• The wire that serve “radiating element”, or the “driven element”
  • There are also things called “parasitic elements” which serve to reflect or direct waves to increase gain
  • In yagi antennas, more reflectors and directors usually means more gain
  • Yagi antennas can also be stacked together to double the gain
• Polarization
  • Vertical → vertical antenna, signal radiates uniformly across the land
  • Horizontal → horizontal antenna, signal has better power “to the sides”, since the ground and the sky are usually not where you’re trying to receive
  • Receive and transmit antennas should have the same polarization
• Ideally an antenna should be the length of its wavelength
• Antenna length calculations: use 300 million m/s unless below 30MHz (then use 286 million)
• Half and quarter length antennas still work because of harmonics, but won’t be as efficient
• Dipole: two 1/4 wave wires separated by an insulator
• Lots of ways to handle multiple frequencies:
  • Coil + capacitor along the length (called a trap)
  • Multiple wires on each side of different lengths
  • All of these suffer from harmonics generation however, causing interference
• How to increase gain without using more space
  • Use a half or quarter wavelength
  • Make shape out of the antenna
    • Cubic quad: a square with each side = 1/4 of the wavelength, usually oriented perpendicular to the ground. If you hook up power to the horizontal side first, you’ll have horizontal polarization. If you hook power up to the vertical side, you’ll have vertical polarization.
    • Delta: a triangle where all sides are 1/3 of the wave
    • Folded dipole (angled dipole): each side is usually 1/4 of the wavelength, and this orientation often gives you more bandwidth than a regular dipole
• SWR: Standing Wave Radio, usually measured with an SWR “bridge”
  • Measures the ratio of voltage transmitted out the antenna to the voltage reflected back
  • 1:1 ratio is perfect, anything below 2:1 is very good
  • Reflected power that ends up getting stored is removed with an antenna tuner
  • Antennas need to be retuned every time you change the frequency
• Dummy load: big resistor used to test without blasting the air with random messages
• Transmitter to antenna connection:
  • Impedance must be matched for transmitter, cable, and antenna
• Cables and connection types:
  • Unbalanced: coaxial cables, usually have an impedance of 50-75 ohms, central wire is ground
  • Balanced: ladder line: two wires, separated by an insulator, usually 300-600 ohms of impedance
  • To switch between balanced and unbalanced cables, we use a balun
  • Keep connectors sealed to prevent water from getting in
  • Bigger number usually means better connector
  • SMA is for small devices like hand helds
• Safety:
  • Antennas should be kept high above ground so that they don’t damage you or your eyes
  • Only coax cables are insulated enough to be buried
  • Ladder lines are better at handling power
• Signal strength is measured with an S-meter
• Gain is measured relative to an isotropic antenna usually
• Gain is measured in dB - its a ratio with logarithms: doubling the power = 3 dB gain increase
• 8dB signal doubled means 8+3 = 11 dB