Antennas 10/18/2017

Forward and Reflected Power Feed line impedance should match load impedance If it doesn’t, some power is reflected (reflected power) The power traveling towards the load is called forward power Reflected power and forward power interfere They create a stationary wave pattern called a standing wave https://www.desmos.com/calculator/dsv35uvokb

SWR SWR Whichever of the following is greater than 1: Load impedance to feed line impedance Feed line impedance to load impedance Lower is better More efficient power transfer to antenna Less is reflected Power going through the feed line generates heat Reflection can damage transmitter 1:1 is a perfect match

Polarization Orientation relative to ground

Dipole antennas 2 conductors going out in either direction Feed point in the center Can move, but signal loss

Wi-Fi antennas are dipoles as shown by the omnidirectional radiation pattern. As length of dipole increases, gain in peak direction (not all directions b/c Law of Conservation of Energy) increases up to about a dipole of 1 wavelength (when lobes in different directions become more numerous), but vertical coverage decreases. Dipoles about ½ wavelength long are resonant. Dipoles of different lengths are no longer resonant and have different feedpoint impedances, so a loading coil can provide impedance matching.

Monopole antennas Half a dipole above a ground plane Ground plane can just be wires Amount of ground plane needed inversely related with frequency

Yagi antennas Reflector(s) (larger than driven element) Driven element Director(s) (smaller) Horizontally polarized Yagis are often used for long-distance communication Especially SSB and CW on VHF/UHF

Exam Questions

T9A03 Which of the following describes a simple dipole mounted so the conductor is parallel to the Earth's surface? A ground wave antenna A horizontally polarized antenna A rhombic antenna A vertically polarized antenna

T9A04 What is a disadvantage of the “rubber duck” antenna supplied with most handheld radio transceivers? It does not transmit or receive as effectively as a full-sized antenna It transmits a circularly polarized signal If the rubber end cap is lost it will unravel very quickly All of these choices are correct

T9A05 How would you change a dipole antenna to make it resonant on a higher frequency? Lengthen it Insert coils in series with radiating wires Shorten it Add capacitive loading to the ends of the radiating wires

T9A07 What is a good reason not to use a “rubber duck” antenna inside your car? Signals can be significantly weaker than when it is outside of the vehicle It might cause your radio to overheat The SWR might decrease, decreasing the signal strength All of these choices are correct

T9A08 What is the approximate length, in inches, of a quarter-wavelength vertical antenna for 146 MHz? 112 50 19 12

T9A09 What is the approximate length, in inches, of a 6 meter ½-wavelength wire dipole antenna? 6 50 112 236

T9A10 In which direction is the radiation strongest from a half-wave dipole antenna in free space? Equally in all directions Off the ends of the antenna Broadside to the antenna In the direction of the feed line

T3A03 What antenna polarization is normally used for long-distance weak-signal CW and SSB contacts using the VHF and UHF bands? Right-hand circular Left-hand circular Horizontal Vertical

T3A05 When using a directional antenna, how might your station be able to access a distant repeater if buildings or obstructions are blocking the direct line of sight path? Change from vertical to horizontal polarization Try to find a path that reflects signals to the repeater Try the long path Increase the antenna SWR

Sources ARRL Ham Radio License Manual from 2010 Dipoles: http://qrznow.com/dipole-and-inverted-v-antenna-basics/ http://www.antenna-theory.com/antennas/dipole.php http://www.techrepublic.com/blog/data-center/80211-time-to-clear-up-some-antenna-misconceptions/ https://ham.stackexchange.com/questions/7200/how-does-wi-fi-antenna-work Monopoles: http://www.vlf.it/cr/differential_ant.htm http://www.antenna-theory.com/antennas/monopole.php Yagi: http://www.antenna-theory.com/antennas/travelling/yagi.php http://lasepattern.net/yagi-radiation-pattern/ https://upload.wikimedia.org/wikipedia/commons/7/74/Yagi_TV_antenna_1954.png?download Exam questions: http://ncvec.org/downloads/2014-2018%20Tech%20Pool.pdf