Complementary wideband antenna

#This invention relates to a wideband antenna, and in particular to a complementary wideband antenna comprising a planar dipole and a shorted patch antenna.

#The success of second generation (2G) mobile phones has motivated and enhanced the development of a wide range of wireless technologies, including for example 3G video phones, WiFi, WIMAX, ZigBee and Bluetooth. For cost effectiveness and space utilization wideband antennas that can accommodate several different communication systems are in high demand. In particular, antennas with unidirectional radiation patterns of various beam widths are of interest as they may be mounted on walls or vehicles without degrading their electrical characteristics and without affecting the aesthetics of the mounting bodies.

#There are a number of techniques available for implementing wideband antennas with a unidirectional radiation pattern. The first approach is to put a dipole in front of a finite ground plane. However, since the size of the ground plane and the distance between the ground plane in terms of wavelength are frequency dependent, the antenna the disadvantages of large variations in gain and beam width over the operating band, different beam widths in the E and H planes, and strong radiation in the back side.

#Some embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which:

#FIGS. 1(a) and (b) are (a) perspective and (b) side views of one example of an antenna according to an embodiment of the invention,

#FIGS. 2(a) and (b) show (a) measured and simulated SWR against frequency and (b) measured and simulated gain against frequency for the antenna of FIG. 1,

#Referring firstly to FIG. 1 there is shown one example of a complementary wideband antenna according to an embodiment of the invention with FIG. 1 (a) being a perspective view and FIG. 1(b) a side view. In the embodiment of FIG. 1 the antenna comprises a horizontal rectangular planar dipole formed by dipole sections 1a and 1b connected to the open end of a vertically-oriented shorted patch antenna comprising vertical sections 2a, 2b and horizontal section 2c. A ground plane 3 is located beneath the antenna for back lobe reduction. In the embodiment of FIG. 1 the horizontal section 2c of the shorted patch antenna is an integral part of the ground plane 3 located between the vertical sections 2a, 2b but the section 2c could be an individual connecting section separate from the ground plane (see FIG. 9 below for examples). The function of the horizontal section 2c is to connect the vertical sections 2a, 2b so that they are electrically short-circuited. Vertical sections 2a, 2b are generally parallel to each other but the skilled person will understand that they may be slightly non-parallel, ie sloping towards or away from each other by a few degrees.

#Each dipole section 1a, 1b is a rectangle having a width W and a length L. The width W is approximately equal to 0.5λ (where λ is the intended operating wavelength) and the length L is approximately equal to 0.25λ. The spacing of the dipole sections 1a, 1b and the height H of the shorted patch antenna is also approximately 0.25λ. For wideband operation the separation S of the vertical sections 2a, 2b of the shorted patch antenna is approximately 0.1λ. The size of the back plane can be adjusted for low back radiation, but a typical size GW×GL is 1λ×1λ. The dipole sections 1a, 1b and the patch antenna may be formed of a suitable conducting material such as copper plate.

#The antenna as described above may be excited in a number of different ways. The excitation method shown in FIGS. 1(a) and (b) uses a Γ-shaped probe feed 4 comprising a vertical first portion 4a, a horizontal second portion 4b, and a third portion 4c formed by folding a straight metallic strip of rectangular cross-section into the desired Γ shape. Vertical first portion 4a has one end connected to a coaxial launcher 5 mounted below the ground plane 3. This portion acts as an air microstrip transmission line which transmits an electrical signal from the coaxial launcher 5 to the second portion 4b of the feed 4. Vertical portion 4a is parallel to the vertical portion 2a of the patch antenna and spaced therefrom by a small amount c such as approximately 0.008λ. The second portion 4b is horizontally located and serves to couple the electrical energy to the planar dipole and the shorted patch antenna. The input resistance of the antenna is controlled by the length p of this second portion so the length of the second portion 4b has to be selected accordingly. Typically p may be approximately 0.07λ. The second portion 4b itself acts as an inductive reactance which can cause the antenna to be mismatched, but the third portion 4c in combination with patch antenna vertical portion 2b forms an open circuit transmission line with the equivalent circuit of this line being a capacitor. By selecting the appropriate length q for the third portion 4c (eg approximately 0.18λ) the capacitive reactance can be used to compensate for the inductive reactance caused by the second portion 4b. The third portion is preferably parallel to vertical patch antenna portion 2b, but may be angled thereto slightly as shown in FIG. 1(b) where third portion 4c is at an angle θ to the vertical.