We know that a dipole on FR4 PCB material is about 8 centimeters in length, just a little shorter than the average phone platform.
Ignoring this requirement, some manufacturers simply print a monopole on the main PCB, and put up with the coupling, losses, and pattern deficiencies that arise. Some while ago, we measured the gain of such an arrangement at about dB relative to the reference dipole. So designers have turned to size-reduced antennas, either by using higher dielectric materials to form them, or by using complex shape and feed derivatives such as the PIFA in Figure 2.
Another combo idea is to use the communications antenna. So this is not really too attractive, especially when measurements show a dB performance relative to our dipole, along with a poor coverage pattern. In this model, removing the whip and leaving the ferrule to which it connects provided a 6 dB improvement in performance for GPS only; obviously it spoils the communications function.
Figure 3. A more conventional approach is to fit an off-the-shelf GPS antenna. The problem here is that any component-type antenna will have been tested with some standardized ground plane, and most are reliant on the ground plane for both tuning, and pattern and gain.
Although these antennas have found favor in personal navigation devices for their superior performance, they are not usually considered for mobile phones because of cost and size considerations. This antenna did, however, give us a reference device against which we could make comparative measurements when undertaking the practical test campaign.
Figure 4. A more usual selection is the patch type, long standard in the GPS industry. We measured the gain of this antenna at about 28 dB compared to a dipole antenna connected to the same point in the circuit, which is actually at the better end of the performance range that we see. In this phone, the communications antenna is again at the bottom of the PCB.
Figure 5. Figure 6. Edge view of GPS antenna, top of phone removed. This phone includes an external GPS antenna input connector seen here mounted below the patch antenna. Interference and Isolation. The related characteristics of interference and isolation are difficult to specify and model, leading to practical measurements as the only way of accurately characterizing them. Of course, since the mechanical arrangement including plastics, screen, battery, and PCB components plays such a large part in determining the levels of interference and isolation, these tests can only be carried out once the phone is at the prototype stage, when major surgery to improve any particular aspect is not really an option.
This also creates a problem when considering new approaches, as the result may not resemble the stand-alone tests, unless the antenna element chosen really has no significant interaction with the rest of the phone.
Most interference we see in mobile phones gets into the GPS receiver at the antenna. Typically this is followed by an RF filter of some sort, which although it spoils the noise figure, does eliminate the out-of-band transmissions from the other radios on the platform.
Usually we see a plethora of self-generated in-band signals that have entered the GPS receiver via the antenna. One effect seen in current offerings is that the GPS antenna may actually be much better at coupling to interferers than it is at extracting GPS signals from free space, thus making the problem worse.
To get a view of the coupling between antennas, we tested a few available phone types to see what was the actual coupling in the antenna band of interest see TABLE 1.
Of course, one advantage of a poor antenna is that its coupling is likely to be less to adjacent antennas. Coupling is also seriously affected by the user holding the phone or the surface on which it is placed. Phones in a pocket seem to be more affected in this way. To develop requirements for a better antenna implementation, we need to consider the factors discussed above, and to develop numerical specifications against each.
Given the variables involving user interaction, mechanical changes from model to model, use cases and the ever-increasing pressure on cost and size, this is far from straightforward.
Our team has spent considerable time defining requirements, and a short synopsis is reported here. In addition to the coexistence requirements see the next section , the antenna should fulfill the following criteria:.
Coexistence and Cohabitation. Initially we aim to define the parameters affecting interaction with other services on the phone platform. By coexistence, we mean the ability to share a platform with the other radios and antennas and only be marginally affected by them, whatever they are doing such as transmitting full power, low power, or idling, and with any frequency choice.
This produces a straightforward immunity table see TABLE 2 once we have determined the basic isolation between all of the elements. For the purposes of Table 2, we have chosen 15 dB as the minimum isolation value between any two antennas.
A glance at Table 2 will tell the reader that the modern mobile phone implements a vast number of transmit and receive frequencies, modulation types, and standards.
Of particular concern to the GPS designer is the advent of wideband CDMA signals, which can cause intermodulation products to appear in band at the intermediate frequency of the GPS receiver. Special receiver techniques are required in this case, but the antenna is unable to help except by being of naturally narrow bandwidth. Cohabitation is a newer concept that describes the isolation between functions of the same device.
This is a fairly natural development, since these functions are all add-ons to a conventional phone platform, and there is a space-saving advantage in the combination. Since Wi-Fi and Bluetooth share the same band at 2. As a precursor to forming some specifications, our team measured a commercially available combined antenna, and TABLE 3 shows the isolation results. A number of tests were carried out on available solutions to gain some information and experience about current offerings and platforms.
A GTEM cell is an expanded transmission line within which a uniform electromagnetic field can be generated for determining antenna properties such as gain and bandwidth. The internal space at the septum 40 centimeters is big enough to handle antenna sizes used by GPS. It has a small side door and some feedthroughs coaxial to the bottom plate.
Figure 7. Differential vs. Single-Ended Antennas. The first test conducted concerned comparison of balanced and unbalanced antennas, the theory being that a balanced antenna would help with interference because it would be presented to the GPS receiver as a common mode signal that is, balanced on the positive and negative inputs. The trial began with calibration of the test setup using the balanced antenna shown in Figure 4, against which we measured a printed dipole antenna and a monopole equivalent, arranged to incorporate a balun to make it of the same size as the dipole see FIGURE 8.
Once this calibration had been made, we sought to generate an interfering signal on the GPS receiver test board so that comparisons of interference rejection could be made. This was done in two different ways, in case the method of exciting the GPS board was subject to resonances or peculiar standing-wave modes. The jamming created in this manner was increased until a predetermined drop in GPS sensitivity was reached.
A number of frequencies were tried and the results compared. In the second setup, we directly applied an RF signal across the ground plane of the GPS board, using a coaxial feed to excite the ground plane, and repeated the stages described above. Figure 8. Antennas used in the balanced vs. Results for both tests were within 2 dB of each other, and showed that the differential approach could reduce local jammer pickup by only 4—6 dB.
This is probably due to the differential structure being of similar size to the test platform chosen to be similar to a phone platform , and therefore not achieving true differential coupling to the on-board radiated jammer. With this marginal advantage, we concluded that the benefit was barely justified by the extra complexity and size involved in differential antennas.
The peak absorption must be less than 1. The location and type of antenna significantly affect the SAR value, and if the SAR is too high you can't sell your phone. In addition, there are specs for the TRP when the phone is mounted on a mannequin head. TRP is a function of the radios transmit power and the antenna efficiency. The receiving requirements for the primary cell antenna are: TIS - In addition to TRP, the cellular carriers set minimum specifications for the amount of Total Isotropic Sensitivity TIS for every frequency band the phone will operate in.
TIS is a function of the radio's conducted sensitivity, the antenna efficiency, and desense. We also need to know what frequencies the antenna will work at. The primary cellular antenna will typically have both a lowband somewhere between and MHz , and a highband somewhere between and MHz. OnePlus 8 GB. OnePlus 9 Android GB. Apple iPhone 12 64 GB. Apple iPhone 12 GB. Apple iPhone 12 Pro GB.
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