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antenna. Correspondingly, the receiver, i.e. rectenna is composed of an antenna element, an AC-to-DC
conversion circuit, and a load. Data transmitter/receiver circuitry can be included into the WPT system if a
data link needs to be established.

A common problem for all applications utilising WPT is that due to the limitations in the transmitted
power allowed according to regulations, the received power is relatively low. As a result, the conversion
efficiency of the rectenna is low, because rectifier diodes operate more efficiently at higher input power
levels. In addition, the received power varies if the distance or relative orientation in respect to the trans-
mitter changes. A CP rectenna operating in two frequency bands is demonstrated here in order to diminish
some of these disadvantages. Circular polarisation enables nearly constant output independent of the rota-
tional angle of the rectenna, whereas dual-band operation increases the operational diversity of the rec-
tenna. The structure of the proposed rectenna is first explained and the measurement results for dual-band
operation are then represented. Some of the performance issues are also discussed and the effect of a sim-
ple electromagnetic band-gap (EBG) structure on the antenna performance is finally demonstrated.

DUAL-BAND CP RECTENNA

Layout and cross-section of the rectenna is shown in Figure 1. The three sections, i.e. antenna, high band
rectifier, and low band rectifier, were first designed and measured separately and then combined to form
the complete rectenna. Long dotted divider lines in Figure 1 indicate the interface between the antenna
and rectifier circuits.

Structure
The dual-band CP antenna was formed as a combination of two shorted annular ring-slot structures de-
signed for operation at 2.45 GHz (low band) and 5.8 GHz (high band). The short section of the low band
structure (angle aX) provides a continuous ground to the feed of the high band antenna, whereas the short
section of the high band structure (angle a2) provides a continuous ground to the feed of the low band an-
tenna. The feed for the high band antenna was composed of a transmission line 7X1 and quarter-wave
transformers 7X2 and 7X3, which were also utilized to match the input impedance of the antenna into
50£l Correspondingly the feed for the low-band antenna was composed of a transmission line TL6 and a
quarter-wave transformer 7X7.
Rectifiers were composed of HSMS-2862 microwave Si Schottky detector diode pair D, a bypass/storage
capacitor C, a load resistor 7? and a choke inductor X. The diode pair was connected as a voltage-doubler
circuit in order to maximise the output voltage. The choke inductor reduces the effect of output DC volt-
age measurement wires on the performance of the rectifier. The impedance matching circuit of the high
band rectifier was composed of transmission lines 7X4 and 7X5 and two sections of an open stub (Stub\
in Figure 1) between them. Equally, the impedance matching circuit of the low band rectifier was com-
posed of transmission lines 7X8 and 7X9 and two sections of an open stub (Stub!) between them.

Performance
Measured return loss of both high band and low band antenna and rectifier is represented in Figure 2. All
measurements were performed using -5 dBm input power. Good impedance match between antenna and
rectifier can be observed in both frequency bands. A second resonant frequency between 6 GHz and 7
GHz for both low band antenna and rectifier can also be noticed.