Chapter 4 The RF Link

Transcription

1 Chapter 4 The RF Link The fundamental elements of the communications satellite Radio Frequency (RF) or free space link are introduced. Basic transmission parameters, such as Antenna gain, Beamwidth, Free-space path loss, The basic link power equation

2 The concept of system noise and how it is quantified on the RF link is then developed, such as -Noise power, -Noise temperature, -Noise figure, -Figure of merit -The carrier-to noise ratio

3 5.1 Transmission Fundamentals Figure 5.1 Basic communications link The basic parameters of the link are p t = transmitted power (watts); p r = received power (watts); g t = transmit antenna gain; g r = receive antenna gain; r = path distance (meters).

4 *An electromagnetic wave, referred to as a radiowave at radio frequencies, is nominally defined in the range of 100MHz to 100+GHz. The frequency and wavelength in free space are related by

5 Figure 5.2 Definition of wavelength Table 5.1 Wavelength and frequency

6 Figure 5.3 Inverse square law of radiation *Consider a radiowave propagating in free space from a point source P of power p t watts. *The wave is isotropic in space, i.e., spherically radiating from the point source P,

7 The power flux density (or power density), over the surface of a sphere of radius ra The density over a sphere of radius r b is The ratio of power densities is given by

8 5.1.1 Effective Isotropic Radiated Power (eirp) The effective isotropic radiated power, eirp is eirp p t g t EIRP = P t + G t in db

9 Figure 4.4 Power flux density Power Flux Density (pfd) The power density, in watts/m 2, at the distance r from the transmit antenna with a gain g t, is defined as the power flux density (pfd) r

10 Or, in terms of the eirp, The power flux density expressed in db, will be With r in meters,

11 5.1.3 Antenna Gain *Isotropic power radiation is usually not effective for satellite communications links, because the power density levels will be low for most applications *Some directivity (gain) is desirable for both the transmit and receive antennas. *Consider first a lossless (ideal) antenna with a physical aperture area ofa(m 2 ). *The gain of the ideal antenna

12 *So the ideal antennas are not practical, because some energy is reflected, some energy is absorbed by lossy components (feeds, struts, subreflectors). *To account for this, an effective aperture, Ae, is defined in terms of an aperture efficiency physical antenna gain as g,

13 Circular Parabolic Reflector Antenna *The circular parabolic reflector is the most common type of antenna used for satellite earth station and spacecraft antennas. * It is easy to construct, and has good gain and beamwidth characteristics. *The physical area of the aperture of a circular parabolic aperture is given by

14 From the antenna gain Equation

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16 Beamwidth *Most antennas have sidelobes, or regions where the gain may increase due to physical structure elements or the characteristics of the antenna design *Sidelobes are a concern as a possible source for noise and interference,

17 *This for satellite ground antennas located near to other antennas or sources of power in the same frequency band as the satellite link. The beamwidth for a parabolic reflector antenna is the 1/2 power beamwidth in degrees, d is the antenna diameter in meters, f is the frequency in GHz.

18 5.1.4 Free-Space Path Loss The power p r intercepted by the receiving antenna will be Where Is the effective aperture (eq 15) p t transmitter power in watts g t transmitter antenna gain Then Eq. 5-22

19 Rearranging Equation (5.22) in a slightly different form, inverse square loss

20 Free space path loss is reciprocal of inverse square loss For the range r in meters, and the frequency f in GHz

21 For the range r in km,

22 5.1.5 Basic Link Equation for Received Power We now have all the elements necessary to define the basic link equation for determining the received power at the receiver antenna terminals for a satellite link

23 Sample Calculation for Ku-Band Link A 0.55

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