The question of how many Radio Frequency (RF) sensors are needed sounds simple, but the situation is a bit like Goldilocks' porridge dilemma: you can have too many of the wrong type of sensors, or you can have too few of the right ones. The 'just right for your project' number of sensors is achievable, but you need to know which questions to ask.
Some careful analysis needs to be carried out to define how many RF sensors will be required. Alongside the project's RF requirements, you need to understand the full system, electrical, mechanical, space envelope, cost requirements, etc. The outlined process to follow is:
- Identify what RF properties you need to detect
- Confirm what information you need to know about the signal
- Understand where the system needs to work
- Check the system is buildable
Before we go into these steps in more detail, let's first consider what an RF sensor is.
RF sensors
An RF sensor is a device that receives and analyses a signal in the radio part of the electromagnetic spectrum. Depending on the sensor, it could tell you how strong the signal is, where it is coming from, or what information is included in it. To recover this information, downstream signal processing is required. This is often included in the RF sensor itself, but it can be done through external proprietary systems or some combination of the two.
It helps to think of RF sensing being split into three different areas: the antenna, the RF hardware, and the signal processing software.
The antenna part of the RF sensor receives the signal and couples it from a free space radio wave to an electrical current, which the RF hardware can then analyze. Antennas have a limited range of frequency over which they can operate properly, and their construction dictates what kind of signal you can receive with it. It is common for the antenna to be physically separate from the rest of the sensor system and connected via an RF cable. This gives the user access to the full range of use of the RF sensor by connecting different antennas and allows equipment re-use.
Behind the antenna is the RF and electrical hardware, which takes the electrical signal and does the required initial processing. In modern systems, this signal is digitalized at an early stage, allowing several improvements to be made because it has replaced sections of the RF hardware with software processing. For example, this has allowed the use of Software Defined Radio (SDR) receivers, which can handle many tasks with a software update rather than an expensive hardware change.
The signal processing software analyses the signal and pulls out the pertinent pieces of information that the project needs. Normally, the signal processing is split between the RF sensor and a proprietary system bespoke to the project. Typically, the RF sensor gets the standard information about the signal, and the proprietary system gets the project-specific information. It is important that the RF sensor you use has an Application Programming Interface (API) to integrate the information into the proprietary system so you can streamline processes with automation.
An RF sensor will tell you about the signal received but not the object sending the signal. For this, you would need to use a radar system that transmits and receives its signal.
What can RF sensors do?
RF sensors can fulfill a variety of functions, for example:
- Get information from a transmitted signal.
- Spectrum monitoring – to find when and where the frequency is being used and potentially by whom.
The good news is that the number of receivers each function needs can be straightforward. For example, three receivers are needed for two-dimensional TDoA and four for three-dimensional TDoA.
Unfortunately, this still does not answer the question: 'How many sensors do I need?' Adopting the approach of 'having as many sensors as possible' does not solve the problem. This is like Goldilocks' hot porridge—resulting in a system that is overly complex, expensive (or cheap and ineffective), and ultimately inefficient.
Likewise, an approach of 'we can manage with one RF sensor,' or 'there's no time to complicate the project with detailed calculations' is equally flawed. This approach, the cold porridge option, often leads to a system that either underperforms or fails to collect the necessary data, leaving critical gaps in information.
What do I need to consider?
The following stages outline considerations to help determine the number and type of RF sensors required for a system.
STAGE ONE is to define what you are trying to detect. For example: 'I want to detect the location of a hidden communications antenna' or 'I want to detect transmissions in a certain frequency range, and when they are being used.'
The physical quantity needing to be measured will dictate which RF sensors are appropriate for your system. Other general signal considerations will influence the choice and number of RF sensors, such as:
- What is the frequency range?
- Is it a static or moving signal?
- How many signals are to be detected?
STAGE TWO is to consider what information you need to get from your signal. There are different levels of detection, from 'it exists' to 'I need to know the exact encoded message'. This means you need some initial information about the signal. Also, consider how the information needs to be accessed. Is it in a field on a mobile test range, in the middle of a city, or a permanent installation in an airfield?
What are the parameters of the received signal? For example:
- Modulation characteristics
What performance do you require from the RF sensor, and is it of the correct standard to work with the signal given the needs of the project? For example:
- I/Q signal capture requirements
- Is geolocation a requirement?
How does the project need to monitor, capture, and analyze the data? For example:
- HDMI console port for quick access to results
- Integration into a proprietary system with an API for automation
STAGE THREE is to complete an onsite propagation analysis so you understand the operating conditions of the system both in the short and long term. Outside factors can significantly affect RF propagation and impact how sensors work. Things to consider:
- Obstructions, natural and manmade, which can block or reflect the signal through RF multipath effects. These obstructions can be static or moving.
- Atmospheric conditions over which the antenna has to operate. Changing atmospheric conditions alters the attenuation of the RF signal significantly.
- Electromagnetic interference from the surroundings. This can be natural or manmade, constant or intermittent.
- Environmental disruption over the lifetime of the system installation.
STAGE FOUR moves onto considerations outside of the pure RF requirements of the project. It can be thought of as 'can this system be built?'. Considerations such as can/are the RF sensors:
- Fit within the installation volume?
- Be placed in their correct position and orientation to effectively receive the required transmissions?
- Be placed on the hardware so that they will not degrade the performance of each other?
- Within the financial budget?
- Within the electrical power budget?
- Within the environmental operating requirements?
- Within the mass budget for the installation?
- Be easily installed and set up?
Conclusion
Once you have worked your way through the different stages, only then will you get your answer as to how many RF sensors are needed for the project to function properly, arriving at the 'just right' number of RF sensors.
There are many things to understand before the choice and number of RF sensors can be finalized. Fortunately, the answers to each question should be found in the specification documentation for the project. Once all the information is collected, saying, 'you need X number of RF sensors' should be easy with the right guidance.
