102L5antb - Chapter 5 Section B Other RF Elements Other RF...

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May 27, 1997 RF Engineering 102 v1.0 (c)1997 Scott Baxter 5B - 1 Other RF Elements Other RF Elements Chapter 5 Section B
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May 27, 1997 RF Engineering 102 v1.0 (c)1997 Scott Baxter 5B - 2 Antenna Systems n Antenna systems include more than just antennas n Transmission Lines Necessary to connect transmitting and receiving equipment n Other Components necessary to achieve desired system function Filters, Combiners, Duplexers - to achieve desired connections Directional Couplers, wattmeters - for measurement of performance n Manufacturer’s system may include some or all of these items Remaining items are added individually as needed by system operator F R D u p l e x e r Combiner BPF TX RX TX Transmission Line Jumper Jumpers Directional Coupler Antenna
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May 27, 1997 RF Engineering 102 v1.0 (c)1997 Scott Baxter 5B - 3 Characteristics Of Transmission Lines Physical Characteristics n Type of line Coaxial, stripline, open- wire Balanced, unbalanced n Physical configuration Dielectric: –a i r – foam Outside surface – unjacketed –j a c k e t e d n Size (nominal outer diameter) 1/4”,1/2”, 7/8”, 1-1/4”, 1- 7/8”, 2-1/4”, 3” Foam Dielectric Air Dielectric Typical coaxial cables Used as feeders in wireless applications
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May 27, 1997 RF Engineering 102 v1.0 (c)1997 Scott Baxter 5B - 4 Characteristics Of Transmission Lines, Continued Electrical Characteristics n Attenuation Varies with frequency, size, dielectric characteristics of insulation Usually specified in dB/100 ft and/or dB/100 m n Characteristic impedance Z 0 (50 ohms is the usual standard; 75 ohms is sometimes used) Value set by inner/outer diameter ratio and dielectric characteristics of insulation Connectors must preserve constant impedance (see figure at right) n Velocity factor Determined by dielectric characteristics of insulation. n Power-handling capability Varies with size, conductor materials, dielectric characteristics d D Characteristic Impedance of a Coaxial Line Z o = ( 138 / ( ε ε 1/2 ) ) Log 10 ( D / d ) ε = Dielectric Constant = 1 for vacuum or dry air
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May 27, 1997 RF Engineering 102 v1.0 (c)1997 Scott Baxter 5B - 5 Transmission Lines Special Electrical Properties n Transmission lines have impedance- transforming properties When terminated with same impedance as Z o , input to line appears as impedance Z o When terminated with impedance different from Z o , input to line is a complex function of frequency and line length. Use Smith Chart or formulae to compute n Special case of interest: Line section one-quarter wavelength long has convenient properties useful in matching networks Z IN = (Z o 2 )/(Z LOAD ) Z o =50 Z LOAD = 50 Z IN = 50 Matched condition Z o =50 Z LOAD = 83 -j22 Z IN = ? Mismatched condition Z o =50 Z LOAD = 100 Z IN =25 λ /4 Z IN = Z O 2 / Z LOAD Deliberate mismatch for impedance transformation
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May 27, 1997 RF Engineering 102 v1.0 (c)1997 Scott Baxter 5B - 6 Transmission Lines Some Practical Considerations n Transmission lines practical considerations Periodicity of inner conductor supporting structure can cause
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102L5antb - Chapter 5 Section B Other RF Elements Other RF...

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