Antenna Size matters!
Wireless Sensor Networks (WSN) are key parts of the Internet of Things (IoT), and are taking more and more advantage of longer range and less interference in the sub-GHz frequency bands compared to 2.45 GHz. A challenge that then arises is the size of the antenna and understanding why the antenna size matters for the final design, because the lower the frequencies, the bigger the antenna needs to be due to the longer wavelength.
We want to keep the antenna as small as possible, but still pay attention to the “link budget”. The link budget is the total signal gain and loss between the transmitter and the receiver. It is determined by the output power of the transmitter, the sensitivity of the receiver, as well as the antenna gain at both sides, and the “Path loss”.
“Path loss” is a measure of how much signal power is lost from the transmitter to the receiver. The “Path loss” is the loss of signal in an ideal link between two isotropic antennas due to the spreading of power from the source into space. It also depends on the radio frequency due to the area of the antenna which scales with the wavelength. Therefore, the Path loss increases with higher frequencies.
Obstacles and reflections will also impact the signal loss. Lower frequencies tend to creep around corners and are therefore less affected by buildings or hills. Also note that 2.45 GHz is attenuated in humidity, such as raindrops on leaves or humidity in concrete walls. That is the reason the microwave oven uses 2.45 GHz to heat up the water molecules in the food instead of for satellite communication where this frequency is deemed unsuitable.
The RF module characteristics that are important for the link budget are sensitivity and TX power. A larger negative number for sensitivity is better, and a larger positive number for the TX is better.
The last item that impacts the link budget is the antenna, which also has a large impact on the industrial design of the final product. Often a compromise must be made between an appealing design and the antenna performance. The lower the frequency, the larger the antenna should be to achieve the same gain. So, for a given link budget, the better the RF performance of the RF module, the smaller the antenna can be. A 1-2dB extra sensitivity and TX power, can reduce the antenna size by 50% which is several centimeters in the sub-GHz domain. Those centimeters will, in many cases, play a key role in allowing for a customer appealing industrial design. These reasons among others are why antenna size matters.
Radiocrafts has released a White Paper which is intended to guide you to the best antenna selection for your system. You will also find several high performing RF modules on the website.