By Mark W. Ingalls | Machine2Wireless LLC
Michael D. Glover | University of Arkansas
Ed Liang | MCV Microwave, Inc.
Typically, an internal antenna may be custom designed and integrated into a radio’s printed wiring board (PWB) or case, implemented as a separate component plus matching elements that are then attached to the PWB or remotely connected to the radio by means of a cable and connectors. These alternatives suffer from both inflexibility and poor economy. A new hexa-band cellular antenna compatible with mass production has been designed and tested that is partly integrated into the PWB. The new design thus offers the benefits of both customization and high-volume production without requiring a separate matching network.
There is a broad trend toward connecting machines to the internet and to each other wirelessly using radios that have been integrated into the machine’s packaging. Equipment manufacturers, wanting to provide a product that is generally serviceable at a reasonable cost, naturally desire that their products in general, and the installed antenna in particular, to be able to operate as flexibly and economically as possible.
In addition to being capable of receiving and transmitting wireless information flexibly and economically, the antenna must also be designed so as to be compatible with the radio equipment to which it is connected. It is thus said that the antenna and the radio equipment must be matched. The essential characteristic of this compatibility, or match, is that information will be passed from antenna to radio and from radio to antenna without reflection of the electromagnetic waves upon which the information is superposed.
One must ensure that all elements are matched one to another, for example, by selecting and inserting extra circuit elements (Figure 1), or by modifying the design of the elements themselves. This process is not too difficult when it comes to the non-radiating elements of the signal path, because these elements are relatively immune to influence by other system changes, that is, to the package that contains the radio equipment. Once the circuit connections to and from the non-radiating elements in the signal path are quantified and their influences on the match between those elements and the rest of the network have been matched, no further tuning is usually necessary.
For example, a system designer may specify that an amplifier, a filter, a switch, a mixer, connectors, cables and other elements of a radio all match when incorporated into a system. The vendors of these elements will be able to provide the same components, be they amplifiers, filters or so on, to many different system designers for integration into many different systems, as long as the systems matching requirements are similarly defined. But this is not necessarily the case with the antenna.
The antenna, by virtue of its unique role in the transmission and reception of information-bearing waves, is susceptible to the influence of local but non-connected elements, such as the package that contains the antenna or the PWB that supports other system elements. One way to deal with this problem is to place the antenna far away from anything that might influence its matching characteristics and connect it to the radio with a cable. Another way is for the antenna manufacturer to precisely define the characteristics of the package that houses the antenna. However, forcing these definitions on the equipment manufactures is not “flexible.” A third way to deal with the influence of non-connected elements on the antenna is to custom-tune the antenna for each and every particular situation; for example, every possible style of packaging. This requires specially designing different antennas for different packages. Unless the radio equipment will be manufactured in large quantities, this option is not “economical.”
There is thus a need for an antenna that is “customizable” to a particular installation at minimum cost and without the need for additional matching elements to be purchased and installed into the radio.
An antenna may be incorporated into a packaged radio by integrating it into the PWB or package of the radio itself (Figure 2). In this instance, the antenna cost is low, because the PWB and package have to be manufactured anyway. But the antenna’s performance may be compromised because the materials used to manufacture the PWB or package are not necessarily suited to the requirements of antenna design. The main drawback, however, is that custom designing an integrated antenna involves a high design cost.
Another way that an antenna may be incorporated into a packaged radio is to design it as a discrete element that is then attached or connected to the PWB (Figure 3). This usually results in better performance, but at higher cost. The antenna cost is slightly higher, and it typically requires a custom matching network, as described previously, which must be counted toward the total manufacturing cost of the antenna. The design cost is lower but now includes the matching network.
A third way that an antenna may be incorporated into a packaged radio is by providing a short ‘pigtail’ with the antenna (Figure 4). This method allows the antenna to be placed away from the PWB, allowing more design flexibility, but the cost of the pigtail, a board connector, and the inevitable matching network must still be assigned to the antenna cost.
Multiband Hybrid Design
In the proposed design some of the antenna’s radiating functionality is designed into the PWB, and some of the functionality is designed into a mass produced portion that is attached to the PWB portion during assembly of the radio. The radiating functionality is shared between an integrated, customized PWB portion and a separately mass-produced portion which is added to the first portion in a later step when the radio is assembled. The radiating functionality of the antenna is thus realized by the composite of the PWB portion and the mass-produced portion.
Unlike prior designs which divided the antenna into a PWB portion that operates in one sub-band and an attached portion that operates in another sub-band, i.e., in an uncooperative manner, both the PWB portion and the attached portion operate together cooperatively over the entire frequency range of the proposed design. Tuning is still required for the proposed design to be matched to the packaged radio, but all the necessary tuning features are incorporated into the “cost-free” PWB portion, while the mass produced assembled portion is designed to afford the proposed design the improved performance associated with the discrete antenna of prior art. Thus, the proposed design is a composite of a tunable, low cost, integrated portion and an efficiently radiating mass-produced portion (Figure 5).
The PWB portion of the design may be tuned at one or more separate locations for optimum matching while the attached portion is designed to be economically mass produced. The hexa-band embodiment is able to work in a wide variety of packaged cellular radios by virtue of the fact that it is a composite of a mass produced portion that radiates efficiently like the typical antenna that is mounted on the PWB and a highly customizable portion that is essentially free of manufacturing cost like the typical antenna that is integrated into the PWB.
The applied, mass-produced hardware presented here was realized from a metal stamping, but it may also be realized from metallization applied on or in a supporting structure that is a monolithic or assembled structure made from ceramic, plastic, glass, or the like. So the applied, mass-produced metal portion may be self-supporting or it may be supported with a non-metallic support structure. The key feature of the design is that the antenna’s functionality is hybridized, with tunable matching functionality incorporated into the integrated PWB portion or portions and radiating functionality shared by both the integrated PWB portion and the applied mass-produced portion or portions.
Performance of Hybrid Design
In order to gauge the performance of the design, testing was performed using a ‘real’ board layout and components as shown in Figure 6. The grounded coplanar waveguide RF trace was interrupted at the radio module’s RF pin and a backside SMA wave launcher was soldered to the modified board.
The measured return loss of five prototype samples are presented in Figure 7. During the simulation process, it was noted that adding a shunt inductor to the design improved the return loss of the upper frequency range to -15 dB. This strategy would have improved the input match but at the expense of Ohmic loss in the inductor, plus added manufacturing cost.
The measured return loss of the new design in the upper frequency was -7.5 dB, or 17.8 percent magnitude. If we improve the return loss to -15 dB with the addition of a shunt inductor (3.2 percent magnitude), the additional power delivered to the antenna would potentially be 10*log10(1-.146) 0.7 dB. But if we allow for the insertion loss of a lumped inductor, say 0.2 dB, the potential benefit of improving the match amounts to less than a half decibel at the matched frequency. For many applications, the slight improvement in match does not justify the additional cost and complexity of adding the matching element to the bill of materials.
The modeled peak gains of the new design are presented in Table 1. Typical modeled antenna radiation patterns are presented in Figures 8 and 9.
A new hexa-band cellular antenna that is partly integrated into the PWB and partly a massed produced component has been designed and tested. The new design demonstrates the benefits of both customization and high-volume production but without the need for a separate matching network. Measured and modeled data of the prototypes were in good agreement. This low cost, high performance antenna alternative is now commercially available.
The assistance of the staff at the University of Arkansas High Density Electronics Center (HiDEC) is greatly appreciated in building and testing the prototypes.
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