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RF Propagation No. 1  Seattle Pacific University Basic RF Transmission Concepts.

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Presentation on theme: "RF Propagation No. 1  Seattle Pacific University Basic RF Transmission Concepts."— Presentation transcript:

1 RF Propagation No. 1  Seattle Pacific University Basic RF Transmission Concepts

2 RF Propagation No. 2  Seattle Pacific University Radio Systems InformationModulatorAmplifier Ant Feedline Transmitter InformationDemodulatorPre-Amplifier Ant Feedline Receiver Filter RF Propagation This presentation concentrates on the propagation portion

3 RF Propagation No. 3  Seattle Pacific University Waves from an Isotropic source propagate spherically As the wave propagates, the surface area increases The power flux density decreases proportional to 1/d 2 At great enough distances from the source, a portion of the surface appears as a plane The wave may be modeled as a plane wave The classic picture of an EM wave is a single ray out of the spherical wave

4 RF Propagation No. 4  Seattle Pacific University Real antennas are non-isotropic Most real antennas do not radiate spherically The wavefront will be only a portion of a sphere The surface area of the wave is reduced Power density is increased! The increase in power density is expressed as Antenna Gain dB increase in power along “best” axis dBi = gain over isotropic antenna dBd = gain over dipole antenna Gain in this area

5 RF Propagation No. 5  Seattle Pacific University Antenna Gain Ratio of antenna’s maximum radiation intensity to maximum radiation intensity from a reference antenna with same input power dBi – If reference antenna is isotropic source of 100% efficiency dBd – If reference antenna is simple dipole of typical efficiency Gdip (gain with respect to dipole antenna) is 2.15 dB less than Gi (gain with respect to isotropic antenna)

6 RF Propagation No. 6  Seattle Pacific University Max gain = 12dBi (Largest circle represents Max gain. Gain specified separately for this model)

7 RF Propagation No. 7  Seattle Pacific University Antenna Beam Width Antenna achieves gain by concentrating its radiation pattern in a certain direction The greater the gain, the narrower the beam width Beam width is width of radiated pattern where signal strength is one-half that of maximum signal strength At this point, signal is 3 dB less than that of the maximum Angle between left and right points that are 3 dB down from maximum is beam angle or beam width For unidirectional antennas, resulting major lobe of radiation pattern has a certain width

8 RF Propagation No. 8  Seattle Pacific University Beam width is between -3dB points. 44 degrees for this antenna.

9 RF Propagation No. 9  Seattle Pacific University Antenna Front-Back Ratio Measure of antenna’s ability to focus radiated power in intended direction successfully And not interfere with other antennas behind it Referred to as f-b ratio or f/b ratio Ratio of radiated power in intended direction to radiated power in opposite direction Ratio of the two gains is the f/b ratio:

10 RF Propagation No. 10  Seattle Pacific University “Back” level is around 25dB less than front. F/B = 25dB

11 RF Propagation No. 11  Seattle Pacific University Dipole Antenna From Douglas A. Kerr A dipole antenna is simply a wire of length approximately λ /4, driven in the middle. I has a radiation pattern shaped light a doughnut, with the most gain perpendicular to the orientation of the antenna.

12 RF Propagation No. 12  Seattle Pacific University Collinear Array of Dipoles From Douglas A. Kerr Stacking two or more dipoles gives a more compressed radiation pattern. The dipoles must be in phase.

13 RF Propagation No. 13  Seattle Pacific University Directional Array Antenna λ /4 delay = 90 degrees =  /2 From Douglas A. Kerr Placing a dipole ¼ wavelength behind another dipole gives a directional antenna. The rear element should be driven 90 degrees earlier than the primary.

