Rectenna

A rectenna ( rectifying antenna from the English language ), a circuit arrangement called which receives high frequency electromagnetic waves and then converted into a DC voltage. The purpose is to provide downstream electronics circuits with energy. The big advantage of this is that you no more power - like a battery or a connection to the power grid - needed.

Historical reference

In the early days of broadcasting technology detector receivers were widespread. These consisted of an antenna, often a resonant circuit, a rectifier and a high-resistance and sensitive headphones. These headphones have received his power directly from the received broadcast signal. Thus, no external power supply was necessary, and a radio receiver was already possible before the development of the first amplifier tubes.

In the 1960s, there were special transistor - self- receiver circuits for long, medium and short waves, which also covered the energy for the operation of a simple transistor amplifier from the antenna; at least in Germany the operation, however, was prohibited because the broadcast receiving permit only the exploitation of the modulation, but not (except with pure detector receivers ) allowed the transmission energy. The ban was enacted after it was found that residents operated in the vicinity of active transmitters their lamps using simple wire antennas with transmit energy.

Recent Developments

To provide an electronic circuit, a sufficient voltage must be provided. In order to operate the transistors and possibly integrated circuits, voltages of at least 0.5 V or more is required. Furthermore, a sufficient amount of electric charge ( current times time ) must be provided.

However, the benefits received are usually only quite small. An already quite high received power of -60 dBm - corresponding to 1 nW - thus a long time must (4.2 hours in the following example ) are buffered until an amount of energy would be available that would be sufficient, an electronic circuit, for example, 10 ms to operate with 1 mA and 1.5 V (ie, 1.5 mW).

This results in the requirement to design circuits that have a high overall efficiency and record the highest overall performance. For this purpose antennas are initially useful, which cover a larger area, possibly an interconnection of multiple antennas. Typically, the reception of a specific frequency band is desired, this it is useful to operate the antenna to resonate, i.e., tune to this frequency range.

Usual frequency ranges

To keep the antenna size small and compact, it is useful to operate at higher frequencies, is conceivable, for example, the 2.4 GHz ISM band in which transmitters are also available according to cost. However, the 868 MHz ISM band are interesting, especially because here even more powerful transmitter up to 500 mW are allowed in Germany.

The longer term, even the 24 GHz ISM band and this is very interesting, currently the corresponding transmitter modules, however, are still quite expensive. The advantages are here but very small receiving antennas and an allowable transmit power of 500 mW to call.

Need for a supply station

Usually found today in interiors and even more in the free field everywhere frequency energy fields of diverse channels. The existing services in the area of the receiver, however, are so small that a supply external electronics is not possible. This is true even if an electronics should only be activated briefly and it takes several days for waiting times.

Instead, it is necessary to provide an external transmitter, which provides sufficient power available at the receiver. As a minimum required received power is to call on the current development status, a value of about -20 to -30 dBm. Even then, however, only output voltages in the tens of millivolt range are directly realizable.

It should be noted that the supply channel must have only one power output 0-27 dBm, depending on the frequency band used. Thus, a link budget of 20 is up to max. 60 db available, which is sufficient only to cover short distances, which max. are in the range of a few meters.

Technical circuit realization

In the simplest case one uses a dipole - matched and aligned with the source transmitter. The high frequency signal is then rectified with a Schottky diode, because it has a particularly low threshold voltage of only about 0.2 volts.

More sophisticated antennas are implemented as a patch antenna on a PCB substrate. Also conceivable are numerous other forms of antenna, also resonant magnetic antennas can be useful. These can be implemented on a PCB carrier with correspondingly high frequencies. Their vote is then performed with a capacitor to the corresponding transmission frequency of the power transmitter.

The intermediate storage of energy is then carried to a condenser. Basically, the receiving voltage will always fluctuate, so may need a wider voltage stabilization, but this requires a minimum voltage. Furthermore, the use of voltage multiplier circuits, it is conceivable to increase the output voltage. Here, too, it should be noted that a minimum voltage for their operation is required.

Demarcation from other methods

The method given here to high frequency power supply is preferably suitable for higher frequencies, and offers only a low output power available if the recipient is not near a very strong transmitter.

For lower frequencies and higher transmission services, in contrast, offers an inductive method for energy transfer. Also by a loose inductive coupling can be several meters or devices in the range of a few milliwatts permanent supply.

There are however while correspondingly high magnetic fields and there are large spools necessary, which must completely enclose the area to be served.

• Wireless technology