Figure 4-30: Free Space Loss at 2.4 GHz and 5.8 GHz Chapter Four a straight line between the two antennas. The volume of space around this straight line affects signal propagation as well, and any obstructions that come close to the direct line-of-sight will also cause signal loss. Fresnel Zone Theory The theory used to calculate the effect of obstructions is called Fresnel zone theory. The Fresnel zone is a region between the two antennas with an oval shape similar to a rugby ball. There are actually a series of such regions, called the 1st, 2nd, 3rd, etc. Fresnel zones (Figure 4-31), and at the mid point between transmitter and receiver, the radius of the nth Fresnel zone in metres is calculated from the formula: R = 0.5 (nxAxD) (4.10) where the wavelength and the transmitter to receiver distance are also in metres. For a 2.4 GHz signal, with a wavelength of 12.5 cm (0.125 m), the first Fresnel zone has a mid-point radius of 1.8 m for a 100 m range, or 5.6 m for a 1 km range. Any obstructions within the first Fresnel zone will cause signal loss through reflection, refraction or diffraction. Multipath Fading Multipath fading occurs when reflected, refracted or diffracted signals travel to the receiver along different paths, resulting in a range of different arrival times known as the multipath delay spread. Signals arriving along different paths will be phase-shifted with respect to the direct path signal, First Fresnel Zone Second Fresnel Zone Figure 4-31: The Fresnel Zones Around a Propagation Path Radio Communication Basics Other paths omitted for simplicity Out-of-phase arrival of two signals Figure 4-32: Multi-path Fading in an Indoor Environment as shown in Figure 4-32, and will therefore cause some degree of destructive interference at the receiving antenna. This is familiar in UHF TV reception as a ghost image caused by an interfering signal reflected from a nearby building or other large object. Interference between these multiple delayed signals can substantially reduce the signal strength at the recei