Radio Frequency by Steve Winder and Joe Carr - HTML preview

11.2 Propagation

The path loss is higher at microwave frequencies than at VHF and UHF: Figure 1.5 charts the free space loss between isotropic radiators for microwaves at frequencies of 1, 2, 3.5 and 7 GHz over distances of 10 to 1000 km. The free-space loss between isotropic radiators is given by

Free space loss,dB = 32.4+ 20 log₁₀ d + 20 log₁₀ f

 

where d is in km and f in MHz.

 

The free-space loss between practical antennas is given by:

 

(4πd)² 1 Loss, dB = 10 log_{10 λ2 GtGr}

where
d = path length, metres
λ = wavelength, metres

G_t = power gain of transmitting antenna
G_r = power gain of receiving antenna
The antenna gains are expressed relative to an isotropic radiator (not in dB).

Absorption varies with atmospheric humidity, and as energy is also absorbed by the ground the path height therefore has an effect

159 on the losses. Absorption by rain is a factor at the higher frequencies but, although considered to be insignificant below about 3 GHz, it has an indirect effect. Wet foliage, for example, produces considerable absorption at frequencies as low as 450 MHz.

Waves of millimetric lengths are special cases and narrow bands of very high absorption due to resonance effects exist at 22 and 183 GHz for water vapour, and 60 and 119 GHz for oxygen. Non-resonant attenuation occurs due to scatter from rain, hail and snow. The attenuation increases with frequency as the wavelength approaches the dimensions of a raindrop. Bands of very low absorption, ‘atmospheric windows’, where the water vapour and oxygen attenuations are very low occur at 37, 97, 137 and 210 GHz.

Objects close to a path may severely affect the received signal and a proposed path must be examined for the likely effects of these at the planning stage. The effects may be due either to diffraction bending the wave away from the line of sight between the antennas, or reflection causing multi-path signals.

The additional losses in a microwave path caused by objects either intruding into the first Fresnel zone or close to it, and which exhibit ‘knife-edge’ or ‘smooth-sphere’ characteristics, are shown in Figure 11.1. A negative F/F₁ indicates an intrusion. When the path clearance exceeds 0.6 times the first Fresnel zone radius, the free space loss is achieved.

0
−4
₋8
₋₁₂ Knife-edge
₋₁₆ diffraction
₋20 Smooth-sphere ₋₂₄ diffraction −28
−32
−36

^{− 40}−2.5 −2.0 −1.5 −1.0 −0.5 0 +0.5 F₌ Path clearance ^F First Fresnel-zone radius₁

Figure 11.1 Knife-edge and smooth-sphere diffraction

Refraction also affects microwave propagation and temperature inversions may cause ducting resulting in a loss of signal and, possibly, interference. When ducting occurs, a layer of air of low refractive index is formed between two highly refractive layers and a wave may be trapped between them. Under these conditions a signal can be carried an abnormal distance before it can return to Earth resulting in loss of signal at the intended receiver and possibly interference at a distant one on the same frequency.