Communications: radio wave antennas – Antennas – Loop type
Reexamination Certificate
2001-05-11
2002-06-04
Wong, Don (Department: 2821)
Communications: radio wave antennas
Antennas
Loop type
C343S742000
Reexamination Certificate
active
06400337
ABSTRACT:
TECHNICAL FIELD
The present invention relates to three dimensional polygon antenna designs.
BACKGROUND ART
There are various types of antennas which are known in the art. These types include the full-wave (FW) square loop antenna, the rectangle class of antennas (of which the square is a member), the multi-loop antenna, and the large perimeter loop antenna which results from the use of thick wires in its construction.
The basic full-wave (FW) or 1 &lgr; loop element is a known antenna. A four-sided rectangular full-wave (FW) loop antenna can have many shapes, ranging from a folded dipole at one extreme to a 0.5 &lgr; transmission line which is terminated by two minute Hertzian dipole elements at the other extreme. The square variant was the first of this class to be developed.
FIG. 1
illustrates a square full-wave (FW) loop antenna, also known as a Quad loop. In
FIG. 1
, a square loop is shown that is nominally 1 &lgr; in perimeter, or 0.25 &lgr; per side. The design may be visualized as being comprised of two 0.5 &lgr; dipoles, separated in height by 0.25 &lgr;, with their ends folded down and touching. The antenna may also be visualized as illustrated in
FIG. 2
, which shows two U-shaped 0.5 &lgr; dipoles with truncated central radiating sections of 0.25 &lgr; and folded ends of 0.125 &lgr;.
Basically, then, rectangle-based antennas are comprised of two short parallel dipole radiating elements with the ends of the dipoles connected to each other by way of a pair of transmission line wires. The square is a unique case which has radiators and transmission line wires of equal size.
Only a single feedpoint is necessary and this port may be located at the center of any of the four sides. The polarization of the antenna depends on whether the feedpoint is placed at the center of a vertical or a horizontal wire. The wire opposite and parallel to the fed wire also radiates, because it carries in-phase and co-directional currents of equal magnitude.
The wires orthogonal to the radiators act as transmission lines, have a 180 degree phase shift at their centers (current minima), and carry currents which are anti-directional; therefore, for all practical purposes, these wires do not radiate. In the case of a square where only one radiating element is fed, the physical transmission line connections between the two radiators are necessary in order to maintain the proper phase relationships in the radiator currents.
In all subsequent discussions of rectangle-based antennas, the radiators will be referred to as including the fed wire and those elements which are parallel to it. “Transmission line wires” will refer to the wires connecting the radiator ends to each other. The sizes of the radiators and transmission line wires are inversely related (i.e., as one lengthens the other must shorten) to maintain the overall loop perimeter somewhere at approximately 1 &lgr;.
The radiation resistance R
in
of such a square loop is in the order of 120 ohms and is a product of the self resistance R
self
of each of the truncated dipole radiators and the mutual resistance R
m
induced by the parallel radiator. A close approximation of R
in
for the square or any rectangular-shaped variant of a FW loop may be represented by the formula R
in
=2(R
self
+R
m
).
These loops are only nominally 1 &lgr; in perimeter. Due to the capacitive reactance induced by the proximity of their high-voltage/low-current points at the centers of their transmission line wires (i.e., the points where the folded back dipole tips touch each other), the feedpoint reactance X
in
of an antenna consisting of two such folded-down 0.5 &lgr; dipoles is highly negative. In order to resonate (X
in
=0) such an antenna, the sides must be increased in size beyond 0.25 &lgr; and therefore the perimeter beyond 1 &lgr;.
The overall loop perimeter or length per side depends on the thickness of the wire composing the antenna: the greater the diameter of the wire, the greater the negative X
in
and the greater the perimeter at resonance. With very thick wires (e.g., having diameters exceeding 0.03 &lgr;) the loop perimeter exceeds 1.3 &lgr;.
The bandwidth (BW) for all antennas discussed herein is defined by the standing wave ratio (SWR) 2:1 limits when referenced against R
in
. This is also dependent on the wire thickness since it is a function of the Q-factor; the thicker the wire, the wider the bandwidth.
The square FW loop has broadside radiation from its two radiators, which are in phase and which have equal currents. It has a gain improvement over that of one of its constituent dipoles in the order of slightly more than 1 dB. This is due to the “stacking effect,” an aperture overlap between the radiation patterns of the two truncated dipoles separated by 0.25 &lgr;. Depending on the wire thickness, the overall gain of a square FW loop is in the order of 3.1-3.4 dBi.
There are two extremes in the shape of rectangular FW loops. One extreme is the folded dipole (FD), where the two radiators are almost 0.5 &lgr; long and the interconnecting transmission lines are minuscule. The R
in
, derived from the formula above, is in the order of 288 ohms. The gain is that of a simple dipole but there is an improvement in the bandwidth.
The other extreme is that of two minuscule “Hertzian dipole” radiators connected by a 0.5 &lgr; transmission line. The R
in
approaches 0 ohms while the modeled gain, using Numerical Electronics Code (NEC), exceeds 6 dBi.
In between these two extremes in the shape of a FW loop, there are an infinite number of possible antennas with intermediate properties. The narrower the radiator, the lower the radiation resistance, the greater the gain, and the narrower the bandwidth. The gain is a function of the separation of the radiators. The bandwidth bears a direct relationship to the R
in
. Other, well-known, antennas which function similarly are slot antennas.
Any FW loop can be attached directly to another. If two such loops are conjoined at a common radiator, a planar antenna results, with two equal-sized loops consisting of three radiators, as shown in FIG.
3
. Since there are now three radiating elements with equal element separation, there is an increase in gain. NEC modeling of one of maximum gain, at the dimensional extreme of three Hertzian dipole radiators connected via 0.5 &lgr; transmission lines, yields a gain in excess of 7.1 dBi.
These double-loops have properties similar to the simple rectangles of which they are comprised. Their loop perimeters increase with wire diameter, their gain is a function of radiator separation, their radiation resistance is related to the self resistance of the radiators as well as the mutual resistance contributed by the two other parallel radiating wires, and their bandwidth is related to the input resistance.
There is a need for more compact antennas with improved broadband design capabilities and wider bandwidths, both in impedance and gain.
SUMMARY OF INVENTION
The antenna design of the present invention is a three-dimensional (3-D) arrangement of full-wavelength (FW) or 1 &lgr; loops. A major advantage of this antenna design is that it is very amenable to broadband design with a range of operating frequencies or bandwidth (BW) in excess of 3:1. Furthermore, the antenna design is relatively compact, with the maximum dimension being less than that of a half-wave dipole at the lowest frequency. The gain of this antenna design exceeds that of a dipole over the entire bandwidth by 1-1.5 dB. These antennas may be used by themselves or as individual elements in high-gain wideband arrays.
Variants of these antennas may also be designed for use as compact higher gain vertical scanning arrays where the beam pattern may be rotated electronically over a 360 degree azimuth. In addition to the increased forward gain, the other advantage in this type of use is in the very deep nulls off the rear which serve to minimize interference. Other variants, whether horizontally or vertically polarized, may be used for controlled squint of their elevation lobes.
With dimensional chan
Cooper & Dunham LLP
Nguyen Hoang
Wong Don
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