The following is excerpted from Chapter 3: Radio Basics for UHF RFID from the Book, The RF in RFID: Passive UHF RFID in Practice by Daniel M. Dobkin. Order a copy of The RF in RFID: Passive UHF RFID in Practice before December 31, 2007 to receive an additional 20% off! Visit www.newnespress.com or call 1-800-545-2522 and use code 91090.
While this book excerpt from The RF in RFID:Passive UHF RFID in Practice, focuses on RFID applications, it is an excellent primer for RF basics.
Part 1 covers electromagnetic waves, signal voltage, and power.
Part 2 covers modulation and multiplexing.
Part 3 covers backscatter radio links and introduces link budgets.
Part 5 focuses on the effect of antenna gain on range.
Part 6 covers antenna polarization.
Part 7 covers antenna propagation.
This part reveals how to determine the link budget.
In order to find the forward link budget, we need to know the following:
- How much power can the reader transmit?
- How much power does the tag receive as a function of distance from the reader?
- How much power does the tag need to turn on?
- How much power does the tag need to decode the reader signal?
Let's examine each question in turn.
Reader Transmit Power
The reader transmit power is set by a combination of practicality and regulation. Most RFID equipment operates in spectrum set aside for unlicensed use by the governmental body that regulates radio operation in a given jurisdiction. For example, in the United States, the FCC allows operation in the band 902"928 MHz without requiring that the person operating the equipment have a license to do so. However, the equipment itself must obey certain operating limitations in order to allow unlicensed use. Relevant for us at the moment is the maximum transmit power, which cannot exceed 1 W. While not all readers will deliver a watt, and in some applications, we may intentionally reduce transmitted power, in many cases a UHF reader will be operated at the legal limit. So let's assume we transmit 1 W of total power.
The difference between the power delivered to the transmitting antenna and that obtained from the receiving antenna is known as the path loss. In general, finding the path loss requires knowing something about the details of the antenna operation, and we shall discuss the relevant measurements and terminology shortly. However, to get started, we will use the simplest possible (not very accurate) approach: let us assume that the transmitting antenna radiates in all directions with the same power density, that is the transmitter is isotropic. We can picture the radiated power as being uniformly distributed over a spherical surface at any given distance r from the reader antenna (Figure 3.21). Some of this power can be collected by a tag antenna. It is reasonable to guess that the amount of power collected should be proportional to the density of power impinging on the tag and dimensionally necessary that the constant of proportionality be an area, often known as the effective aperture Ae of the tag antenna.
Since in the isotropic case the power density at a distance r is the ratio of the transmitted power PTX to the sphere area, we can find the power received by the tag PRX:
3.21. An Isotropic Antenna Radiates Power Uniformly Over the Surface of a Sphere.