IEEE Microwave Magazine - May 2017 - 128

TABLE 1. A comparison of NF-WPT and FF-WPT.
NF-WPT
Resonant

Nonresonant

Transmission mechanism

Coupling, no wave propagation

Coupling, no wave propagation

Wave propagation

Interacting device

Coils/electrodes

Coils/electrodes

Antennas

Tx-Rx antenna interaction

Strong interaction

Medium interaction

No interaction

Operating frequency

LF, HF

LF, HF

Microwave, millimeter-wave

Power level

Medium (mW-W), High (kW)

Medium (mW-W), High (kW)

Ultralow (nW-mW) High (MW)

Efficiency

High (70-90%)

Medium (30-60%)

Low (10-50%)

Commercial applications

Yes

Yes

No

between colocated coils/wires or electrodes/metallic
plates, respectively. Being in the reactive field region,
the power delivered to the field by the Tx coil/plate
returns back in cases where the Rx coil/plate is absent
and thus depends on the Rx coil/plate's position. This
dependency is extremely strong in the case of resonant
NF-WPT, where a narrow-band resonance is established between the transmitter and receiver [26]. Consequently, several solutions are proposed to minimize/
cancel this dependency [26], [29], [30]. The reciprocal
position (including link coverage) is less critical in the
nonresonant NF-WPT transfer mechanism. Here, the
(typically) coils are different in size (the transmitter is
larger), and, being far from resonance, the Tx/Rx input
impedances are almost constant by varying (within
certain limits) their reciprocal position; thus, satisfying matching conditions can be easily guaranteed.
With regard to commercial research applications,
the far-field approach is mainly deployed when low
(FF-WPT) or ultralow (EH) power applications are envisioned. Only in space and military targets, where cost
is not an issue, do high-power FF-WPT activities exist
[31], [32]. NF-WPT has, up to now, more commercial
applications than FF-WPT. Nonresonant NF-WPT has
been widely applied for many years in HF RFID, where
medium to high power levels are involved; resonant
NF-WPT can also manage high power levels, mainly
due to a more efficient link (thanks to the resonance)
with reduced costs as well as less stringent safety exposure limits at lower frequencies (<100 MHz).
The classification described previously is expanded
by a more recent wireless transfer mechanism resorting to the middle field provided by microwave sources.
This technique is mainly seen as an alternative to NFWPT for wirelessly powering implanted devices [33].
Unlike conventional near-field approaches, mid-field
WPT (MF-WPT) relies on an actual field propagation
and does not suffer from rapid decay with distance [as
(distance)−3], which is the case of the far field. Here,
environmental/tissue losses play a fundamental role;
for this reason, intensive investigations on the optimal
frequency for maximizing the power transfer have

128

FF-WPT

been carried out, providing the interesting result that
the optimal frequency is in the gigahertz range for
a millimeter-sized Tx antenna and shifts to the subgigahertz range for a centimeter-sized Tx antenna [34].
Based on all this, it become clear that defining figures of merit capable of summarizing wireless link
quality is essential. Despite of the significant differences among these approaches, the wireless link building blocks can, in all cases, be depicted as in Figure 3:
planar coils and planar microstrip antennas are representative of the two transmission mechanisms, but
capacitively coupled links or other antenna types can
be adopted equivalently.
As a consequence, in terms of efficiencies, all wireless powering mechanisms share the main figures of
merit as demonstrated by
P
h LINK = h DC - TF $ h TF - TF $ h TF - DC = PTX $ PRX $ DC . (1)
PBIAS PTX PRX
In fact, h LINK represents the efficiency of the entire
wireless power system from the transmitter dc bias to
the receiver dc output. The first term, h DC - TF, is the
ratio between the power PTX at the transmission frequency used in the energy transmission (microwave or
millimeter-wave for cases of the far and middle fields,
and LF or HF in the case of the near field) available
at the Tx antenna/coil input port and the dc power
required at the transmitter side ^PBIAS h : this contribution is mainly due to the conversion efficiency of the
RF power source and of the amplifier at the transmitter side.
The second term, h TF - TF, is the efficiency in the transmission frequency of the wireless path covered during
the energy transfer operation (ranging from a few millimeters to a few centimeters in the case of resonant NFWPT, a few centimeters to tens of centimeters in the cases
of nonresonant NF-WPT and MF-WPT, a few meters to
tens of meters in the case of low-power FF-WPT, and up
to hundreds of kilometers in the case of high-power FFWPT). It consists of the ratio between the power received
by the antenna/coil at the receiver side ^PRX h and PTX
and is strongly related to the medium in between the

May 2017



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