Flat three-dimensional antenna

A flat three-dimensional antenna is built in three planes. In the first plane is a base plate, in the second plane is a slot divider bent in a U-shape, and in the third plane is resonant structure above the slot divider. The slot divider has a middle part with a length of preferably .lambda./4 and two limbs of .lambda./8 of the same length. With the base plate the slot divider forms a .lambda./2 antenna slot, while the resonant structure with the slot divider defines a shorter second antenna slot. The antenna is characterized by a large bandwidth and omnidirectional radiation characteristic. Perpendicular to the base plate there is essentially no radiation. Feeding takes place preferably via a stripline which is routed between two limbs to a middle part. Impedance matching of the antenna is achieved by suitable dimensioning of the stripline. The antenna can be built equally well in air as well as in a dielectric such as a ceramic block. Several of these antennas can be combined into an ultracompact diversity antenna system.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a three-dimensional antenna suitable for 
wireless transmission of digital data in local area networks. 
2. Description of the Background Art 
In wireless communications in local area networks (LAN) new stipulations 
are being added to conventional requirements (such as matched input 
impedance, good radiation characteristic, efficiency). Thus, for example, 
it is desirable for the antenna or a diversity antenna system to have 
space on a PCMCIA card. In laptop computers with communications capacity 
there are horizontal plug-in slots for these cards. An antenna system 
integrated on a PCMCIA card should therefore radiate roughly equally well 
in the horizontal plane in all directions. So that an antenna can be 
integrated on a card of this type, it must not exceed the height allowed 
by standards. Therefore, it is not possible in many frequency ranges to 
use a simple monopole antenna for these communications. 
SUMMARY OF THE INVENTION 
An object of the invention is to devise a flat, compact, three-dimensional 
antenna which is suitable for wireless transmission of digital data in 
local area networks. The antenna should have a radiation pattern as 
omnidirectional as possible with low dependency of matching on adjacent 
external articles. 
In a preferred embodiment, the antenna is built in three planes. In the 
first plane is a base plate, in the second is a slot divider bent in a 
U-shape, and in the third is a resonant structure. The slot divider is 
bent in a U-shape in the second plane so that a middle part and two side 
limbs are formed. 
This antenna is extremely compact and radiates primarily in spatial 
directions which are defined by the base plate (i.e., "horizontal"). Due 
to the resonant structure, the antenna has an extremely large bandwidth 
(for example, 20% to 30%). In this way the effect of adjacent ambient 
articles can be kept low. The existence of a conductive base plate also 
supports this advantage. 
Preferably the antenna is supplied by a stripline which is routed in the 
second plane between the two limbs and which contacts the slot divider on 
the middle part. The input impedance of the antenna can be matched by 
varying the width and length of the stripline. The stripline can for 
example completely fill the area between the limbs. The length of the 
stripline is preferably less than the length of the limbs, so that more 
space than is needed by the antenna is not required by the feed. However, 
it is also possible to make the stripline longer (i.e., to route it more 
or less out of the antenna in the second plane and for example to reduce 
the width). The antenna feed can be accomplished, depending on the 
embodiment, via a microstrip line or coaxial line (which is routed through 
the base plate). 
The middle part of the slot divider for example has length .lambda./4 
(.lambda.=wavelength at the resonant frequency). The two limbs are then 
each .lambda./8 long. On the ends of the limbs the slot conductor is 
joined to the base plate. The length of the middle part can also be 
somewhat longer or somewhat shorter. Accordingly the antenna becomes more 
or less elongated. 
The resonant structure is supported by (electrically conductive) flank 
elements on the limbs of the slot divider. If the antenna is embedded in a 
dielectric medium, the mechanical support function is in principle assumed 
by the dielectric medium. The flank elements can then be suitably attached 
metal coatings for connecting the resonant structure to the slot divider. 
For the case in which the antenna or at least the resonant structure is to 
be in air, the entire antenna can in principle be implemented by bending a 
plate with a suitable cross-sectional pattern. The resonant structure can 
for example have a gap in the middle so that it is formed by two 
plate-shaped, mirror-symmetrical elements. The gap is unimportant in 
electrical terms, since there is a current node in the middle of the 
resonant structure anyway. 
Preferably a first antenna slot formed between the base plate and slot 
divider is larger than a second antenna slot formed between the slot 
divider and resonant element. The length of the second antenna slot can be 
varied, the antenna bandwidth changing accordingly. In the extreme case it 
is possible to build an antenna with two separate resonances (dual 
frequency mode). Conversely, the resonances can also be brought very near 
one another, enabling narrow bandwidth. 
