RFID near field linear antenna
View Patent ↗A near field linear element microstrip antenna is disclosed which is configured to read an RFID label such that a localized electric E field emitted by the antenna at an operating wavelength resides substantially within a zone defined by the near field. The localized E field directs a current distribution along an effective length of the antenna corresponding to a half-wave to a full-wave structure.
1. A near field RFID antenna assembly comprising a substantially linear element microstrip antenna comprising:
a substrate having a thickness H;
a feed point at an end region of the substantially linear element microstrip antenna;
a terminating resistor at an end region of the substantially linear element microstrip antenna opposite the feed point;
a linear microstrip line, the linear microstrip line electrically connecting the feed point and the terminating resistor, the linear micro strip line having a width W;
the antenna configured such that:
a localized electric E field emitted by the antenna resides substantially within a zone defined by the near field;
the localized electric E field directs a current distribution along a length of the antenna corresponding to a half wave to a full-wave structure;
an input impedance Z in ohms at the feed point is substantially equal to the impedance of the terminating resistor; and
a ratio of W/H ranges from about 1 to about 5.
2. The antenna assembly of claim 1 , wherein the substrate further has a first surface and a second surface and the thickness defined there between; and wherein the substantially linear element microstrip antenna further comprises:
a ground plane,
wherein the linear microstrip line is disposed upon the first surface of the substrate and the ground plane is disposed upon the second surface of the substrate.
3. The antenna assembly of claim 2 , said terminating resistor being electrically coupled to the ground plane.
4. The antenna assembly of claim 3 , wherein the linear microstrip line has the width W and the substrate has the thickness H such that the input impedance Z in ohms of the antenna assembly is substantially equal to the following equation:
Z
=
120
π
ɛ
re
[
W
H
+
1.393
+
0.667
ln
(
W
H
+
1.444
)
]
-
1
where
ɛ
re
=
(
ɛ
r
+
1
2
)
+
(
ɛ
r
-
1
2
)
(
1
+
12
H
W
)
-
1
2
and “∈ r ” is the relative dielectric constant for the substrate.
5. The antenna assembly of claim 4 , wherein the substrate and ground plane each have a width of at least five times the width W (5W).
6. The antenna assembly of claim 5 , wherein the linear microstrip line has first and second lengthwise edges and the linear microstrip line is substantially centered on the substrate such that an edge of the substrate and an edge of the ground plane each extend a distance of at least two times the width W (2W) from the first and second lengthwise edges.
7. The antenna assembly of claim 4 , wherein the relative dielectric constant for the substrate “∈ r ” ranges from about 2 to about 12.
8. The antenna assembly of claim 3 , wherein the linear microstrip line has a length L extending from the feed point to and including the terminating resistor, the length L given by the following equation:
L
=
n
c
f
ɛ
re
where c is the speed of light in m/s (about 3×10^8 m/s), f is the operating frequency in
Hz
,
ɛ
re
is
_
ɛ
re
=
(
ɛ
r
+
1
2
)
+
(
ɛ
r
-
1
2
)
(
1
+
12
H
W
)
-
1
2
,
and n ranges from about 0.5 for an equivalent half-wave dipole antenna to about 1.0 to an equivalent full-wave dipole antenna.
9. The antenna assembly of claim 3 , wherein input impedance of the antenna at the feed point is about equal to a characteristic impedance of a cable supplying a feed signal at the feed point.
10. The antenna assembly of claim 2 , wherein the linear microstrip line trace has a thickness ranging from about 10 microns to about 30 microns.
11. The antenna assembly of claim 2 , wherein the substrate has first and second edges along a length of the substrate; and wherein the ground plane is disposed upon at least a portion of the first surface of the substrate and not in contact with the linear microstrip line, the ground plane being disposed on the first and second edges of the substrate and on the second surface of the substrate.
12. The antenna assembly of claim 11 , wherein the linear microstrip line has the width W and the substrate has the thickness H such that input impedance Z in ohms of the antenna assembly is substantially equal to the following equation:
Z
=
120
π
ɛ
re
[
W
H
+
1.393
+
0.667
ln
(
W
H
+
1.444
)
]
-
1
where
ɛ
re
=
(
ɛ
r
+
1
2
)
+
(
ɛ
r
-
1
2
)
(
1
+
12
H
W
)
-
1
2
and “∈ r ” is the relative dielectric constant for the substrate.
13. The antenna assembly of claim 12 , wherein the substrate and ground plane each have a width of at least five times the width W (5W).
14. The antenna assembly of claim 2 , wherein the ground plane of the antenna assembly is electrically coupled to a conductive housing.
15. The antenna assembly of claim 14 , wherein the conductive housing is separated from the antenna via at least one dielectric spacer.
16. The antenna assembly of claim 15 , wherein the dielectric spacer includes an air gap.
17. The antenna assembly of claim 2 , further comprising a capacitive load electrically coupled to the linear microstrip line.
18. The antenna assembly of claim 1 , wherein the antenna assembly is configured such that the localized electric E field of the antenna assembly couples to an RFID label that is oriented lengthwise along a length of the antenna assembly.