1、5 (1978-1982) CCIH VOLUME*V 90 4855232 0500330 9 = Distancc - 40t 50 60 70 80 1 90 = 100 - - - - - 300 - - 800 - 900 = 1000 = - - - 8000 - 9000 = 10000 = Rec 525-1 Equivalent isotropic Free-space field radiated power 600 E 400 300 - - 40 30 = 20 10 8 6 4- 31 2 - - 800 600 400 = 300 = - - - 40 - 30 2
2、0 10 0 6 - - 4z 3= - -. 400 - 300 5 FIGURE 1 - Electro-magtletic field in free space CCIR VOLUME*V 90 W 4855232 0500333 O Rec. 525-1 7 2.2 Point-to-point links With a point-to-point link it is preferable to calculate the free-space attenuation between isotropic antennas, also known as the basic free
3、-space transmission loss (symbols: Lbf or AO), as follows: in which: Lbf: the free-space basic transmission loss in dB, d : the distance, h : the wavelength, and d and h are expressed in the same unit. Expression (3) can also be written using the frequency instead of the wavelength. Lbf = 32.5 + 20
4、log fMHz + 20 log,&, dB (4) Expressions (3) and (4) are shown in Fig. 2. 2.3 Relations between the characteristics of a plane wave There are also relations between the characteristics of a plane wave (or a wave which can be treated as a plane wave) at a point: E 4xpr s=- - 120n h2 in which: S : is t
5、he power flux-defisity in watts per square metre, E : is the r.m.s. field strength in volts per metre, p, : is the power in watts available from an isotropic antenna located at this point, h : is the wavelength in metres. Expression (5) is shown in Fig. 3. 3. The free-space basic transmission loss f
6、or a radar system (symbols: Lbr or AO,) Radar systems represent a special case because the signal is subjected to a loss while propagating both from the transmitter to fhe target and from the target to the receiver. For radars using a common antenna for both transmitter and receiver, a radar free-sp
7、ace basic transmission loss, Lbr, can be written as follows: Lbr = 103.4 20 log f + 40 log d - 10 log CI dB (6) in which: CI : the radar target cross-section in m2, d : the distance from the radar to the target in km, and f: the frequency of the system in MHz. The radar target cross-section of an ob
8、ject is the ratio of the total isotropically equivalent scattered power . to the incident power density. - 8 CCIR VOLUME*V 90 4855232 0500332 2 Distance 2 km 3 4 5 6 7 8 E9 = 10 - F 20 - - 200 300 400 500 800 700 800 900 1000 - - - Ret 525-1 Attenuation between isotropic antennae Frequency Wavelengt
9、h FIGURE 2 - Free-space attenuation between isotropic antennae CCIR VOLUME*V i0 4855232 0500333 4 Rec 525-1 Power receivcd Power Voltage. at isotropic flux-density Field receivcd across antcnna dBOY/m*) /.I V/m 5Oohms dBW Frequency Wavelength -80 -90 -100 - 110 -120 -130 -140 2000 - - _ : 1000 800 600 500 400 300 200 100 80 60 50 40 30 20 - - - - lo 8 6 5. 4 3 2 $ -70 400 300 200 pv 100- 60 -140 0.6 $5 0,2 -150 0,006 0,004 0,003 0,o o 1 0,0006 0,0004 9 FIGURE 3 - Characteristics of a plane wave
