Rectenna

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Inductive antenna for supplying energy to the transponder in an RFID

A circuit arrangement which receives high-frequency electromagnetic waves and then converts them into a direct voltage is referred to as rectenna (from the English word rect ifying antenna , rectifying antenna) . The purpose is to supply downstream electronic circuits with energy. The big advantage here is that no additional power supply - such as a battery or a connection to the mains - is required.

Historical reference

In the early days of broadcast technology, detector receivers were widespread. These consisted of an antenna , often also an oscillating circuit , a rectifier and high-impedance and sensitive headphones . These headphones got their energy directly from the radio signal received. This meant that no external power supply was necessary and radio reception was possible even before the development of the first amplifier tubes.

In the 1960s there were special transistor self-sufficient receiver circuits for long, medium and short waves, which also obtained the energy for operating a simple transistor amplifier from the antenna; At least in Germany, however, the operation was prohibited, since the broadcast reception permit only allowed the use of the modulation, but not (except for pure detector receivers) the transmission energy. The ban was issued after it was found that residents in the vicinity of powerful transmitters operated their fluorescent lamps using simple wire antennas with transmitting energy.

Current developments

Sufficient voltage must be made available to supply an electronic circuit . In order to operate transistors and possibly also integrated circuits, voltages of at least 0.5 V or more are required. Furthermore, a sufficient amount of electrical charge (current intensity times time) must be made available.

However , the services received are usually only very small. An already quite high received power of −60  dBm - corresponding to 1 nW - must therefore be temporarily stored for a long time (4.2 hours in the following example) until an amount of energy would be available that would be sufficient to power an electronic circuit e.g. Operate at 1 mA and 1.5 V (i.e. 1.5 mW) for 10 ms.

This results in the requirement to design circuits which have a high overall efficiency and which consume the highest possible overall power. Antennas that cover a larger area are initially useful for this purpose, and possibly also an interconnection of several antennas. Usually, the aim is to receive a certain frequency band, for this purpose it makes sense to operate the antenna in resonance, i. H. to match this frequency range.

Usual frequency ranges

In order to keep the antenna dimensions small and compact, it makes sense to work with higher frequencies. B. the 2.4 GHz ISM band , in which transmitters are also available at low cost. However, the 868 MHz SRD band is also of interest, especially because more powerful transmitters up to 500 mW are permitted here in Germany.

Need for a supply transmitter

Today, high-frequency energy fields from a wide variety of transmitters can usually be found indoors and, even more, in the open air. However, the available services in the area of ​​the receiver are so small that it is not possible to supply external electronics. This even applies if electronics are only to be activated for a short time and waiting times of several days are accepted.

Instead, it is necessary to provide an external transmitter that provides sufficient power at the receiving location. According to the current state of development, a value of around −20 to −30 dBm should be mentioned as the minimum required reception power. Even then, however, only output voltages in the double-digit millivolt range can be realized directly.

It should be noted that the supply transmitter may only have an output power of 0 to 27 dBm, depending on the frequency band used. Thus there is a link budget of 20 to max. 60 db available, which is only sufficient to bridge short distances, which max. are in the range of a few meters.

Circuit implementation

In the simplest case, a dipole is used - tuned and aligned with the supply transmitter. The high-frequency signal is then rectified with a Schottky diode , because this has a particularly low threshold voltage of only about 0.2 V.

More sophisticated antennas are implemented as patch antennas on a PCB substrate . Numerous other antenna shapes are also conceivable; resonant magnetic antennas can also be useful. These can also be implemented on a PCB carrier at correspondingly high frequencies. They are then matched to the corresponding transmission frequency of the supply transmitter using a capacitor.

The intermediate energy storage is then carried out with a capacitor . In principle, the received voltage will always fluctuate, so further voltage stabilization may be necessary, but this requires a minimum voltage. The use of voltage multiplier circuits is also conceivable in order to increase the output voltage. Here, too, it should be noted that a minimum voltage is required for their operation.

Differentiation from other procedures

The method for high-frequency energy supply specified here is particularly suitable for higher frequencies and offers only a low output power available if the receiver is not in the vicinity of a very powerful transmitter.

On the other hand, an inductive method for energy transmission is available for lower frequencies and higher transmission capacities. Even with a loose inductive coupling , devices in the range of a few milliwatts can be permanently supplied over several meters.

However, correspondingly high magnetic fields occur and large coils are necessary which must completely enclose the space to be supplied.