Technician Class License Course Chapter 4 Practical Antenna Systems Sec 4.4 26 Jan 18 K0NK The Dipole Pg 4-11 • The most basic antenna — The Dipole – Two conductive, equal length parts. – Feed line connected in the middle. • Dipoles are easy to make and easy to use Typical HF Dipole Installation Pg 4-11 Most dipoles on the lower bands are oriented horizontally and radiate a horizontally polarized signal. Broadside Radiation Pattern Pg 4-11 Dipoles radiate the strongest signal broadside to the axis of the dipole Broadside Radiation Pattern Pg 4-11 Azimuthal Pattern Computer Generated Sketch Dipole Length Pg 4-12 Total length is ½ wavelength (1/2 l ). Estimated Length (in feet) = 468 / Frequency (in MHz). • For 146 MHz: 468 / 146 = 3.2 ft = 38.5 inches • For 50 MHz: 468 / 50 = 9.33 ft = 112 inches • For 28.5 MHz: 468 / 28.5 = 16.4 ft 7 Dipole Length Typical lengths for common ham bands We shorten the antenna to raise the frequency Building a Dipole Hint: Make the element a few percent longer than needed, then check the SWR. Shorten the antenna until it is resonant at the desired frequency. SWR Measurements 4-10 The length of each leg of the antenna is adjusted to provide a low SWR on your favorite part of the band. 10 Common Dipole Configuration Fan Dipole The Ground-Plane Pg 4-12 The Ground-Plane • Is simply a dipole that is oriented perpendicular (vertical to the Earth’s surface). • One half of the dipole is replaced by the ground- plane. – Earth – Car roof or trunk lid or other metal surface. – Radial wires. • Length (in feet) = 234 / Frequency (in MHz). – For 146 MHz: 234 / 146 = 1.6 ft = 19.2 inches Quarter Wavelength Vertical A very common antenna is the ¼ λ vertical The 5/8 λ Ground Plane More than twice the length of a ¼ λ. About 48 inches Focuses more energy towards the horizon. Uses and inductive loading coil at the base to make the antenna electrically longer. Ground Plane Gain A 5/8 λ antenna focuses more energy toward the horizon Rubber Duck The short flexible antenna on a Handie Talkie is often called a Rubber Duck Rubber Duck Performance Pg 4-13 The built in Rubber Duck antenna on a Handie Talkie (HT) performs poorly – It does not transmit or receive as effectively as a full sized antenna--especially inside a car. Connecting to an external mobile antenna DRAMATICALLY improves the operation of your HT. = Handheld Antennas Rubber duck operating tips Hold the transceiver vertical Keep antennas aligned in polarity Misaligned antennas can reduce signals by 100 times For best HT performance, Abandon the rubber duck Connect to an external antenna Especially in a car Mobile Antenna Types Mobile Antenna Mounts Mag- Mount Through-hole mount Trunk/hatch lip mount Number of Bands Single Band Dual Band (2m/70cm) Multi Band (2m/1.25m/70cm) Loop Antennas – Dipole Variations • Quad • Delta • Horizontal Directional (Beam) Antennas Pg 4-14 • Beam antennas focus or directs RF energy in one direction. – Gain • An apparent increase in power in the desired direction (both transmit and receive). • Yagi (rod-like elements – TV antennas). • Quad (square wire loop elements). Directional (Beam) Antennas Pg 4-14 Yagi Quad Horizontally polarized Yagis and Quads are used for long-distance VHF and UHF communications. Horizontal polarization has less ground losses. Directional (Beam) Antennas Pg 4-15 • All beam antennas have parts called elements. – Driven element connected to the radio by the feed line. – Reflector element is on the back side. – Director element(s) is on the front side toward the desired direction. Directional (Beam) Antennas Pg 4-14 • Beam antennas are usually pointed at the station you want to talk to. • Sometimes a beam can be pointed at a distant object or structure to reflect the signal around a building or hill that is obstructing a direct line-of-sight signal. The Dish is a Directional Antenna Pg 4-14 Used at frequencies above 1 GHz. Focus energy Provides gain World’s Worst Antenna! However it has a very good SWR The Dummy Load Is used to prevent radiating signals when making a test transmission. Dissipates the power into heat instead of radio waves. Coax Feed Lines Pg 4-16 • RG-58 • RG-8 • RG-213 • RG-174 • Hardline • All these are 50 Ω impedance Coaxial Cable (Coax) Has an outer jacket to keep water out. Prolonged exposure to ultraviolet light can cause the plastic jacket to degrade (crack). Water degrades the effectiveness of the shield. Water damage dramatically increases loss. All coax cables have loss Loss increases at the higher frequencies Coax Loss Pg 4-16 Remember: 3 dB of loss = ½ of your power is lost or LMR-400 See Table 4-1, Page 4-16 RG-8 will have lower loss than the smaller RG-58 Coax Connectors Pg 4-17 • UHF – SO-239/PL259 – Used at HF frequencies • BNC • SMA • N – Used at frequencies above 400 MHz Seal the Coax Connectors Pg 4-17 • Water in the coaxial cable degrades the effectiveness of the braided shield • Air-core coaxial cable requires special techniques to prevent water absorption Soldering http://stomach.v2.nl/projects/Basic_Electronics/Soldering.pdf http://w6rec.com/duane/bmw/solder/ Soldering Pg 4-17 Antenna/coax work may require soldering For antennas and other electronic use, use only rosin-core solder Heat the connection well with a soldering iron. Good joints are shiny Bad or “cold” solder joints have a grainy or dull finish DO NOT USE ACID CORE SOLDER Feed Line Devices • Balun • Duplexer • Antenna switches • SWR meter • Antenna analyzer • Antenna tuners SWR Meter / Directional Wattmeter Pg 4-18 SWR Meter Directional Reads out SWR Wattmeter • Forward W • Reflected W Reads out SWR and Forward and Reflected Power at the same time Pay attention to the specified usable frequency range. SWR Meters • Measure SWR directly by sensing power flow in the line • Usually installed at the transmitter 2014 Technician License Course Antenna Analyzer Pg 4-19 An antenna analyzer can be used to measure the SWR of an antenna. Does not require a transmitter Antenna Tuner Antenna Tuner Schematic 42 How to use an Antenna Tuner Pg 4-18 • Monitor the SWR meter. • Make adjustments on the tuner until the minimum SWR is achieved. – We are matching the antenna system’s impedance to the transceivers output impedance All-Band Dipole 80 – 10 m G5RV All-Band Dipole Random Wire Antenna Antenna Tuner Pg 4-18 • One way to make antenna matching adjustments is to use an antenna tuner. • Antenna tuners are impedance transformers (they actually do not tune the antenna). – When used appropriately they are effective. – When used inappropriately all they do is make a bad antenna look good to the transmitter…the antenna is still bad. Duplexer Allows two radios (or a dual-band radio) to share one antenna. Many dual band radios have the duplexer built in 2 m 70 cm Antenna Tuners • Don’t really “tune the antenna” • Matches the antenna system impedances to the transmitter’s output impedance • Reduces the SWR the transmitter sees • Antenna feed point impedance unchanged • Feed line SWR unchanged • Also called impedance matchers, transmatches, matchboxes, other trade names 2014 Technician License Course How to Use an Antenna Tuner • Transmit a low-power signal • Monitor the SWR meter • Adjust the tuner until minimum SWR is achieved 2014 Technician License Course Antenna Supports • Trees. • Towers or masts. • Covenants and antenna restrictions must be considered. • Stainless steel hardware resists corrosion. .