standard broadcasting range as 88 - 108 MHz it is important that the transmission power is very low as e.g. 11 nW in US.

Throughout the present application we consider that the term "small antenna" refers to an antenna having its physical dimensions much smaller that the wavelength of the signal transmitted via said antenna i.e. the carrier. Let us observe that in known broadcasting systems the antenna dimensions is a fraction of the wavelength of the carrier e.g.1/4, and that in this application we refer to antenna as small as 1/10 or less of the wavelength of the carrier. The problem with small antennas is that they have a very low radiation resistance, which determines in practice a very low efficiency. For example a full size dipole antenna having a length of two times 750 mm and operating at 95 MHz with 50-ohm source impedance has an efficiency of 95 % and a source voltage of 20OmV will generate a maximum effective radiated power 100 μW. If we consider a short dipole of two times 50 mm, loaded with coils, we obtain an efficiency of 0.32 % depending on the quality factor of the antenna and the position of the coils, the antenna generating an effective radiated power of 250 nW. The radiation resistance for the full size dipole is 70 ohms while the radiation resistance for the short dipole is 0.0052 ohm. Since some kind of electrically loading is required for the short antenna, the antenna becomes narrowband. This is the reason why this kind of antenna radiates a different amount of power at different frequencies.

If this problem is not solved the received signal strength by a FM radio for frequencies besides the resonance frequency is too low for good quality reception and noise will be introduced in the audio signal or it can happen that the received signal will not be detected by the FM radio.

SUMMARY OF THE INVENTION

Hence, it is a need to provide an antenna system suitable for low range and low power communication, which is integrable in a single transmission device. The invention is defined by the independent claims. Dependent claims define advantageous embodiments. This scope is achieved in an antenna system for transmitting a RF signal comprising an antenna feeding circuit and a small antenna coupled to the feeding circuit. The system has a first resonant circuit at a first resonant frequency inside a first frequency band and at least a second resonant circuit at a second resonant frequency outside said first frequency band for providing the RF signal having a substantial same power in said first frequency band. The word substantial refers to signals differing ±1.5 db from each other.

The first resonant circuit improves the transmission properties of the system inside the frequency band. The second resonant circuit improves the increase of the current through the antenna inside the first frequency band where there is a degradation of the power of the RF signal. The resonant circuits may share some of their components and may provide different types of resonance e.g. series or parallel.

In an embodiment of the invention the small antenna is a loop antenna. In this case the antenna behaves more like an inductor and in order to radiate a relatively constant output power i.e. compensating the degradation of the radiated RF power by the antenna system, a solution may be lowering the current through antenna as the transmitted frequency increases.

Alternatively, in another embodiment the small antenna is a capacitive antenna. The capacitive antenna behaves like a capacitor and in order to radiate a relatively constant output power i.e. compensating the degradation of the radiated RF power by the antenna system, a solution may be lowering the current through antenna as the transmitted frequency increases.

Preferably the feeding circuit comprises inductors and capacitors as they are relatively easy to be integrated in a single chip when the RF frequencies lie in tenths of MHz.

In an embodiment, when the small antenna is a loop antenna the feeding system comprises a first capacitor in parallel with the loop antenna for determining the first

resonant frequency. The first capacitor is further coupled in series with a second capacitor for determining the second resonant frequency.

In another embodiment, when the small antenna is a capacitive antenna the feeding system comprises a first capacitor in series with a second capacitor. This series combination is in parallel with an inductor, the inductor being in parallel with the capacitive antenna. The inductor, the first capacitor and the capacitive antenna determines the second resonance frequency and the inductor, the first capacitor, the second capacitor and the capacitive antenna determines the first resonant frequency.

In order to avoid interferences with other transmitters, which may be in the same device comprising the antenna system as e.g. a portable phone, the wherein the feeding circuit comprises band - reject filters having their resonant frequencies in the frequency bands allocated to higher wireless frequency standards.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other advantages will be apparent from the exemplary description of the accompanying drawings in which:

Fig.l depicts a block diagram of an antenna system as claimed in present invention;

Fig. 2 depicts a loop antenna; Fig. 3 depicts the necessary current for a loop antenna depending on frequency;

Fig. 4 depicts a feeding circuit for a loop antenna according to an embodiment of the invention;

Fig. 5 depicts another feeding circuit for a loop antenna according to another embodiment of the invention;

Fig. 6 depicts a capacitive antenna;

Fig. 7 depicts a feeding circuit for a capacitive antenna according to an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 depicts a block diagram of an antenna system as claimed in present invention.

