Patent Application
20170237174
The invention relates to antennas. More particularly, the invention relates to a broad band diversity antenna system.
A log-periodic antenna (LP), also known as a log-periodic array or log-periodic aerial, is a multi-element, directional, antenna designed to operate over a wide band of frequencies.
The most common form of log-periodic antenna is the log-periodic dipole array or LPDA, The LPDA consists of a number of half-wave dipole driven elements of gradually increasing length, each consisting of a pair of metal rods. The dipoles are mounted close together in a line, connected in parallel to the feed line with alternating phase. Electrically, it simulates a series of two or three-element Yagi antennas connected together, each set tuned to a different frequency.
In radio and telecommunications a dipole antenna or doublet is the simplest and most widely used class of antenna. It consists of two identical conductive elements such as metal wires or rods, which are usually bilaterally symmetrical. The driving current from the transmitter is applied, or for receiving antennas the output signal to the receiver is taken, between the two halves of the antenna. Each side of the feed line to the transmitter or receiver is connected to one of the conductors. This contrasts with a monopole antenna, which consists of a single rod or conductor with one side of the feed line connected to it, and the other side connected to some type of ground. A common example of a dipole is the “rabbit ears” television antenna found on broadcast television sets.
The most common form of dipole is two straight rods or wires oriented end to end on the same axis, with the feed line connected to the two adjacent ends. This is the simplest type of antenna from a theoretical point of view. Dipoles are resonant antennas, meaning that the elements serve as resonators, with standing waves of radio current flowing back and forth between their ends. So the length of the dipole elements is determined by the wavelength of the radio waves used. The most common form is the half-wave dipole, in which each of the two rod elements is approximately ¼ wavelength long, so the whole antenna is a half-wavelength long. The radiation pattern of a vertical dipole is omnidirectional; it radiates equal power in all azimuthal directions perpendicular to the axis of the antenna. For a half-wave dipole the radiation is maximum, 2.15 dBi perpendicular to the antenna axis, falling monotonically with elevation angle to zero on the axis, off the ends of the antenna.
Antenna diversity, also known as space diversity or spatial diversity, is any one of several wireless diversity schemes that uses two or more antennas to improve the quality and reliability of a wireless link. Often, especially in urban and indoor environments, there is no clear line-of-sight (LOS) between transmitter and receiver. Instead the signal is reflected along multiple paths before finally being received. Each of these bounces can introduce phase shifts, time delays, attenuations, and distortions that can destructively interfere with one another at the aperture of the receiving antenna.
Antenna diversity is especially effective at mitigating these multipath situations. This is because multiple antennas offer a receiver several observations of the same signal. Each antenna experiences a different interference environment. Thus, if one antenna is experiencing a deep fade, it is likely that another has a sufficient signal. Collectively such a system can provide a robust link. While this is primarily seen in receiving systems (diversity reception), the analog has also proven valuable for transmitting systems (transmit diversity) as well.
While log periodic and dipole antennas are known, it might be desirable to use both log periodic and dipole antennas in applications where the benefits of a diversity antenna system are advantageous.
Embodiments of the invention provide a broad band diversity antenna system which, in a presently preferred embodiment, comprises a system of log periodic antennas (LPA) and dipole antennas. Embodiments of the invention use two LPAs to form a balanced dipole by feeding the back ends of the LPA. In embodiments of the invention, the feed is improved by the addition of a balun to ensure RF balance. Because a dipole only requires narrow bandwidth, a simple balun is constructed from coax cable or a transmission line, thus providing low cost construction.
Embodiments of the invention provide a broad band diversity antenna system which, in a presently preferred embodiment, comprises a system of log periodic antennas (LPA) and dipole antennas. Embodiments of the invention use two LPAs to form a balanced dipole by feeding the back ends of the LPA. In embodiments of the invention, the feed is improved by the addition of a balun to ensure RF balance. Because a dipole only requires narrow bandwidth, a simple balun is constructed from coax cable or a transmission line, thus providing low cost construction.
A presently preferred embodiment of the invention provide a dual band antenna comprising a low band 700 MHz to 960 MHz top loaded dipole and a high band 1.4 GHz to 2.7 GHz broad band LPA. Embodiments of the invention allow the antenna system to rotate physically. Those skilled in the art will appreciate that the invention is not limited to the foregoing bands and/or frequencies, and that any number of bands and/or range of frequencies can be provided in accordance with the invention disclosed herein.
Embodiments of the invention are fixed and use two oppositely pointing LPAs for the high band. Each LPA has a cardioid pattern in both the vertical and horizontal directions, where polarization is vertical in the azimuth plane. The LPAs can be placed back to back to ensure orthogonality, i.e. isolation between the antennas.
