Dielectrically-loaded antenna
View Patent ↗A dual-band dielectrically loaded helical antenna for circularly polarised signals has two groups of helical antenna elements. In each group there are at least four such elements and they are connected at their distal ends to a respective feed coupling node and at their proximal ends to a common linking conductor. Each group includes pairs of neighbouring such antenna elements, each pair having one electrically short element and one electrically long element, and the arrangement of the elements is such that in each group the number of pairs in which, in a given direction around the core, the short element precedes the long element is equal to the number of pairs in which, in the same direction, the long element precedes the short element.
1. A dielectrically loaded helical antenna for operation at first and second frequencies above 200 MHz and with circularly polarised radiation, wherein the antenna comprises an electrically insulative dielectric core of a solid material that has a dielectric constant greater than 5 and occupies the major part of the interior volume defined by the core outer surface, a pair of feed coupling nodes, and an antenna element structure that includes a plurality of elongate conductive antenna elements and a common interconnecting conductor, the antenna elements being in the form of elongate conductors distributed around the core on or adjacent the outer surface thereof,
wherein the antenna elements comprise a first group of at least four substantially co-extensive antenna elements extending from one of the feed coupling nodes to the common conductor and a second group of at least four substantially co-extensive antenna elements extending from the other feed coupling node to the common conductor, the said groups containing electrically short antenna elements associated with a circular polarisation resonance at the first frequency and electrically long antenna elements associated with a circular polarisation resonance at the second frequency, and
wherein each of the said groups includes pairs of neighbouring antenna elements, with each pair comprising one electrically short antenna element and one electrically long antenna element, the arrangement of the elements being such that, in each group, the number of pairs in which, in a given direction around the core, the electrically short antenna element precedes the electrically long antenna element is equal to the number of pairs in which, in the said direction, the electrically long antenna element precedes the electrically short antenna element.
2. The antenna according to claim 1 , wherein the second frequency is spaced from the first frequency by a frequency difference no greater than 12 percent of the mean of the first and second frequencies.
3. The antenna according to claim 1 , wherein the core outer surface has oppositely directed transversely extending end surface portions and a side surface portion extending between the end surface portions, and wherein the feed coupling nodes are located on or close to one of the end surface portions and the common interconnecting conductor is an annular conductor or sleeve encircling the core on or adjacent the side surface portion at a location spaced from the feed coupling nodes in the direction of the other end surface portion.
4. The antenna according to claim 1 , having first and second resonant modes each associated with circularly polarised radiation, the first mode being centred on a first frequency and associated with the electrically long elements, and the second mode being centred on a second frequency spaced from the first frequency by a frequency difference no greater than 12 percent of the mean of the first and second frequencies and being associated with the electrically short elements, wherein the core outer surface has oppositely directed transversely extending end surface portions and a side surface portion extending between the end surface portions, and wherein the feed coupling nodes are located on or close to one of the end surface portions and the common interconnecting conductor is an annular conductor or sleeve encircling the core on or adjacent the side surface portion at a location spaced from the feed coupling nodes in the direction of the other end surface portion the electrical length of the common interconnecting conductor being λ g where λ g is the guide wavelength of currents on the conductor at the second frequency.
5. The antenna according to claim 1 , wherein the said antenna elements are generally helical and formed as conductive tracks on the outer surface of the core.
6. The antenna according to claim 5 , wherein the electrically long antenna elements are meandered about respective pure helices.
7. The antenna according to claim 1 , wherein each of the said groups of antenna elements has at least five co-extensive antenna elements.
8. The antenna according to claim 1 , in which the core is cylindrical and the said antenna elements are helical and substantially uniformly distributed over the cylindrical outer surface portion of the core.
9. The antenna according to claim 1 , constructed as a backfire antenna.
10. The antenna according to claim 9 , wherein the feed coupling nodes are located on or adjacent a distal end surface portion of the core and the common interconnecting conductor sleeve encircles the core and is connected at a proximal end surface portion of the core to a feeder structure passing through the core between the distal and proximal end surface portions.
11. The antenna according to claim 1 , constructed as an endfire antenna.
12. A dielectrically loaded helical antenna having a pair of neighbouring circular-polarisation resonant modes, wherein the antenna comprises two groups of at least four axially co-extensive conductive helical antenna elements with a common radius, a pair of feed coupling nodes and an annular linking conductor, the antenna elements of one of the groups extending from one of the feed coupling nodes to the common conductor and those of the other groups extending from the other feed coupling node to the common conductor, characterised in that, in each group, the antenna elements form at least part of respective conductive paths of at least first and second different electrical lengths, one of the pair of resonant modes being associated with the paths of the first electrical length and the other of the pair of resonant modes being associated with the paths of the second electrical length, wherein the pattern formed by the paths is such that the sequence of the different electrical lengths within each group is mirrored about a centre line associated with that group, wherein each said group has at least two antenna elements of the first electrical length and at least three antenna elements of the second electrical lenghth.
13. A dielectrically loaded helical antenna having a pair of neighbouring circular-polarisation resonant modes, the antenna having two groups of at least four axially co-extensive conductive helical antenna elements with a common radius, a pair of feed coupling nodes and an annular linking conductor, the antenna elements of one of the groups extending from one of the feed coupling nodes to the common conductor and those of the other groups extending from the other feed coupling node to the common conductor, characterised in that each group of antenna elements has at least two antenna elements of a first electrical length and a least two antenna elements of a different, second electrical length, the resonant modes being centred on first and second respective frequencies between which the frequency spacing is between 2 percent and 12 percent of the mean of the first and second frequencies.
14. The antenna according to claim 13 , having at least ten helical antenna elements.