Anti-Jamming Technology in Antennas
Polarization in Anti-Jamming Antennas
Another trick of antennas is polarization. Signals from satellites are RHCP, meaning right-hand circular polarization. With every reflection, the polarization changes to the opposite, from right to left, from RHCP to LHCP and vice versa. Helical (Helix) and Cross-dipole antennas are designed to receive RHCP polarization much better than LHCP. This feature is called anti-jamming.
Signal Reception Efficiency
In the photo, you can see how RHCP signals (black) are received much better than reflected LHCP signals (red). Additionally, it’s noticeable how the reception level drops as the angle from the vertical increases.
The Role of Patch Antennas and Groundplane
patch antennas are not very effective at distinguishing RHCP from LHCP. Moreover, the larger the groundplane, the weaker the distinction. This does not undermine the advantages of a large groundplane; it simply does not contribute to filtering reflected signals in the upper hemisphere.
Reducing Interference with Anti-Jamming Antennas
Interference comes in arbitrary polarizations—linear, right-handed, or left-handed. Therefore, anti-jamming antennas receive fewer interference signals.
Measuring Signal Quality with Axial Ratio
The quality of the antenna is measured using a peculiar parameter called the Axial Ratio, which shows how close the antenna is to ideal polarization, where only RHCP is received. For an ideal antenna, it is 0 dB (i.e., 1). For a good antenna, it is 1-2 dB (i.e., 1.12-1.26). If this parameter is very high, it is not specified.
Understanding Axial Ratio
To put it into perspective, an Axial Ratio of 1 dB means a 25 dB difference in the strength between the direct and reflected signal, i.e., about 18 times in voltage and 316 times in power. An Axial Ratio of 2 dB means a 19 dB difference in signal strength, i.e., about 9 times in voltage and 80 times in power. An Axial Ratio of 3 dB means a 15 dB difference in signal strength, i.e., about 5.5 times in voltage and 30 times in power.
Choosing the Best Anti-Jamming Antennas for Precision Applications
If you are involved in precision agriculture, drone navigation, RTK positioning, or GNSS surveying, investing in high-performance anti-jamming antennas is essential for maintaining accuracy and reliability. In our store, you can purchase the modern, high-tech SMA Multi-Band GNSS Embedded Cross Dipole Active GNSS Antenna (ELT0148)
If you need assistance in choosing the right solution, please contact our support team.
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