Antenna & Feed Lines Chapter 2 Lesson 2.5
Antenna Basics Two basic types used by hams • Ground plane antenna: radiates a signal from the vertical wire attached to the base • 2. Yagi-style beam
Antenna Parts • Elements • Radio signals are transmitted and received on this portion of the antenna • Array • An antenna with more than one element • Feed Line • Used to deliver the radio signal to or from the antenna • Feed Point • The connection of antenna and feed line • Polarization • Simply the direction in which the electric field of a radio wave is oriented (or pointed) • Gain • The greatest concentration of radio signals in a specific direction • Gain focuses power – it cannot create power
Feed Line • Coaxial Cable (or Coax) • Most popular feed line used by amateurs Radio-grade flexible coaxial cable.A: outer plastic sheathB: copper screenC: inner dielectric insulatorD: copper core
Feed Lines • Open wire, ladder line or twin lead • A feed line of two parallel wires separated by insulating material
Coax and Twin Lead Impedance • Impedance • Measurement of how energy is carried through a feed line • Coax impedance • 50 ohms • Twin lead impedance • 300 – 450 ohms • Your power, feed line and antenna need to match this impedance as close as possible. What ever amount of measure you are off will result in some of your power being reflected by the antenna • Forward moving towards the antenna is called: Forward Power! • Power that is reflected back by the antenna is called: Reflected Power! • The reflected power causes an interferance pattern. • This interference wave is stationary and is called a Standing Wave.
Standing Wave Ratio or SWR • SWR • The ration of maximum value to minimum value of interference • SWR of 1:1 is called a Perfect Match • SWR greater than 1:1 is called a Mismatch • Most amateur equipment is built to work at full power and with a SWR of 2:1 or less. • If your SWR is high (greater than 2:1) then your equipment will usually shut down automatically. If not, your equipment will be damaged! • Causes of high SWR • Antennas that are too long or too short • Bad\damaged feed lines or connectors • Cure for high SWR • Use of antenna tuner (also called transmatch) • Replace bad connectors • Adjust length of antenna
SWR Meter • SWR Meter • Used to measure SWR! • SWR meter is placed between the transmitter and the antenna tuner.