14 RF Propagation No. 14  Seattle Pacific University Parasitic Array Antenna An undriven element is called “parasitic”. It absorbs and re-radiates the E/M energy. The phase alignment of a parasitic element can be adjusted by changing its length. By picking the lengths and placement of parasitics carefully, the directionality of an antenna can be altered. From Douglas A. Kerr

15 RF Propagation No. 15  Seattle Pacific University Panel Antenna If a dipole or collinear array is placed in front a reflector, an “image” antenna will appear to exist behind the driven element. This behaves just like the reflector element in a parasitic array antenna. From Douglas A. Kerr Collinear array Metal reflector plane Image of antenna

16 RF Propagation No. 16  Seattle Pacific University Transmitted Power Radiated power often referenced to power radiated by an ideal antenna P t = power of transmitter G t = gain of transmitting antenna system The isotropic radiator radiates power uniformly in all directions Effective Isotropic Radiated Power calculated by: G t = 0dB = 1 for isotropic antenna This formula assumes power and gain is expressed linearly. Alternatively, you can express power and gain in decibels and add them: EIRP = P(dB) + G(dB) The exact same formulas and principles apply on the receiving side too!

17 RF Propagation No. 17  Seattle Pacific University Propagation Models Large-scale (Far Field) propagation model Gives power where random environmental effects have been averaged together Waves appear to be plane waves Far field applies at distances greater than the Fraunhofer distance: D = largest physical dimension of antenna = wavelength Small-scale (Near Field) model applies for shorter distances Power changes rapidly from one area/time to the next

18 RF Propagation No. 18  Seattle Pacific University Free Space Propagation Model ERIP Rcv Gain Spreading Loss Isotropic Antenna Aperture Substituting λ = c/f P t = transmitted power P r = received power G t = transmitter antenna gain G r = receiver antenna gain d = transmitter-receiver separation λ = wavelength in meters f = frequency (Hz) c = speed of light λ, d and c must be in the same units

19 RF Propagation No. 19  Seattle Pacific University Free Space Model EIRP Path gain (negate for path loss) In dB, assuming meters for d and m/s for c: Free space path loss (dB): P r (dBm) = P t (dBm) + G t (dB) – path loss (dB) + G r (dB) P r (dBm) = EIRP (dBm) – path loss (dB) + G r (dB)

20 RF Propagation No. 20  Seattle Pacific University Applying formulas to real systems A transmission system transmits a signal at 960MHz with a power of 100mW using a 16cm dipole antenna system with a gain of 2.15dB over an isotropic antenna. At what distance can far-field metrics be used? = 3.0*10 8 m/s / 960MHz = 0.3125 meters Fraunhofer distance = 2 D 2 / = 2(0.16m) 2 /0.3125 = 0.16m What is the EIRP? Method 1: Convert power to dBm and add gain Power(dBm) = 10 log 10 (100mW / 1mW) = 20dBm EIRP = 20dBm + 2.15dB = 22.15dBm Method 2: Convert gain to linear scale and multiply Gain(linear) = 10 2.15dB/10 = 1.64 EIRP = 100mW x 1.64 = 164mW Checking work: 10 log 10 (164mW/1mW) = 22.15dBm

21 RF Propagation No. 21  Seattle Pacific University Applying formulas to real systems A transmission system transmits a signal at 960MHz with a power of 100mW using a 16cm dipole antenna system with a gain of 2.15dB over an isotropic antenna. What is the power received at a distance of 2km assuming free-space transmission and an antenna with gain 4.2dB at the receiver? Path Loss(dB) = 20 log 10 (960MHz) + 20 log 10 (2000m) – 147.56dB = 179.6dB + 66.0dB – 147.56dB = 98.0dB Received power(dBm) = EIRP (dBm) – path loss (dB) + G r (dB) = 22.15dBm – 98.0dB + 4.2dB= -71.65dBm Received power(W) = EIRP(W)*G r (linear)/loss(linear) = 164mW * 10 4.2dB/10 / 10 98.0dB/10 = 6.8 x 10 -8 mW = 6.8 x 10 -11 W Checking work: 10 -71.65dBm/10 = 6.8 x 10 -8 mW P r (dBm) = EIRP (dBm) – path loss (dB) + G r (dB)

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