The antenna system for transmitting a RF signal comprises an antenna feeding circuit 10 and a small antenna 20 coupled to the feeding circuit 10. The system has a first

resonant circuit at a first resonant frequency inside a first frequency band and at least a second resonant circuit at a second resonant frequency outside said first frequency band for providing the RF signal having a substantial same power in said first frequency band. The first frequency band could be e.g. 88 - 108 MHz. The first resonant circuit improves the transmission properties of the system inside the first frequency band. The second resonant circuit improves the increase of the current through the antenna inside the first frequency band where there is a degradation of the power of the RF signal. The resonant circuits may share some of their components and may provide different types of resonance e.g. series or parallel. A suitable antenna for such an application is a loop antenna as shown in Fig. 2.

The loop antenna has a length and a width, which are substantially smaller that the wavelength of the RF signal generated by the antenna. In Fig. 3 it is shown that if it is necessary to obtain a transmission system having a constant output field it is necessary to supply the loop antenna with a variable current i.e. the lower the frequency the larger the current. In order to equalize the dependency shown in Fig. 3, it is necessary that the current through the antenna is decreasing with a frequency of the RF signal.

A suitable circuit for this is shown in Fig. 4. Fig. 4 shows the circuit that gives a solution to the problem. Ll is the small loop antenna of Fig. 2, which is tuned inside the frequency band with Cl i.e. they determine the first resonant frequency. The small loop antenna Ll in combination with C2 is the second resonant circuit having the purpose of increasing the current in the lower band frequencies. The second resonance frequency, defined by Ll and C2, can be chosen in that way that the resulting current curve has a shape that compensates for the shape of the E-field degradation due to the frequency performance of the small loop antenna Ll. Additionally, the circuit may comprise the feeding circuit comprises band - reject filters having their resonant frequencies in the frequency bands allocated to higher wireless frequency standards, as shown in Fig. 5. Signals situated in the higher frequency bands may influence the sensitivity of the transmitter and may produce interferences with a transmitter comprising such an antenna as a GSM telephone. In Fig. 5 two frequency bands are suppressed i.e. one determined by L2 and C3 and another determined by L3 and C4. These frequency bands might be e.g. 800 to 900 MHz and 1700 to 1900 MHz. Additional frequency bands can be suppressed if more LC tank circuits are provided.

If we consider a capacitive antenna as shown in Fig. 6, then the feeding circuit may be as the circuit shown in Fig. 7. Fig. 6 shows an example of a short capacitive antenna

element. The size is small compared with the wavelength. One side is the feeding point. The other side is open and thus not connected. The input power is supplied between the feeding point of the short capacitive antenna and the ground of the electronic circuit.

For the capacitive antenna, the current necessary at lower frequencies is higher than that necessary at higher frequencies in order to provide a quasi-constant emitted power.

To cope with this problem a method is used to equalize the radiated electromagnetic field by implementing a small capacitive antenna in combination with a feeding network that increases the supply current for the lower frequencies and decreases the supply current with increasing frequency as shown in Fig. 7. Fig. 7 shows the feeding network that provides an equalization of the electromagnetic field. There is provided a second resonance, outside the frequency band of interest and a first resonance, somewhere in the frequency band of interest. L, Cl and the antenna capacitance Ca mainly define the second resonance, while L, Ca, Cl and C2 mainly define the first resonance. The second resonance is chosen below the frequency band so that there is an increase of antenna current, the current through Ca. Resistor R has the purpose to lower the quality factor at the second resonance. In this way the shape of the decreasing current in function of frequency (in the frequency band of interest) can compensate better for the frequency dependence of the small capacitive antenna.

Such a combination of short antenna and feeding network can be easily integrated into a mobile phone. For example the short antenna can be a printed conducting track on a Flex material, which is very thin, and fixed at the inner side of the plastic cover. In this way the antenna occupies no additional volume.

Additional parallel circuits can be inserted between the supply source and the supply network for rejecting unwanted signals that are available at the output of the power amplifier. This is required to prevent that the spurious signals from the power amplifier interferes with the mobile phone's receiver and other present receiver functions like for example the GPS receiver.

It is remarked that the scope of protection of the invention is not restricted to the embodiments described herein. Neither is the scope of protection of the invention restricted by the reference numerals in the claims. The word "comprising" does not exclude other parts than those mentioned in the claims. The word "a(n)" preceding an element does not exclude a plurality of those elements. Means forming part of the invention may both be implemented in the form of dedicated hardware or in the form of a programmed purpose processor. The invention resides in each new feature or combination of features.