Passive, active, and smart antennas

Lost? Use the clue

The simplest antennas are passive, meaning they don't have an amplifier. There are spiral antennas for helix antennas and ceramic plates for patch antennas. To avoid losing weak signals in long cables, they are placed close to the receiver, for example, in mobile phones and trackers. All amplification is done in the radio frequency (RF) cascade of the receiver. This is an inexpensive and straightforward solution, but the quality might not be the best.

The problem is that the receiver is a microchip, which means not all elements can be implemented within it. For example, it's difficult to create a frequency filter, which is very useful for preventing the amplification of various interferences on neighboring frequencies. GNSS signals are already 100 times weaker than natural noise per 1 Hz of the spectrum. And if there is interference, say, the third harmonic of television broadcasting or satellite communication, it can cause problems.

That's why filters are used. The classic example is the surface acoustic wave (SAW) filter, a distant relative of ultrasonic delay lines from tube-based color TVs and quartz generators. However, a SAW filter cannot be integrated into a microchip. It can't be grown out of silicon, and naturally, it introduces losses, meaning it weakens useful signals.

So, SAW filters are used together with amplifiers, and antennas with built-in amplifiers are called active antennas. Typically, the configuration is antenna → amplifier → filter → amplifier, but other variations exist.

The advantage of active antennas is that they produce a stronger signal, which can be transmitted over long cables. Some antennas have amplification up to 40 dB, meaning 10,000 times stronger, allowing for the use of 100-meter coaxial cables. The downside is that the antenna requires power, which is supplied through the same coaxial cable. Note that it's incorrect to assume that an antenna with higher amplification is better than one with lower amplification. The primary criterion is not the signal level but the signal-to-noise ratio. The more an amplifier amplifies, the more noise is added to the signal. Therefore, the amount of noise introduced by the amplifier is more important than the amplification factor, especially if the cable is not hundreds of meters long.

Power supply... Supplying power through a long cable can be a pain. The cable has resistance, and part of the voltage is lost in the cable. It's not always clear what portion of the voltage is lost. Therefore, it's preferable to have a voltage regulator in the antenna, and the power supply voltage should be something like "3-5 volts."

The power supply also has an advantage: by checking the current flowing to the antenna, the receiver can determine if there's a short circuit in the cables or if the antenna is connected. To do this, the receiver has certain limits for what is considered too little current (not connected) and too much current (short circuit). The antenna's supply current should fall between these boundaries, otherwise the automatic system will not work correctly.

The next thought is why do we need an expensive coaxial cable? Why not place the receiver and antenna in one large housing and get the digital signal from the receiver through a cheap twisted pair?
That's how SMART antennas emerged, which are essentially more receivers than antennas. As I mentioned earlier, SMART antennas are used everywhere except for timing applications.

"To my shame, when we made our first SMART antenna, we didn't sufficiently isolate the receiver from the antenna itself. The local oscillator in the receiver emits radiation, and the digital part generates noise at the same frequencies. As a result, interference from the receiver was induced directly onto the RF amplifier of the antenna, amplified by it, and sent back to the receiver. No, it didn't cause direct interference, but the receiver's amplifier was operating in a mode it was not designed for and overheating. This led to the receivers failing after a couple of months."

The point is, if you place the antenna and receiver in the same housing, separate their ground planes. And on the side where the antenna is located, don't mount any high-frequency or digital circuits on the board.

© Eltehs SIA 2023