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 6 covers antenna polarization.
This part covers antenna propagation.
Propagation in the Real World
All the calculations we've performed so far assume that a wave leaves the antenna and strikes the tag, interacting with no other objects. This kind of calculation is very sensible if the tag and reader are placed in a specially designed anechoic chamber, or perhaps suspended high in the air from (nonmetallized) balloons. In the actual circumstances in which most readers and tags are used, the wave emitted from a reader antenna is likely to interact with many other objects besides the tag.
The interaction between waves that travel along the direct path between the reader and the tag, and those that are scattered or reflected, is of counter-intuitively large importance because it is voltages not powers that add. Let us consider, for example, the addition of a direct beam and two reflected beams, perhaps from the floor and a distant wall (Figure 3.35), each of which contains only 1/10 of the power of the direct signal. We can write the resulting voltage as:
Here the δs are the phase differences between the reflected waves and the direct wave. The phase difference depends on the relative length of the path traveled by each wave; a change in that path of 8 cm (a quarter of a wave) corresponds to a 90-degree phase shift (from a maximum value to zero or vice versa) for the beam traveling that path. It is unlikely (!) that we can measure or control the position of every object in the room to within a couple of centimeters, so we must consider these phase delays as being generally unpredictable and uncontrollable.
3.35. Direct and Reflected Beams Can Interfere.
Thus, the best we can do is to examine the extreme cases. First of all, what if the reflected beams happen to both be in phase with the direct beam (δ = 0°)? We get:
The received power is about 4 dB higher than in the absence of reflections. On the other hand, if the reflected beams are exactly out of phase (that is, δ = 180 deg;), we find: