RFID Basics: Antenna Polarization

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 4 reveals how to determine the link budget.
Part 5 focuses on the effect of antenna gain on range.
Part 7 covers antenna propagation.

This part covers antenna polarization.

Real tag antennas have some gain, but it is typically modest (around 2 dBi, since they are usually dipole-like), and since we don't always control the exact orientation of the tag antenna and may not be able to guarantee that the main beam of the tag antenna is pointed at the reader, it is prudent to count on minimal gain from the tag antenna.

Using the Friis equation, we can also, after a bit of algebra, provide a couple of convenient range equations that can be useful for quick estimates. First, defining the minimum power, the tag requires as P_min,tag we obtain the forward-link-limited range:

and defining the minimum signal power for demodulation at the reader as P_min,rdr, we obtain the reverse-link-limited range:

One additional antenna parameter is of vital importance in RFID. The radiated magnetic vector potential occurs in the direction of the current from which it radiates. The vector potential has a direction at each point in space. The electric field, which is derived from the vector and scalar potentials, describes the effect these potentials have on electrons in a wire. It is always pointed along that part of the vector potential that is perpendicular to the direction of propagation.