Antenna feed network arrangement

In accordance with the present invention, there is provided a linear antenna array comprising a number of antenna elements and a feed network, wherein the feed network is operable to apply the cumulative effect of a progressive phase shift across the antenna elements of the array and a stepped complex operator shift to selected groups of antenna elements of the array, whereby a down tilted and null-free coverage by a resulting radiation pattern can thereby be provided. The complex operator can be phase, amplitude or a combination of both. The antenna array can be a layered antenna and the phase shifts in the feed network can be provided by differing length transmission paths, whilst any amplitude shift can be provided by unequal power dividers. In order to provide no down tilt and just null fill-in, then the progressive phase shift can be specified to be zero.

This invention relates to a base station arrangement as used in cellular 
radio communications systems and in particular relates to an antenna feed 
network arrangement having a null-free coverage and more particularly to 
an antenna arrangement having a null-free coverage and down-tilt 
capabilities. 
Cellular radio systems are used to provide telecommunications to mobile 
users. In order to meet the capacity demand, within the available 
frequency band allocation, cellular radio systems divide a geographic area 
to be covered into cells. At the centre of each cell is a base station 
through which the mobile stations communicate with each other and with a 
fixed (wired) network. The available communication channels are divided 
between the cells such that the same group of channels are reused by 
certain cells. The distance between the reused cells is planned such that 
co-channel interference is maintained at a tolerable level. 
When a new cellular radio system is initially deployed operators are often 
interested in maximising the uplink (mobile station to base station) and 
downlink (base station to mobile station) range. Any increase in range 
means that less cells are required to cover a given geographic area, hence 
reducing the number of base stations and associated infrastructure costs. 
The downlink range is primarily increased by increasing the radiated power 
from the base station. National regulations, which vary from country to 
country, set a maximum limit on the amount of effective isotropic radiated 
power (EIRP) which may be emitted from a particular type of antenna being 
used for a particular application. In Great Britain, for example, the EIRP 
limit for digital cellular systems is currently set at +56 dBm. Hence the 
operator is constrained and, in order to gain the maximum range allowable, 
must operate as close as possible to the EIRP limit, without exceeding it. 
In cellular radio base stations, the antennas are generally arranged to 
cover sectors, of typically 120.degree. in azimuth--for a trisectored base 
station. The antenna arrays comprise a number of vertically oriented 
layered antenna arrays to provide an M.times.N array to serve a sector. 
Each vertically oriented antenna array is positioned parallel with the 
other linear antenna arrays. The radiating antenna elements of a vertical 
array cooperate to provide a central narrow beam coverage in the elevation 
plane and broad coverage in azimuth, radiating normally in relation to the 
vertical plane of the antenna array. In the elevation plane the radiation 
pattern consists of a narrow "main" beam with the full gain of the antenna 
array, plus "side lobes" with lower gains. With a uniform phase excitation 
for the antenna array, there are deep "nulls" between the main lobe and 
the first side lobes on either side. These produce undesirable "holes" in 
the base station coverage. 
Downtilt in the cellular radio environment is used to decrease cell size 
from a beam shape directed to the horizon to the periphery of the cell. 
This provides a reduction in beam coverage, yet allows a greater number of 
users to operate within a cell since there is a reduction in the number of 
interfering signals. The antennas used in a base station can be of a 
layered or tri-plate form and each antenna radiating element of an antenna 
array is formed on the same layer. 
This tilt can be obtained by mechanically tilting the antenna array or by 
differences in the electrical feed network for all the antenna elements in 
the antenna array. Electrical downtilt can be used to controllably steer a 
radiation beam downwardly from an axis corresponding to a normal subtended 
by an array plane and results from a consecutive phase change in the 
signal fed to each antenna element in an antenna array. Mechanical 
downtilting is simple yet requires optimisation on site; electrical 
downtilting allows simple installation yet requires complex design. 
However, neither forms of downtilting compensate for nulls which are 
formed between lobes in the radiation pattern. 
The present invention seeks to overcome or reduce the above mentioned 
problems. 
In accordance with the present invention, there is provided a linear 
antenna array comprising a number of antenna elements and a feed network, 
wherein the feed network is operable to apply the cumulative effect of a 
progressive phase shift across the antenna elements of the array and a 
stepped complex operator shift to selected groups of antenna elements of 
the array, whereby a downtilted and null-free coverage by a resulting 
radiation pattern can thereby be provided. 
