Abstract:
A spiral antenna (100) having a feed-point end and a termination end for use within a portable two-way radio housing includes a ground substrate (102) and a number of spiral elements (103, 105) having a number of segments that form two or more spiral shapes. A shorting stub (107) connects the planar elements at a termination end for effectively increasing the feed-point impedance of the spiral antenna (100). The spiral elements (103, 105) may be positioned in a planar arrangement (FIGS. 1 and 2) or may be stacked in separate planes (FIGS. 3 and 4) for forming a limited space antenna having a substantially 50 ohm feed-point end impedance at resonance.

Description:
TECHNICAL FIELD 
     This invention relates in general to antennas and more particularly to antennas occupying limited space. 
     BACKGROUND 
     Conventional antennas used on portable two-way radio equipment typically are operated as a whip or helix type antenna and are designed to resonate at one or more desired wavelength. Antennas of this type are generally designed to operate at a 50 ohm input impedance. As is well known, these types of antennas generally extend out from the radio housing which significantly increases the perceived size of the radio housing. 
     It should be recognized that at a given center frequency, a significant reduction in the height of the conventional antenna will greatly decrease the antenna input impedance from a 50 ohm nominal value. This mismatch ultimately will cause a higher reflected power to the radio&#39;s power amplifier and a loss of the radio&#39;s transmitter power efficiency. Although circuitry can be used to match a lower antenna impedance to a 50 ohm nominal value, this circuitry can be complex, introducing significant insertion loss while ultimately adding additional manufacturing time and expense. 
     Thus, the need exists for a space efficient antenna structure that can be easily used within a radio housing having a 50 ohms impedance at resonant frequency in view of its limited size. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of a single layer spiral antenna according to the preferred embodiment of the invention. 
     FIG. 2 is a top perspective view of that shown in FIG. 1 showing the additional use of a tuning stub. 
     FIG. 3 is a top perspective view of an alternative embodiment to that shown in FIG. 2 wherein the single layer spiral antenna is fed at it&#39;s opposite end. 
     FIG. 4 is a top plan view of a two layer spiral antenna according to an alternative embodiment of the invention. 
     FIG. 5 is a top perspective view of that shown in FIG. 3 showing the additional use of a tuning stub. 
     FIG. 6 is a top perspective view of an alternative embodiment to that shown in FIG. 5 wherein the two of the spiral radiators are in one plane and a third spiral radiator is in a second plane. 
     FIG. 7 is a top perspective view of a three layer spiral antenna according to an alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIGS. 1 and 2, a planar folded spiral antenna 100 for a portable two-way radio transceiver includes a feed-point 101 and 101&#39; positioned on one edge of a ground substrate 102. The antenna 100 includes a first spiral element 103 and a second spiral element 105 with each element comprised of a plurality of substantially linear segments. The segments are inter-connected in a substantially rectangular configuration successively reduced in size so as to form each respective spiral element. Although FIGS. 1 and 2 show the antenna 100 in a substantially rectangular shape, it will be evident to those skilled in the art the other shapes such as a substantially square or circular configuration can be also used. Furthermore, although FIG. 2 shows the antenna 100 in a homogeneous background above the ground substrate 102, it will be evident to those skilled in the art the other background configurations such as layered dielectric materials can be also used above the antenna and/or between the spiral structure and the ground substrate. Thus, the configuration shown in FIG. 1 could be positioned on one side of a single supporting substrate (such as a PC board) above the ground substrate in order to conserve space and provide an ease in manufacturing. Furthermore, it will be evident to those skilled in the art the ground substrate can also take other forms such as a two-way radio or a cellular phone. 
     The plurality of linear segments forming the first spiral element 103 and the plurality of segments forming the second spiral element 105 are positioned in a parallel relationship such that each of the respective segments are in the same plane. As best seen in FIG. 2, the folded spiral antenna is constructed as a uni-planar structure permitting the antenna to occupy a very limited space within a portable two-way radio housing. Conductive runners or traces are used as radiators and form both the first spiral element 103 and the second spiral element 105. Both the first spiral element 103 and the second spiral element 105 have a predetermined width and are separated by a predetermined distance. 
     At the terminating ends of both the first spiral element 103 and the second spiral element 105 a shorting strip or stub 107 is used to electrically interconnect both of the first spiral element 103 and the second spiral element 105 together. Since the second spiral element 105 is grounded at the feed-point end 101&#39;, this has the effect of increasing the feed-point impedance where it can be adjusted to substantially 50 ohms in order to properly match the required load impedance of a radio power amplifier (not shown). Although 50 ohms would be a typical value, the shorting stub 107 and the respective distance of the each spiral element 103, 105, above the ground substrate 102, permit this value to be easily adjusted. 
     The shorting stub 107 is generally one quarter of a wavelength away from the feeding point 101 to ensure that the current flow on the vertical sections 109 and 109&#39; are in the same direction and thus maximize the antenna efficiency since the sections 109 and 109&#39; are the main radiators of this antenna. Moving the shorting stub 107 further away from the feeding point 101 will add an effective capacitive load to the antenna impedance and thus increase the resonant frequency and the impedance at the resulting resonant frequency. On the other hand, moving the shorting stub 107 toward the feeding point 101 will add an effective inductive load to the antenna impedance and thus lower the resonant frequency and the impedance at the resulting resonant frequency. 