The antenna may be built in different ways. It is conceivable for example 
that the antenna can be formed from a punched or etched sheet and soldered 
onto a base plate (for example, a metal-coated printed board). Between the 
first and second plane of the antenna there can be a dielectric. Thus, for 
example, the slot divider can be printed onto the top side of a suitably 
thick circuit board as a printed conductor structure, the base plate being 
formed by the metal coating on the back of the substrate. The resonant 
structure in the third plane can be made for example as a flat, inverted 
U-section (board with two flanks opposite on the end side, the flanks 
being soldered onto the printed conductor structures). 
According to one especially preferred embodiment, the antenna is formed on 
a ceramic block. The resonant structure is then a metal coating on the 
first (top) main surface of the ceramic block. The slot divider in the 
second plane is represented for example by a metal coating on the narrow 
lateral surface of the ceramic block. The base plate can be formed by a 
metal coating on the second (bottom) main surface of the ceramic block or 
by a metal surface onto which the ceramic block is soldered. Between the 
two main surfaces there can be a metal-coated slot in the ceramic block in 
which the stripline for feeding the antenna is located. An antenna with 
this structure is not only extremely compact (due to the relative 
dielectric constant .di-elect cons..sub.r &gt;1), but also very durable. It 
can be handled and soldered like any other electronic component 
(SMD=surface mounted device). Because the antenna is small, the danger of 
damage is prevented (no antenna projecting out of the housing). 
Under certain circumstances there can be an inductance for antenna 
matching. It is preferably integrated in or in front of the stripline. 
The antenna is also well suited for diversity reception. This relates both 
to space and angular diversity, often also called pattern diversity. 
Sectorizing angular diversity is achieved by placement directly next to one 
another, which is noteworthy. This means that each of the two antennas is 
especially sensitive in one direction in which the other has only 
extremely low sensitivity. By switching or combining the two antenna feeds 
the performance of a receiver can be enhanced (diversity gain). For 
example, there is switching from one antenna to the other when the signal 
of the first one becomes too weak. If the antenna signals are additionally 
phase-shifted against one another, the sensitivity pattern can be turned 
in space. 
To achieve space diversity, several antennas can be placed next to one 
another at a certain distance (for example, .lambda./3 to .lambda./2). 
With the antenna element described below, for example, a 3-x space 
diversity antenna system can be built which can be packed into a volume of 
54.times.28.times.5.2 mm.sup.3 (which corresponds to the extension of a 
PCMCIA card). 
The antenna is suited preferably for HIPERLAN application and hand 
radiotelephones (including cordless phones). The frequency ranges provided 
for these applications are typically above 1 GHz (for example, at 5.2 GHz 
in the European Telecommunications Standard HIPERLAN). 
The antenna is also suited for use in an antenna array since the large 
bandwidth also allows matching in the vicinity of adjacent antennas. 
Further scope of applicability of the present invention will become 
apparent from the detailed description given hereinafter. However, it 
should be understood that the detailed description and specific examples, 
while indicating preferred embodiments of the invention, are given by way 
of illustration only, since various changes and modifications within the 
spirit and scope of the invention will become apparent to those skilled in 
the art from this detailed description.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 shows an embodiment of the antenna in air. It is built in three 
planes or layers. The first plane is defined by base plate 1. It can be a 
wall of a metal box or the metal coating on a circuit board. 
In the second plane is the slot divider. It is in principle a U-shaped 
metal strip with middle part 2 and two limbs 3, 4. The length of middle 
part 2 is preferably .lambda./4, that of limbs 3, 4 is .lambda./8. The 
slot divider is shorted with base plate 1 on both ends of limbs 3, 4 via 
two legs 5, 6. 
In the third plane is a resonant structure. In this example it is formed by 
two symmetrical plates 9, 10. They are supported by vertical side surfaces 
12, 13 on the outside of bent limbs 3, 4 of the slot divider. Two plates 
9, 10 are separated by gap 11. Viewed electrically, this is irrelevant 
since it lies in a current node. As FIG. 1 clearly shows, it conversely 
enables the forming of the antenna from a flat, suitably cut sheet metal 
section. 
To feed the antenna there is for example stripline 7 which is joined via 
leg 8 to a coaxial connection under base plate 1. If the base plate is 
made as a circuit board, another microstrip line can take the place of the 
coaxial connection. The stripline completely fills the area formed between 
two limbs 3, 4 according to the necessary impedance matching (in which it 
is separated only by two gaps 14, 15 from limbs 3, 4). 
Regarding dimensioning the following can be stated: 
Two plates 9, 10 essentially cover the surface stretched by the slot 
divider which is bent in a U-shape. The distance between the resonant 
structure and the slotted divider is preferably less than the distance 
between the slotted divider and base plate 1. In this sense, for example, 
the second plane can be located at a height of 2.6 mm (.lambda./8) and the 
third plane at a height of 4.2 mm (.lambda./20) over the base plate 
(medium frequency f.sub.0 =6.4 GHz, .lambda..congruent.4.7 cm). 