The complex operator can be phase, amplitude or a combination of both. The 
antenna array can be a layered antenna and the phase shifts in the feed 
network can be provided by differing length transmission paths, whilst any 
amplitude shift can be provided by unequal power dividers. In order to 
provide no downtilt and just null fill-in, then the progressive phase 
shift can be specified to be zero. 
In accordance with a further aspect of the invention, there is provided a 
method of operating an antenna array comprising a number of antenna 
elements and a feed network; the method steps comprising the application 
of a progressive phase shift in the signals fed to consecutive antenna 
elements in the array and a stepped complex operator shift to selected 
groups of antenna elements of the array, whereby a resultant radiation 
distribution is downtilted and the distribution between the main lobe and 
first sidelobes is null-free. 
The complex operator can be phase, amplitude or a combination of both. The 
antenna array can be a layered antenna and the phase shifts in the feed 
network can be provided by differing length transmission paths, whilst any 
amplitude shift can be provided by unequal power dividers. If null fill-in 
is required, but downtilt is unnecessary, then the progressive phase shift 
can be specified to be zero.

FIG. 1 shows, in section, a linear antenna array 10 operating over a cell 
12 which forms a beam having a main lobe 14 normal with respect to the 
array. Since the array is tilted downwardly, the central lobe serves the 
far-field, with the sidelobes serving the near-field. The feed network 
provides equal phase and amplitude paths from an input of the antenna 
array to each of the antenna elements. The nulls between the lobes can be 
seen to provide a non-uniform coverage. The beam provided by this 
arrangement has an intensity distribution as shown in FIG. 2a--there is a 
central lobe with sidelobes of reduced intensity, which sidelobes are 
separated from adjacent lobes by instances of low power or nulls. 
Electrical downtilt will have much the same effect, with the nulls being 
steered together with the radiating lobes. 
FIG. 3 shows an array wherein the feed network 34 provides varying paths 32 
from an input 36 to each of the antenna elements 35 of the antenna array 
30. The varying paths introduce differences by way of unequal power 
division at path splits or by differences in path length. The beam shapes 
represented in FIGS 5a to 7a are provided by feed networks having the 
amplitude and phase distributions as shown in FIGS. 5b to 7b and FIGS. 5c 
to 7c respectively. The phase shifts in the feed paths for the antenna 
elements have been effected progressively across the antenna array (also 
known as a phase taper) together with a phase shift or amplitude shift for 
a group of antenna elements. This progressive series of phase shifts along 
the antenna array has the primary result of effecting downtilt. Typically, 
a phase taper for an array will be 10-90.degree. phase difference between 
antenna elements of an array, which elements are spaced 1/2-3/4 
wavelengths apart. A representation of such an antenna in use is shown in 
FIG. 4, wherein the antenna array 40 provides an electrically downtilted 
beam 44 operating over a cell sector 42, with null fill-in. The many 
benefits in the design and installation of such antenna arrays in 
comparison with mechanical downtilting can easily be envisaged; moreover, 
the coverage defined is near uniform by reason of the nulls between lobes 
not being significant. 
The linear antenna arrays of FIGS. 5 to 7 comprise 16 antenna elements. In 
a layered or flat plate arrangement the antenna arrays are arranged 
vertically to provide a beam which is narrow in elevation. The microwave 
signals from the base station transmitter are introduced or coupled to an 
antenna array feed network printed upon a dielectric substrate of an 
antenna by, typically, a coaxial line arrangement. The feed network 
provides a signal for each antenna element. The radiation pattern provided 
by each antenna element cooperates with the radiation pattern provided by 
the other antenna elements within an antenna array whereby the resulting 
radiation intensity distribution is the sum of all the radiation 
distributions of all the antenna elements within the antenna array. The 
antenna array can be deployed mounted on a mast or other type of suitable 
structure. 
In one embodiment of the invention, the feed paths between the first to 
sixteenth antenna elements comprise, in addition to the progressive phase 
change, a series of a first group of antenna elements having a phase 
difference with respect to a second group of antenna elements. The feed 
network for each antenna element can be arranged such that the phase of a 
further group of antenna elements is different. FIG. 5 shows a radiation 
distribution for such a case in which nulls between the first two side 
lobes and the central lobe are absent. The elements of the antenna array 
can also be grouped as in FIG. 6, to provide null fill-in between first 
and second side lobes as well. 