     The resonant frequency and the impedance of the antenna are increased by increasing the distance of spiral elements 103, 105 above the ground substrate because of the increased radiation of the antenna and the decreased capacitive coupling between the antenna and the ground substrate. The impedance of the antenna depends not only on the structure of the two spirals but also on the way the antenna is fed. Alternatively, the planar folded spiral antenna 100 may be fed by switching the feeding point 101 and grounding point 101&#39; such that spiral element 105 is directly fed and spiral element 103 is grounded. This has the effect of lowering the antenna input impedance. 
     An alternative embodiment to FIG. 2 is shown in FIG. 3, where the feeding point 101 and grounding point 101&#39; are moved to the inside of each spiral and the shorting stub 107 is also moved to the opposite end of each spiral radiator. FIGS. 2 and 3 differs from FIG. 1 in that a tuning stub 107&#39; is attached to the shorting stub 107 and may be used for fine tuning the folded spiral antenna 100 to a specific resonant frequency. Increasing the length of the tuning stub 107&#39; will lower the antenna resonant frequency and vice versa. 
     In a second embodiment as shown in FIGS. 4 and 5, a multi-planar folded spiral antenna 200 includes a feed-point 201 and 201&#39; positioned on one edge of a ground substrate 202. A first spiral element 203 and a second spiral element 205 each are comprised of a plurality of linear segments. The first spiral element 203 and the second spiral element 205 are positioned such that the second spiral element 205 is positioned in a plane beneath the first spiral element 203. Both the first spiral element 203 and second spiral element 205 are formed into a plurality of substantially rectangular spirals and are separated by a predetermined distance. Although FIG. 5 shows the antenna 200 in a homogeneous background above the ground substrate 202, it will be evident to those skilled in the art the other background configurations such as layered dielectric materials, such as a single or multi-layered supporting substrate, can be also used above the antenna, between the two layers of the spirals and between the spiral structure and the ground substrate. Thus, the two layers of spirals shown in FIG. 5 could be positioned on opposite sides of a single substrate (such as a PC board) above the ground substrate in order to conserve space and provide an ease in manufacturing. 
     At the terminating end of both the first spiral element 203 and the second spiral element 205, a shorting bar or stub 207 is used to electrically interconnect both elements. Since the second spiral element 205 is grounded to the ground substrate 202 at its feed-point end 201&#39;, this has the effect of increasing the feed-point impedance. Like the embodiment shown in FIGS. 1 and 2, this effectively raises the input impedance so it can be properly matched to a radio power amplifier output. Although 50 ohms would be a typical value, the shorting stub 207 and the height of the spirals, 209 and 209&#39; above the ground substrate 202 and the distance between the spiral elements 203 and 205, permit this value to be easily adjusted. 
     The shorting stub 207 is generally a quarter of a wavelength away from the feeding point 201 to ensure that the current flow on the vertical sections 209 and 209&#39; are in the same direction and thus maximize the antenna efficiency since the sections 209 and 209&#39; are the primary radiators of this antenna. Moving the shorting stub 207 further away from the feeding point 201 will add an effective capacitive load to the antenna impedance and thus increase the resonant frequency and the impedance at the resulting resonant frequency. Conversely, moving the shorting stub 207 toward the feeding point 201 will add an effective inductive load to the antenna impedance and thus lower the resonant frequency and the impedance at the resulting resonant frequency. 
     The impedance of the antenna 200 is increased by increasing the distance of the spiral elements 203 and/or 205 above the ground substrate 202. The impedance of the antenna 200 depends not only on the structure of the two spiral elements 203, 205 but also on the manner that the antenna 200 is fed. An alternative way of feeding the antenna 200, in FIGS. 4 and 5, is to switch the feeding point 201 and grounding point 201&#39; such that spiral element 205 is directly fed while spiral element 203 is grounded. However, this will result in a lower antenna input impedance. Additionally, FIG. 4 shows the use of a tuning stub 207&#39; that permits the folded spiral antenna 200 to be fine tuned enabling it to operate at a specific resonate frequency. 
     In FIG. 6, a multi-planar spiral antenna 400 is yet another embodiment that is much like the embodiment in FIG. 5 however a first and second spiral element 403, 405 respectively are in one plane while a third spiral element 404 is positioned in a separate plane. The first spiral element 403 is directly fed using a vertical section 409 and the second and third spiral elements 405 and 404 are grounded at the ground substrate 402 using, respectively, vertical sections 409&#39; and 409&#34;. As discussed above, a shorting stub 407 and a tuning stub 407&#39; are used to tune the multi-planar spiral antenna 400 to a desired resonant frequency. Finally, FIG. 7 is another embodiment of a multi-planar spiral antenna 500 where each of the three spiral elements 502, 503 and 505 occupy different planes. The embodiments shown in FIGS. 6 and 7 offer additional advantages in that added antenna gain and efficiency can be achieved due to the additional spiral element acting as a radiator. 
     While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.