The GPS receiving antenna plays a crucial role in converting the electromagnetic energy from satellite signals into electrical current that can be processed by the receiver. The size and design of the antenna are critical, as they directly influence its ability to capture weak GPS signals. Antennas can be designed to operate on a single L1 frequency or both L1 and L2 frequencies, depending on the application. Since GPS signals are transmitted as circularly polarized waves, all receiving antennas must also be circularly polarized. This ensures that the signal is properly aligned with the satellite's polarization, allowing for clear reception without the need to adjust the polarization direction manually.
There are various types of GPS antennas available, including unipolar, bipolar, helical, quadrifilar helical, and microstrip designs. Among these, two common built-in types found in GPS receivers are flat antennas and four-arm helical antennas.
Flat antennas are widely used due to their durability and ease of manufacturing. They typically have a shape resembling a copper-clad printed circuit board, and can be round, square, or rectangular. These antennas are often compact, making them ideal for aerospace and portable applications. However, one key limitation is their directional nature. They perform best when facing directly upward, which can be inconvenient for handheld or vehicle-mounted systems. Additionally, if no satellites are visible at low angles, the accuracy of positioning may suffer significantly.
To overcome these limitations, the four-arm helical antenna was introduced. This type of antenna consists of four curved metal elements arranged in a helical pattern, and it does not require grounding. It combines the benefits of a patch antenna and a vertical antenna, offering a gain of approximately 3dB in all directions. This allows for 360-degree signal reception, making it highly versatile for use with PDAs and other mobile devices. Unlike flat-panel antennas, the four-arm helical antenna can effectively receive signals even when satellites are just 10 degrees above the horizon.
However, this antenna is more susceptible to interference from nearby sources, as its horizontal gain can amplify noise along with the desired signal. Modern four-arm helical antennas, however, have been improved to reduce this issue. Made from ceramic materials, they have a very small near field—only about 3–5mm—which minimizes interference caused by the user’s body when holding the device. In contrast, traditional antennas can have near fields up to 1 meter, leading to greater signal distortion.
Overall, the choice between a flat antenna and a four-arm helical antenna depends on the specific application and environmental conditions. Each has its own advantages and limitations, and selecting the right type ensures optimal performance and reliability in GPS signal reception.
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