Alternatively, the feed paths need not be grouped for antenna elements 
having similar phase shifts, but the power split between tracks of the 
feedback path can be such that, in addition to the progressive phase 
change, an amplitude difference for a group of the antenna elements be 
effected. The effect of changing the amplitude of a feed input for a group 
of antenna elements is in many ways similar to the effect of changing the 
phase of a feed input for a group of elements, since both the amplitude 
and phase are components of the complex excitations of the radiated 
signals. The power splits in the feed paths between the first to sixteenth 
antenna elements may vary for a first group of antenna elements having the 
same amplitude and a second group of antenna elements with a fixed 
amplitude change with respect to the other antenna elements. The feed 
network for each antenna element can be arranged such that the amplitude 
of a consecutive group of antenna elements is different. FIG. 7 shows a 
radiation distribution for a case wherein the antenna elements 7-10 of the 
antenna array have an amplitude of a magnitude three times that of the 
other antenna elements; the nulls between the first two side lobes and the 
central lobe are absent. 
Whilst the principle of increasing transmission path lengths may appear to 
be straightforward the same cannot be said for the realisation of such 
features. Typically antenna arrays are situated up a mast or some other 
suitable structure; weight and size constraints determine what can be 
added to an antenna array. Furthermore components for fabrication are 
expensive. Thus weight, size and manufacturing costs must be minimised. 
Flat-plate or layered antenna technology is such that feed networks are 
arranged on a thin dielectric sheet between two ground planes of the 
antenna with only the portions forming radiative probes being situated 
within apertures or radiating elements formed in the ground planes. The 
feed network for the radiating probes must be situated between the ground 
planes i.e. to the side of the apertures, in order that unintended 
coupling effects do not take place. Thus differences in path length, power 
splits, and the like can only be accommodated if the resulting network 
does not compromise the performance of the antenna elements. A particular 
problem arises in the division of the signals from the input transmission 
line to the antenna. If the signals are input via a coaxial cable then the 
signals can be coupled via a reactive coupling scheme whereby the coaxial 
cable feeds a number of Wilkinson dividers (or other type of divider) the 
outputs of which couple with input arms of the feed network. The use of 
thin dielectric films does not lend itself to simple and cheap fabrication 
of input signal connection since such thin dielectric films cannot easily 
be soldered. The use of reactive coupling schemes (see pending patent 
application GB9506878.9) requires the use of a small substrate of ceramic 
(or similar). Such substrates, by reason of fragility and of expense, must 
be of a small size and any signals coupled from this substrate should be 
of equal amplitude and phase, with the signal power and phase division 
occurring on the tracks defined on the dielectric film. 
For amplitude variations to be implemented in a circuit, it is preferable 
to employ unequal dividers at appropriate junctions such that amplitude 
shifts occur for a group of antenna elements. Phase shifting is preferably 
implemented after signal division to reduce the effects of varying effects 
with amplitude and signal strength. In order that signals carried by a 
ceramic substrate are equal in phase and amplitude, isolated dividers 
should be used on the substrate, such as Wilkinson dividers. It is to be 
noted that the use of such dividers is generally contrary to the 
requirements for a low cost and easy to fabricate arrangement. The 
advantages of an isolated coupler are that no reflections are produced and 
no phase differences arise. If a non-isolated divider were to be used, 
then changes in the complex excitation division will depend upon the 
amplitude and phase of any reflected signals (which vary with frequency), 
which will introduce phase errors and may, in turn, negate any benefit 
that may otherwise have been achieved. Accordingly, the shifts in complex 
excitation to achieve null fill-in are preferably associated with isolated 
dividers in the feed network. The use of a minimum number of shifts is 
therefore advantageous. 
Whilst only embodiments providing both amplitude or phase shifts to a group 
of antenna elements has been shown, the same advantages can be provided by 
a combination of such shifts. In many configurations, it is preferable 
only to effect phase shifting, since unequal power division requires more 
circuit space due to the larger space requirements of unequal power 
dividers. In certain cases, there is no requirement for downtilt but only 
null fill-in; in these cases, the progressive phase shift across the 
antenna elements can be zero.