Not too much digression at the beginning of class today, so notes after the jump.
**Please note, for some reason I cannot add pictures to these anymore. If you need to see the pictures, please see the slideshows until I can get this fixed.**
Log Periodic Antenna
Chapter 4 cont.
Antenna Dimensions
- One-dimensional antennas
- Basically a length of wire or metal
- Monopole antenna
- Straight piece of wire or metal, usually a quarter of the wavelength, with no reflecting or ground element
- Dipoles are commonly built as two monopoles
- Mounted together at the base
- A monopole antenna is less efficient than a dipole
- Ground-plane
- Large metal base
- Simulates the signal-reflecting effect of the ground
- Two-dimensional antennas
- Antennas organized in a two-dimensional pattern
- Examples include patch and satellite dish antennas
- Horn antenna
- Another type of two-dimensional directional antenna
- Resembles a large horn with wide end bent to one side
- Common in telephone networks
- Used to transmit microwave signals between two distant towers
Smart Antennas
- Used primarily in mobile or cellular telephony
- “Know” where the mobile receiver is
- Can track and focus RF energy in specific direction
- Classes of smart antennas
- A switched beam antenna
- Uses several narrow beam antennas pointing in different directions
- Adaptive or phased array antennas
- Divided into a matrix of radiating elements
- Used extensively in ultra-modern radar systems
Specific digression: TCXO, Temperature Compensated Crystal Oscillator
Antenna System Implementation
- Proper installation of antennas requires knowing the user’s requirements
- Challenges
- Physical obstacles
- Municipal building codes
- Other regulatory restrictions
Antenna Cables
- Most antennas are connected to the transmitter or receiver using coaxial cable
- Impedance
- Opposition to the flow of alternating current in a circuit
- Represented by the letter “Z” and measured in ohms
- Combination of resistance, inductance, and capacitance of the circuit
- Cable’s impedance must match that of the transmitter circuit as well as that of the antenna
- You must consider the signal loss caused by the connector and by the cable itself
- Cable loss is measured in relation to the length of the cable
- You can use special low-loss antenna cables to minimize signal loss
RG-58 Digression
RF Propagation
- The way that radio waves propagate
- Depends on the frequency of the signal
- RF waves types
- Ground waves follow the curvature of the earth
- Sky waves bounce between the ionosphere and the surface of the earth
- Line-of-sight used by RF waves transmitted in frequencies between 30 MHz and 300 GHz
Point-to-Multipoint Links
- Point-to-multipoint wireless link
- One transmitter communicates with several mobile clients
- Maximize the signal distance by using an omnidirectional antenna
Point-to-Point Links
- Point-to-point wireless link
- Connects two computers in different buildings
- Directional antennas provide the most reliable method of transmitting RF waves
- Telephone companies make extensive use of point-to-point microwave links
Fresnel Zone
- RF waves have a tendency to spread out
- Space between two antennas would be more accurately represented by an ellipse
- Called the Fresnel zone
- When planning a wireless link
- At least 60% of the Fresnel zone must be kept clear of obstructions
- May affect the height of the antenna tower
Link Budgets
- Link budgets
- Calculate whether you will have enough signal strength
- To meet the receiver’s minimum requirements
- Many link budgeting tools available on the Internet
- Information needed to calculate link budget includes:
- Gain of the antennas
- Cable and connector losses for receiver and transmitter
- Receiver sensitivity
- Free space loss figure
Antenna Alignment
- One of the challenges of implementing a point-to-point link
- Position the antennas at the same height and point them toward one another
- Some basic tools
- A compass to position the antenna at the correct angle
- A spotting scope or binoculars
- A means of communication, such as a walkie-talkie or a cellular phone
- If the distance is reasonably short, a light source, such as a flashlight
- Spectrum analyzer
- Displays the signal amplitude and frequency
- Can also detect interference in a particular frequency or channel
Other Challenges of Outdoor Links
- Radio waves can reflect, diffract, or be absorbed by some materials
- Weather phenomena can affect the performance and reliability of wireless links
- Seasonal changes can impact a wireless link
- While planning an outdoor link:
- Always consider environmental conditions
- Check for short- and long-term plans that may interfere with your intended link
- Consider the possibility of another link that may interfere with your link
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