Between the resonant structure and the slot divider is an antenna slot 
which is bounded in length by side surfaces 12, 13. The length of this 
slot can be varied to fix the bandwidth. If side surfaces 12, 13 are for 
example the same length as limbs 3, 4, the antenna slot is the same length 
as middle part 2. In principle, vertical side surfaces 12, 13 can even be 
guided around the corner onto middle part 2. Conversely, they can also 
claim only a small part of limbs 3, 4 and can be placed near the ends or 
legs 5, 6. Accordingly then the upper antenna slot would be roughly the 
same size as the lower antenna slot between the slot divider and base 
plate 1. 
In principle the antenna according to this embodiment is two bent 
.lambda./2 slots stacked on top of one another, with different slot 
lengths. 
Impedance matching is done via dimensioning of stripline 7. In the 
aforementioned numerical example it has a width of for example 11 mm (0.24 
.lambda.) and a depth of for example 5.5 mm (0.12 .lambda.). Two limbs 3, 
4 each have a width for example of 0.75 mm (0.015 .lambda.). Gap 11 is for 
example 1 mm (.lambda./50) wide. The entire antenna has a width of for 
example 0.28 .lambda. and a depth of for example of 0.14 .lambda.. 
Stripline 7 under certain circumstances can also be less wide and/or run 
out of the area stretched by two limbs 3, 4. In particular it is suited 
for feed via microstrip lines. 
The antenna structure shown in FIG. 1 can be embedded partially or entirely 
in a dielectric medium (of course with matching of the dimensioning based 
on the higher relative dielectric constant .di-elect cons..sub.r &gt;1). 
Thus, for example, the slot divider (limbs 3, 4, middle part 2) and 
stripline 7 can be applied to the dielectric substrate as a printed 
circuit structure (printed board). Base plate 1 can be provided as metal 
coating on the back of the substrate, legs 5, 6, 8 (in the form of pins) 
being routed through the substrate. 
The resonant structure in this case can be a continuous rectangular plate 
which in turn is electrically connected via side surfaces 12, 13 to limbs 
3, 4 and at the same time is supported on the substrate. Most simply a 
piece of sheet metal is cut which allows a surface stretched by limbs 3, 4 
to be covered and which is provided with side brackets to form side 
surfaces 12, 13 (by bending at a right angle). Gap 11 is neither necessary 
nor desired in this embodiment (mechanical stability). 
There can also be a dielectric between the second and third planes. This 
can be achieved for example by selectively laminating on a dielectric 
material of a desired layer thickness. Side surfaces 12, 13 can be applied 
on the corresponding boundary surfaces of the layer which has been 
laminated on. The plate-shaped resonant structure can be imprinted onto 
the surface of the layer which had been laminated on. 
One especially preferred embodiment will be explained using FIGS. 2 and 3. 
FIG. 2 schematically shows ceramic block 16. It has top and bottom main 
surface 17 and 18. On top surface 17 there is metal coating as a resonant 
structure over the entire surface. Lower main surface 18 can likewise be 
metal coated (to form, for example, base plate 1 or to be able to solder 
the ceramic block easily onto a base plate or a metal box). 
Ceramic block 16 has two short and two long side surfaces 19, 20 and 21, 
22. The slotted divider is formed by there being a continuous strip-like 
metal coating for forming a printed conductor which runs peripherally in a 
U-shape on side surfaces 19, 21, 20. This printed conductor is formed by 
strip-shaped area 25, 26 roughly in the center between two main surfaces 
17, 18. On the back end (according to the representation chosen in FIG. 2) 
of side surface 19 metal coating 24 is routed downward to main surface 18. 
The electrical connection between the resonant structure and the slot 
divider is likewise produced by metal coating 27 which is attached on side 
surface 19. Side surface 20 is selectively metal coated 
mirror-symmetrically to side surface 19. It is evident that metal coating 
24 corresponds to leg 6, metal coating 25 to limb 4, metal coating 26 to 
middle part 2 and the blanket metal coating of main surface 17 corresponds 
to two plates 9, 10 in FIG. 1. 
What has been lacking until now is a metal coating corresponding to 
stripline 7. For this reason however there is now flat, continuous slot 
23. It extends from side surface 21 to side surface 22 and is for example 
fully metal coated. For feeding then there can be only one more metal 
coating 32 which is routed to the bottom from slot 23 on side surface 22 
(see FIG. 3). This slot can be attached in the form before hardening or 
can be produced by drilling. But it is also conceivable that two thin 
ceramic blocks be joined to form one thick block, the stripline and 
optionally also the slot divider being formed in a flat version between 
them. 
