Small spherical antennas
An antenna is provided for operating within the electrically small antenna regime (i.e., ka≅0.5), and having bandwidth performance quite close to fundamental limits. The antenna of the invention, in various embodiments, is based upon spherical resonator structures that are characterized by a performance factor (Q/Q chu ,) close to 1.5. The antenna combines a resonator structure determined according to the method of the invention with an appropriate transmission line feeding arrangement, such that the resonator effectively couples the transmission line mode to the radiating spherical harmonic mode in an impedance-matched manner.
1. An antenna, comprising an axially symmetric cluster of two or more open rings electromagnetically coupled to a transmission line, wherein a feed point is incorporated within at least one of the open rings, and the transmission line is directly coupled to said ring at the feed point.
2. The antenna of claim 1 , wherein the antenna has an operating frequency range and at least one resonant frequency within that range associated with a free-space wavelength lambda, and the cluster has a maximum spatial dimension 2 a such that
2
π
a
λ
is less than 0.65.
3. The antenna of claim 1 , comprising two or more coplanar pairs of said open rings arranged in respective planes that intersect in a common axis such that the rings of each pair are equidistant from a common axis.
4. The antenna of claim 1 , wherein the rings are semicircular.
5. The antenna of claim 1 , wherein the rings are semielliptical.
6. The antenna of claim 1 , wherein:
the resonator has at least one resonance with a quality factor Q, a resonant frequency, and a free-space wavelength λ corresponding to said frequency;
the resonator has a maximum spatial dimension 2 a , expressible as a dimensionless parameter A defined by
A
=
2
π
a
λ
;
the resonator has a set of dimensional parameters comprising a diameter, a trace width, and a gap size for each ring and a separation distance for each pair; and
the number of pairs and the dimensional parameters are selected such that A is no greater than 0.6 and Q is no greater than
2.5
×
[
1
A
3
+
1
A
]
.
7. The antenna of claim 6 , wherein the dimensional parameters are selected in respect to a dielectric constant of substrates supporting the rings.
8. The antenna of claim 6 , comprising two or more coplanar pairs of said open rings arranged in respective planes that intersect in a common axis wherein the number of pairs and the dimensional parameters are selected such that Q is no greater than
1.7
×
[
1
A
3
+
1
A
]
.
9. The antenna of claim 1 , comprising two or more coplanar pairs of said open rings arranged in respective planes that intersect in a common axis wherein at least one pair has rings that differ in at least one spatial dimension partially determinative of resonant frequency.
10. The antenna of claim 9 , wherein the spatial dimension is gap size.
11. The antenna of claim 9 , wherein the spatial dimension is trace width.
12. The antenna of claim 9 , wherein the spatial dimension is ring diameter.
13. The antenna of claim 1 , comprising two or more coplanar pairs of said open rings arranged in respective planes that intersect in a common axis, wherein at least one pair is fed by a transmission line that makes electrical contact with one ring, but not the other ring, of said pair.
14. The antenna of claim 1 , comprising two coplanar pairs of said open rings oriented, respectively, in mutually orthogonal planes, wherein the transmission line makes electrical contact with both rings of one pair, but no rings of the other pair.
15. The antenna of claim 1 , comprising two or more coplanar pairs of said open rings arranged in respective planes that intersect in a common axis, wherein the transmission line is conformed near the rings as a parallel pair of strip conductors penetrating one ring of a pair and having a shorted end situated in the interior of the other ring of the pair.
16. Apparatus comprising a radiofrequency resonator, wherein: the resonator comprises a plurality of rings; each said ring is opened by a gap; the rings are arranged symmetrically about an axis coplanar with each of the rings; each ring is oriented with its gap facing away from the axis; and the rings are separated from the axis by a separation distance small enough to permit mutual electromagnetic coupling among the rings at an operating frequency.
17. Apparatus of claim 16 , wherein the rings are semicircular.
18. Apparatus of claim 16 , having a maximum spatial dimension not greater than 1/(2π) times an operating wavelength, wherein the operating wavelength is the free-space wavelength corresponding to a resonant frequency.
19. Apparatus of claim 16 conformed as an antenna by further comprising a transmission line.