The need to receive or listen to exceptionally faint signals is often a requirement in all kinds of applications ranging from SIGINT to communications to regulatory compliance measurements. Active antennas are often used in these applications, as these allow the tiny received signal to be received and ‘boosted’ before relaying it down a long and lossy cable to the receiver. In this two-part series we will delve into how and why active antennas are needed in these applications, and then demystify some of the specifications and plots that are often found on datasheets for these types of antennas.
While there are many discussions on the Internet exploring the mathematical interplay between concepts like signal-to-noise ratio, signal strength and signal quality, we will be steering clear of the maths this time.
Signal strength, SNR and signal quality
To illustrate the concepts related to signal-to-noise ratio (SNR), there are a few concepts that we will explore. To get an intuitive understanding of these concepts, we will use a few images that we will zoom, stretch and manipulate to show noise and signal strength interact, and how it influences our ability to extract information from a received signal.
In typical communication systems, there are various ways of encoding information onto the signal that is transmitted, a process we call modulation. If we are interested in sending digital data (ones and zeroes), we have various options available. Some modulation schemes allow us to encode multiple bits at a time, and hence allowing us to send data at much higher data rates. For this to work well, however, the signal quality needs to be excellent. In essence, higher modulation schemes are more difficult for the receiver to “read”. In our illustrations, we will use the font sizes to indicate the modulation scheme. Smaller font sizes can fit much more text onto a single page, at the cost of being more difficult to read.
Depending on the reader’s screen resolution, the QAM-256 signal will be barely decodable (readable). This represents the highest modulation scheme (smallest font) that the receiver (the reader) can decode for this given signal strength (image size).
Now, suppose the signal that we are trying to decode is rather weak by the time it arrives at the receiver, maybe due to a very long and lossy cable between the antenna and the receiver.
In this image, it is no longer possible to decode (read) the 256-QAM line at all. There simply is not enough signal (resolution) for the receiver (reader) us to make out what it says anymore. The same is true for 128-QAM, and 64-QAM which may be barely readable, although the receiver might suffer from high bit-rate errors (reading the letters incorrectly). Practically speaking, 32-QAM would be the highest modulation scheme useable at this signal level.
To try and solve this problem, we decide to add an amplifier just before the receiver. Logic tells us that, if we can boost the signal (scale up the image) to the same signal level (image size) as the original, we should again be able to decode the higher modulation schemes!
Much to our dismay, however, we notice that we are no better off than we were previously. We are still not able to decode (read) 256-QAM, 128-QAM or even 64-QAM! In fact, we seem to be slightly worse off than we were previously, barely able to read 64-QAM anymore. Upon closer inspection, we notice that our signal (image), while being much stronger (larger) now, is rather noisy (pixelated) compared to the original. It turns out that amplifying the signal not only amplifies the signal of interest, but also any noise that is already on the signal! Our signal-to-noise ratio has not changed compared to the signal we tried to amplify, and hence we have no additional information compared to the weaker signal.
We can thus conclude that amplification does not increase the signal-to-noise ratio of a signal. We cannot recover information from the signal that is already lost by simply amplifying the signal.
Active antennas to the rescue!
To address this issue, we will need to boost our signal before we lose the information. In most receiver systems, there is a rather long and lossy path between the antenna and the receiver. The system noise floor (minimum resolution of the reader’s monitor) will stay the same, but the signal level of our signal of interest will drop due to the losses in the cable. This is the basic mechanism which is responsible for reducing the signal to noise ratio in purely passive systems.
Let’s see what will happen if we can boost the signal right as it arrives at the antenna, and before it makes its way down the lossy cables.
We can see that amplifying the signal at this point again amplified the noise along with the signal, but since we started off with a signal with higher SNR, the 256-QAM signal should still be quite decodable. Now we send this amplified signal down the cable, where we will inevitably lose signal strength.
The signal (image) arriving at the receiver, after having been reduced by 3dB (50%) as it travelled through the lossy cable, is still very clear and the receiver (reader) can still decode (read) the 256-QAM information to some extent, although there may be some bit-errors. The 128-QAM line is, however, quite useable (readable), due to the much-improved SNR compared to the signal that was only amplified at the receiver.
Part 2 of this series will delve deeper into the parameters of active monitoring antennas, and how Systems Engineers can evaluate the specifications of these antennas to choose the appropriate antenna for their system.
Here is a side-by-side comparison of the signal amplified at the antenna (left) vs the signal amplified at the receiver (right).
The ability to successfully demodulate or decode the signal arriving at the receiver is almost entirely determined by the signal-to-noise ratio of the signal, not the overall signal strength. Active antennas, which amplify the signal as it arrives at the antenna, provides the best possible SNR to the receiver by effectively fixing the SNR at the point where the highest possible SNR can be achieved, that is, right after the signal has been received by the antenna. By amplifying the signal at the antenna, any further down-stream losses are effectively eliminated, thus preventing cable losses and other components from further reducing the SNR before the signal reaches the receiver.
In part two of this series, we will look at some examples of specifications and plots that can be found on datasheets of active antennas and see how these can be used to evaluate and compare different products.