In embodiments of the invention, the low band uses a single top loaded vertical dipole and operates in a differential mode between the proximal ends of the two LPAs. A result of the differential feed across the ends of the two LPAs produces a single low band horizontal dipole. Because the low band modes are cross polarized, the modes are orthogonal and therefore isolated.
Embodiments of the invention can include, inter alia, a mechanical rotating beam (direct beam), a 3D LPA, a 2D LPA, a combined LPA and dipole antenna, and a non-rotating dual LPA and dipole antenna.
In embodiments of the invention, the structure for the high band LPA antenna comprises interleaved elements that are printed on two opposing PCBs having copper on facing surfaces (see
In embodiments of the invention, the structure for the low band vertical antenna comprises a top loaded dipole that is coupled by a delta match that uses a series capacitor to complete the match. In embodiments of the invention, the capacitor is split on either side of the differential feed to support the balance.
The low band horizontal antenna also comprises a dipole. Because the two LPA antennas are isolated, the horizontal dipole is formed by driving their common mode in a differential fashion. In embodiment of the invention, the feed uses five discrete matching components comprising one series resonator. The return loss is only 7 dB. The addition of another resonator, while improving the match, might make tuning very sensitive and is not preferred for the horizontal antenna.
While an LPA is used in this example, in other embodiment of the invention other antennas, such as conical, Yagii, dipole, and many other balanced and unbalanced antenna systems can be used. Further, while a balun has been used in this embodiment other combiners, such as delta sigma or discrete circuits can be used to achieve a similar result.
A further improvement allows for combinations of these antennas, such as an LPA with a Yagii, and so on. Such combinations can be provided for various beam pattern and bandwidth requirements.
In this embodiment, the LPA provides substantially omnidirectional horizontal coverage, using one or the other of the LPAs. A further improvement uses higher gain high band antennas. By way of example, the horizontal polarized low band antenna can be duplicated with two more LPAs or similar antennas by duplicating the system and rotating the second system by 90 degrees. This provides for four high band, high gain orthogonal antennas and two low band, horizontal orthogonal dipole antennas. This process can continue as desired. While this embodiment is described in the azimuthal plane, one skilled in the art will appreciate that the system described can operate in any chosen plane.
Using the vertical dipole as described limits the horizontal vertical coverage to omnidirectional and it therefore remains a single antenna. Further improvement can be achieved with a system of directed antennas in its place, thus permitting for additional independent beams. Such the antenna system is necessarily larger in size. This allows the possibility of further system extensions that are not limited to the LPA and dipole described above.
LPAs are typically self supporting structures with only an air dielectric between the top and bottom antenna structures, as shown in the first example. However, embodiment of the invention requires more structural support.
One embodiment supports the two metal structures with a thin PCB material on the outer side for each. This allows the metal structure to be printed on the inner side of the PCBs and, accordingly, the antenna impedance remains unaltered, particularly in the overlapping region. This also allows the efficiency to remain high because there is only air and not a lossy dielectric between the metal layers.
A presently preferred embodiment of the invention prints the two metal layers on a single PCB. This makes the antenna assembly much less expensive to build and assemble. However, there are two issues in this case, i.e. the dielectric material increases the loss in the feed system, and the higher dielectric constant slows down the travelling wave in the feed system. This becomes more of an issue as the LPA bandwidth increases.
A solution to this problem is to offset the feed system by shifting the facing side up and the back side down. As the offset increases the impedance is increased and the group delay is decreased, approaching the effective relative dielectric of 1.0. Accordingly, the performance of the antenna system approaches the air-loaded dielectric, thus improving the efficiency and bandwidth capability. This also allows a much thinner, single-layer PCB material to be used. The thinner PCB material allows the electric field to be dominated by the air dielectric, not by the PCB material, and yet maintain the overall strength and robustness of the antenna system. In some cases, small adjustment to the element lengths may be required. Some tapering of the feed system might also help with the taper corresponding somewhat with the taper of the element lengths.
The following are typical specifications of a preferred embodiment of the invention:
Low band 700 MHz to 960 MHz
High band 1.4 GHz to 2.7 Ghz
Two times low band omnidirectional 1 dBi nominal as for a dipole, where one dipole is vertical and the other dipole is horizontal
Two times high band unidirectional 5 dBi nominal as for an LPA, where both high band antennas are oppositely directed
50% to 90%
Vertical all bands
Horizontal for the second low band antenna
Accordingly, return loss is typically better than 10 dB all bands, isolation is better than 20 dB all bands, low band gain omnidirectional is 1 dBi to 2 dBi, high band forward gain is 6.5 dBi to 7.5 dBi, and the high band front-to-back ratio is 15 dB to 20 dB. The low and high band may be combined with a suitable diplexer for each antenna system.
Although the invention is described herein with reference to the preferred embodiment, one skilled in the art will readily appreciate that other applications may be substituted for those set forth herein without departing from the spirit and scope of the present invention. Accordingly, the invention should only be limited by the Claims included below.