To bring the input resistance to 50 .OMEGA., it may be necessary to provide 
an inductance (of for example 1-2 nH) which can be elegantly integrated. 
One possible version will be explained using FIG. 3. This figure shows 
ceramic block 16 from behind in an overdrawn perspective representation. 
Slot 23 has a rectangular cross section and thus four inner surfaces 28, 
29, 30, and 31 which are all metal coated. There is (aforementioned) 
selective metal coating 32 on side surface 22 for feeding. It touches the 
inner area of slot 23. The inductance is produced by the current being 
routed first in a loop along slot edge 34, 35, 36 before it can flow in 
the through direction of slot 23. To do this there is nonconductive 
line-shaped area 33 which isolates the back end of the slot metal coating. 
FIG. 3 shows a version in which nonconductive area 33 is isolated for 
roughly half the width of inner surface 28, the entire width of inner 
surface 29 and roughly half the width of inner surface 30 from the metal 
coating in the slot. The current must therefore flow around half the slot 
periphery; this produces a corresponding inductance. The size of the 
inductance can be easily varied by appropriately choosing the length of 
nonconductive area 33. 
In principle, inductance can also be forced by corresponding loop routing 
of the current on side surface 22. This means that the current must first 
flow a certain amount around the slot before it is routed into it. 
In the dielectric the antenna becomes smaller at the same frequency. To 
optimize the bandwidth which becomes smaller at the same time within 
physical limits, for example the length of the upper slot (between the 
second and third plane) can be increased. For preferred applications 
however there is also enough bandwidth reserve in the dielectric. It must 
furthermore be watched that the dielectric-induced losses should not be 
too great. In air the antenna according to this embodiment has a very high 
efficiency of more than 90%. Ceramic materials with very favorable 
tan.delta. values are also known. 
Generally the antenna is characterized by a large bandwidth (in air for 
example 20 to 30%) and by radiation with low or negligibly small power 
perpendicular to base plate 1. In the direction of the base plate there is 
a good omnidirectional characteristic. 
One important application of the antenna according to the invention is in 
the area of wireless LANs (for example, HIPERLAN). For this application 
the antenna can be mounted on a PCMCIA card. Here it is especially 
advantageous to position two or more antennas of the described type. In 
this way diversity reception can be accomplished. 
To achieve space diversity, several antenna elements are placed next to one 
another at a certain distance (.lambda./3 to .lambda./2). (A space 
diversity effect itself then arises when the antennas touch). One 
exemplary arrangement of three antennas at a distance of 0.4.lambda. shows 
that the antennas have relatively very little mutual influence on one 
another, i.e., each antenna largely retains its omnidirectional behavior. 
The signals received from the different antennas are comparatively 
independent of one another. In the aforementioned exemplary arrangement 
the antenna system was packed in a volume of 54.times.28.times.5.2 
mm.sup.3 (which corresponds to an extension of the PCMCIA card). 
FIG. 4 shows by way of example a U-shaped arrangement of three antenna 
elements 37, 38, 39 on an extension of PCMCIA card 40. Adjacent antenna 
elements 37 and 38 and 38 and 39 are each placed at a right angle to one 
another. For reasons of space, antenna elements 37, 38, 39 (which each are 
made as shown for example in FIG. 1) are located as near as possible to 
the corresponding edge of PCMCIA card 40. 
To achieve angular diversity two antennas can be set up with the narrow 
sides (i.e., the bent limbs) directly next to one another. 
In this arrangement the two antennas have an angular sensitivity which they 
do not have as individual antennas (or not in a pronounced form). 
Depending on from which direction a strong signal is incident, the 
receiver can be switched to the suitable antenna. The antenna signals can 
also be advantageously combined. The angular sensitivity can also be 
rotated as needed by phase rotation of the signal of one antenna compared 
to that of the other antenna. 
The antenna is also suited as the element for so-called antenna arrays. In 
this case several individual antennas are arranged in isolation or 
preferably in an association to achieve a desired radiation/reception 
characteristic by the combination of their signals. 
The antennas of the above-described embodiments may also be suitable for 
hand radiotelephones (cordless phones, GSM handies, etc.). Especially in 
the ceramic block version, the antenna can be placed as a compact 
component on the hand to exhibit the desired radiation characteristic. It 
is even conceivable that the antenna can be designed for receiving two 
adjacent frequencies (dual frequency mode). 
The described antenna has a large number of advantages. In summary the 
following should be mentioned: large bandwidth, variability of bandwidth, 
good possibilities for impedance matching, small space requirement, 
omnidirectional radiation pattern in one plane and no radiation 
perpendicular to the plane, compatibility with a PCMCIA card (especially 
also as a system consisting of several antenna elements) and suitability 
for diversity reception. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.