T9: Antennas and Feed Lines – Ham Radio Technician License Study Guide

T9 covers the antenna systems that connect your radio to the air. Two exam questions come from this subelement, one from each group.

T9A addresses antennas: what beam antennas are and how they work, antenna gain and how it is defined, polarization of dipoles and vertical antennas, antenna loading as a technique for electrical lengthening, the radiation pattern of a half-wave dipole, practical antennas for handheld and mobile use, and how antenna length relates to resonant frequency. T9B addresses feed lines and SWR: why coaxial cable dominates amateur antenna systems, how loss behaves as frequency rises, the difference between common coax types, what causes erratic SWR readings, how an antenna tuner works, which RF connector is appropriate for different frequency ranges, and what SWR actually measures.

T9A: Antennas

T9A covers the fundamental vocabulary and concepts of amateur antennas. A beam antenna concentrates signals in one direction, providing gain over an omnidirectional antenna — the Yagi is the type offering the greatest gain among the options on the exam. Antenna gain is defined as the increase in signal strength in a specified direction compared to a reference antenna. A horizontally polarized antenna is one oriented parallel to Earth's surface, such as a horizontal dipole. Antenna loading refers to electrically lengthening a physically short antenna by inserting inductors in the radiating elements. A half-wave dipole radiates most strongly broadside to the antenna — at right angles to the wire, not off the ends. Shortening a dipole increases its resonant frequency; lengthening it decreases resonant frequency. A quarter-wavelength vertical antenna for 146 MHz is approximately 19 inches long; a half-wavelength 6-meter dipole is approximately 112 inches. The short flexible antenna on a handheld radio has low efficiency compared to a full-sized quarter-wave antenna, and using a handheld inside a vehicle reduces signal strength because of the shielding effect of the vehicle body. A 5/8-wavelength whip offers more gain than a quarter-wavelength antenna.

T9B: Feed Lines and SWR

T9B covers the cables, connectors, and impedance-matching concepts that connect antenna to radio. SWR (standing wave ratio) is a measure of how well a load — the antenna system — is matched to a transmission line. Low SWR is beneficial primarily because it reduces signal loss. Coaxial cable is the most common amateur feed line because it is easy to use and requires few special installation considerations. The most common impedance for amateur coaxial cable is 50 ohms. As frequency increases, coaxial cable loss increases. RG-213 has less loss at a given frequency than RG-58, making it better for longer runs. Air-insulated hardline has the lowest loss of all at VHF and UHF. Sources of loss in coaxial feed line include water intrusion into connectors, high SWR, and multiple connectors in the line. Erratic changes in SWR are typically caused by a loose connection in the antenna or feed line. An antenna tuner matches the antenna system impedance to the transceiver's output impedance. For frequencies above 400 MHz, the Type N connector is most suitable; PL-259 connectors are commonly used at HF and VHF frequencies.

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