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102L5anta - Chapter 5 Antennas for Antennas for Wireless...

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May 27, 1997 RF Engineering 102 v1.0 (c)1997 Scott Baxter 5A - 1 Antennas for Wireless Systems Antennas for Wireless Systems Chapter 5 Dipole Typical Wireless Omni Antenna Isotropic

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May 27, 1997 RF Engineering 102 v1.0 (c)1997 Scott Baxter 5A - 2 Introduction to Antennas for Wireless Introduction to Antennas for Wireless Chapter 5 Section A
May 27, 1997 RF Engineering 102 v1.0 (c)1997 Scott Baxter 5A - 3 Understanding Antenna Radiation The Principle Of Current Moments n An antenna is just a passive conductor carrying RF current RF power causes the current flow Current flowing radiates electromagnetic fields Electromagnetic fields cause current in receiving antennas n The effect of the total antenna is the sum of what every tiny “slice” of the antenna is doing Radiation of a tiny “slice” is proportional to its length times the magnitude of the current in it, at the phase of the current TX RX Width of band denotes current magnitude Zero current at each end Maximum current at the middle Current induced in receiving antenna is vector sum of contribution of every tiny “slice” of radiating antenna each tiny imaginary “slice” of the antenna does its share of radiating

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May 27, 1997 RF Engineering 102 v1.0 (c)1997 Scott Baxter 5A - 4 Different Radiation In Different Directions n Each “slice” of the antenna produces a definite amount of radiation at a specific phase angle n Strength of signal received varies, depending on direction of departure from radiating antenna In some directions, the components add up in phase to a strong signal level In other directions, due to the different distances the various components must travel to reach the receiver, they are out of phase and cancel, leaving a much weaker signal n An antenna’s directivity is the same TX Maximum Radiation: contributions in phase, reinforce Minimum Radiation: contributions out of phase, cancel Minimum Radiation: contributions out of phase, cancel
May 27, 1997 RF Engineering 102 v1.0 (c)1997 Scott Baxter 5A - 5 Antenna Polarization n To intercept significant energy, a receiving antenna must be oriented parallel to the transmitting antenna A receiving antenna oriented at right angles to the transmitting antenna is “cross-polarized”; will have very little current induced Vertical polarization is the default convention in wireless telephony In the cluttered urban environment, energy becomes scattered and “de-polarized” during propagation, so polarization is not as critical Handset users hold the antennas at seemingly random angles…. . TX

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102L5anta - Chapter 5 Antennas for Antennas for Wireless...

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