IEEE Microwave Magazine - May 2016 - 78

As optimum dc load resistance at
the output port, a value of 1.8 kΩ
was found, which differs slightly
from the simulated value.
taking into account a Yagi-Uda antenna design (which
cannot be considered "flat" but does keep within the
contest rules), a comparatively high EFoM could be
achieved. Such a Yagi-Uda antenna can be built easily in lightweight construction, as long as it consists of
thin wires and a slim boom.
Comparisons and evaluations with different
types of Yagi-Uda antennas were made, and, finally,
a six-element Yagi-Uda antenna with theoretical
gain of 12.2 dBi was found, based on the original
design of O. Oberrender (DL2RSX) [4], [5] but scaled
from 144 MHz to 2,450  MHz. Further adjustments
had to be made, including a change of the boom's

material from metal to plastic (polylactic acid) due
to an additional weight reduction; further, the elements were made out of 1-mm thick silver-plated
copper wire.
The final dimensions of the antenna are shown
in Figure 4(a); Figure 4(b) shows the final harvester
design. The simulated gain of this antenna is 12.2 dBi.
As a result, the antenna outperforms even the 2 × 2
patch array with regard to the EFoM. More information about general Yagi-Uda antenna design can be
found in [6].
The driven element of the Yagi-Uda is realized
as a folded dipole, which has a significantly higher
impedance than a regular m/2 dipole. Further
impedance tuning is possible by varying the spacing distance of the folded dipole. Figure 5 shows the
simulated impedance of the antenna. By attaching a
m/2 coaxial cable balun (with a 50-Ω line), the impedance can be directly transformed with the ratio 4:1,
from (153.7- j12) Ω down to approximately 40 Ω,

51.2

38

Boom
(Plastic)
51.2

37.5

Top View
51.7

36.2

Directors

54.2

14.1

55

30

57.5

Driven
Element
(Folded Dipole)

(b)

Reflector

Front View

2.5

1

3

10

(a)

λ
2
(c)

Figure 4. (a) The dimensions (in mm) of the Yagi-Uda antenna, (b) an image of the final harvester, and (c) the schematic of
the folded dipole with an attached balancing element.

78

May 2016

Table of Contents for the Digital Edition of IEEE Microwave Magazine - May 2016

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https://www.nxtbook.com/nxtbooks/ieee/microwave_november2017
https://www.nxtbook.com/nxtbooks/ieee/microwave_september2017
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