Table 3.

Comparison of UHF sensors proposed in PD detection.

Antenna Configuration	Measurement Bandwidth	Physical Size $(\mathit{L}){}^1$	Electrical Length $({\mathit{\lambda }}_{\mathit{f}}^{*}){}^2$	Radiation Pattern	Ref.
Meander-line antenna	0.3 GHz–1 GHz	70 mm	0.07	Unidirectional	[59]
Vivaldi antenna	0.8 GHz–3 GHz	100 mm	0.27	Omnidirectional	[60,61]
Monopole antenna	0.75 GHz–1.5 GHz	100 mm	0.25	Omnidirectional	[41]
Goubau antenna	0.4 GHz–1 GHz	207 mm	0.276	Omnidirectional	[42]
Conical antenna	0.6 GHz–3 GHz	100 mm	0.20	Omnidirectional	[41,62]
Hilbert fractal antenna	0.3 GHz–1 GHz	100 mm	0.1	Unidirectional	[57]
Peano fractal antenna	0.3 GHz–1 GHz	90 mm	0.09	Unidirectional	[63,64]
Bowtie antenna	N/A	N/A		Unidirectional	[54]
U-shaped UWB antenna	0.5 GHz–1.5 GHz	215 mm	0.36	Unidirectional	[55]
Squared patch antenna	0.35 GHz–800 MHz	232 mm	0.27	Unidirectional	[65]
Log-Spiral antenna	0.7 GHz–3 GHz	150 mm	0.35	Unidirectional	[66]
Single-Arm Archimedean Spiral Antenna	1.15 GHz–2.4 GHz	200 mm	0.77	Unidirectional	[67]
Double-Arm Archimedean Spiral Antenna	0.6 MHz–1.5 GHz	130 mm	0.26	Unidirectional	[41]
Cavity-backed Archimedean Spiral Antenna	0.925 GHz–1.6 GHz	80 mm	0.25	Unidirectional	[50]
Minkowski Fractal Antenna	0.7 GHz–3 GHz	300 mm	0.70	Omnidirectional	[52]
Circular Patch Antenna	0.8 GHz–3 GHz	100 mm	0.27	Omnidirectional	[49]
3D cube antenna	1.25 GHz–3 GHz	85 mm	0.35	Unidirectional	[44]
Koch Snowflake antenna	0.3 GHz–1 GHz	280 mm	0.28	Omnidirectional	[68]

¹ The physical size ($L$) is determined as the longest dimension of the antenna.

² The electrical length (${\lambda }_f^{*}$) is normalized by the physical size of the antenna, which is calculated by:

$${\lambda }_f^{*}=\frac{c_0/f_{min}}{L},$$ (1)

where $c_0$ is the speed of the light; $f_{min}$ is the lowerst working frequency.