Oct. 23, 2006 Midterm Marked. –Average: 73%. 6 below, 9 above –Answer Key posted: s/nten216/Tests/NTEN216_Midterm1WithAns wers2006.pdf s/nten216/Tests/NTEN216_Midterm1WithAns wers2006.pdf s/nten216/Tests/NTEN216_Midterm1WithAns wers2006.pdf Assignment #4 –will be posted tonight –Due next Monday (Oct. 30) Antenna lab today – who brought a can? Sector Antennas, propagation paths, free space loss, link budgets

Sector Antennas Generate “Pie Slice” radiation patterns “Pie Slice” width depends on particular antenna Because of predictability of slice, multiple antennas may be mounted together to give larger coverage area E.g. want 270° of coverage, but have only 90° sector antennas, what do you do.

Sector Antenna Characteristics Gain: 10-21dBi. –A 90° sector antenna can still have variable gain by changing the vertical pattern Beamwidth: Depends on Pie Slice size Polarization: linear Bandwidth: –Effective from

Sector Antenna Radiation Pattern

Increase coverage area by joining three 120° antennas together. Omnidirectional in horizontal plane, controllable in vertical plane.

AntennaGain(dBi) Beam width Polari zation Band width Lower F Upper F λ/2 Dipole 280x360Linear10%None6GHz λ/4 Monopole 2-645x360Linear10%NoneNone Parabolic Linear33%400MHz13GHz Yagi5-1550x50Linear5%50MHz2GHz Patch8-18VariesLinear ArrayvariesVariesLinear10MHz10GHz Sector8-21VariesLinear

Review What are lobes and nulls? If you have a right handed circularly polarized transmission antenna, what is the best kind of antenna to use as the reception antenna? If the gain of an antenna is 5dBi for transmission, and its beamwidth for transmission is 90°x45°, what would the gain and beamwidth be if it was used for reception? What is this principle called? A 1 watt signal is input to an antenna, but 200mW is reflected. The gain of the antenna is 10dBi, and the loss of power due to resistance is 50mW. What is the efficiency of this antenna?

Review What happens if there is an impedance mismatch between the antenna, and the cable coming into the antenna? The center frequency of an antenna is 104.7MHz, and at 104.7MHz it radiates 50 Watts of power. If it radiates a 100MHz signal, it can only output 12.5W of power, and at 110MHz, it can only output 12.5W of power. What is the bandwidth of this antenna?

Review In terms of directionality, what kind of antenna is this ? What are the horizontal and vertical beamwidths? In the vertical plane, what is the signal strength at 45° if the strength at 90° is 50mW?

Review Describe at least two ways to provide full wireless coverage to a large rectangularly shaped warehouse. Explain the types of antennas you would use and why. If you have need for long distance point to point communications, what kind of antenna would you use. What would you use a Yagi antenna for?

Wave Propagation Ground (Surface) Wave Propagation Sky Wave Propagation Free Space Wave Propagation –Line of Sight –Fresnel Zones –Free Space Loss –Link Budget

Ground Wave Propagation EM waves that travel over the Earth’s surface –Best at low frequencies, over conductive surfaces 30kHz – 3MHz follow curvature of Earth

Sky Wave Propagation Transmission of RF signals by bouncing them off the ionosphere Allows for VERY long distance communication – thousands of kms in ideal circumstances Role of ionosphere –Ionized air 50+ km above Earth’s Surface –Reflects 3kHz – 30MHz –Multihop propagation

Ionosphere Opacity Most opaque for RF below 30MHz Opacity can change –Time of day – the solar wind pushes ionosphere in, so lower layer disappears at night –sunspots

Space Wave Propagation Propagation of RF signal through free space Line of sight and reflected wave propagation RF “line of sight” slightly different than visual line of sight –RF signal diffraction –Fresnel Zone

Fresnel Zones “Zones” through which RF signals propagate –Numbered starting at 1 Odd numbered zones are constructive Even numbered zones are destructive

Fresnel Zone Clearance First Fresnel Zone must be mostly clear of obstacles for successful signal propagation –Rule of thumb: 60% clear Minimum unobstructed radius: r = x sqrt(d/4f)

Free Space Loss RF signal weakens as it propagates away from source Overall power remains same, area it covers increase –Actual decrease is Power/unit area Free Space Path Loss: (4πR/λ) 2 = (4πRf/c) 2 or 20log(R) + 20log(f) –R squared. f squared why?

Link Budget System analysis accounting for all sources of signal loss and gain Gains: tx antenna, rx antenna, amplifiers Losses: cables, connectors, free space, polarization Fade Margin –Account for changing conditions –Add 10 to 30 dB to budget