Active vs. Passive Antennas for GPS Systems: A Comparison

Active and passive antennas play vital roles in GPS systems, each optimized for signal reception based on specific needs. Active antennas, featuring integrated amplifiers, boost weak signals, making them suitable for long cable runs or challenging environments. Conversely, passive antennas offer reliable signal transmission in short-distance setups without needing external power, ideal for strong signal areas. Choosing the right antenna hinges on understanding these key differences.

Active Antenna

An active GPS antenna incorporates a built-in Low-Noise Amplifier (LNA) to strengthen received signals. This amplification compensates for signal loss due to lengthy cable runs or weak signals from GPS satellites. Active antennas are commonly used when the antenna is far from the GPS receiver or in areas with poor signal reception.

Typically, the LNA is housed with the antenna circuitry. This offers two advantages:

• Cable losses after the LNA device won’t affect the overall noise figure of the GPS receiver.
• The LNA, combined with the antenna, helps reduce the system’s overall noise figure, improving sensitivity.

Active antennas are generally preferred when the RF cable length between the antenna and receiver exceeds 10 cm. They typically consume 3 to 20 mA in GPS systems.

The primary concern is ensuring the LNA doesn’t overload the receiver.

Advantages of Active Antennas

Here’s what makes active antennas beneficial:

1. Low Receiver Noise Figure: Helps maintain a low noise figure for the receiver.
2. Jamming Resistance: Less susceptible to jamming on the antenna cable (if equipped with a filter).
3. Signal Amplification: Amplifies weak signals, improving overall reception.
4. Long Cable Runs: Suitable for long cable runs without significant signal loss.

Disadvantages of Active Antennas

However, active antennas also have drawbacks:

1. External Power Required: Needs an external power supply to power the amplifier, consuming more power (about 10 to 60 mW) compared to passive antennas.
2. Higher Cost: More expensive than passive antennas due to additional components.
3. Potential for Noise: Can introduce noise or interference if not designed correctly.

Passive Antenna

A passive GPS antenna lacks built-in amplifiers. It simply receives GPS signals and transmits them directly to the receiver.

Passive antennas are typically used when the antenna is close to the GPS receiver and signal attenuation is minimal. These antennas consist only of radiating elements, such as ceramic patches or helix structures.

A passive matching network is often used to match the electrical connection with a 50 Ohm impedance line. Passive antennas do not require a power supply.

Advantages of Passive Antennas

The benefits of passive antennas include:

1. No Power Supply Needed: Simple and easy to deploy as no power supply is needed.
2. Lower Cost: Less expensive than active antennas.
3. Reduced Noise Risk: Minimizes the risk of introducing additional noise or interference.

Disadvantages of Passive Antennas

The drawbacks of passive antennas are:

1. Signal Loss in Long Runs: Signal loss can occur in long cable runs.
2. Degraded Performance in Weak Signals: Performance may suffer in areas with weak GPS signals. The antenna must be connected with a carefully designed microstrip or stripline of max. 10 cm to the GPS receiver to ensure good GPS performance.
3. Limited Environments: Not suitable for environments with heavy obstructions or interference.

Active vs. Passive Antenna Comparison

The following table highlights the differences between active and passive antennas:

Conclusion

Active and passive antennas serve different purposes in GPS applications. Active antennas excel in scenarios with long cable runs or weak GPS signals, while passive antennas are best for short-distance installations with strong signal reception. Understanding these differences is crucial for selecting the appropriate antenna type based on your specific needs.