User manual LDG ELECTRONICS AT-897 VERSION 1.1

[. . . ] AT-897 Automatic Antenna Tuner Manual Version 1. 1 LDG Electronics 1445 Parran Road, PO Box 48 St. Leonard MD 20685-2903 USA Phone: 410-586-2177 Fax: 410-586-8475 ldg@ldgelectronics. com www. ldgelectronics. com LDG AT-897 Automatic Antenna Tuner for the FT-897 Table of Contents Introduction Jumpstart, or "Real hams don't read manuals!" Features Specifications Getting to know your AT-897 Installation Mounting your AT-897 tuner Standard Connections Normal operation Full Tune Bypass mode A word about tuning etiquette Theory of Operation Some basic ideas about impedance Transmitters, transmission lines, antennas and impedance The LDG AT-897 Care and Maintenance Technical Support Warranty and Service Feedback 3 3 4 4 5 6 7 8 9 9 9 9 10 10 10 12 13 13 13 13 2 Introduction Congratulations on selecting the LDG AT-897 tuner. The AT-897 is designed to work as an integrated component of your Yaesu FT-897 transceiver. It will tune dipoles, verticals, Yagis or virtually any coax-fed (unbalanced) antenna. [. . . ] When the tuning cycle ends, the radio will automatically unkey and revert to its previous mode. If a good match is found, the setting will be stored to memory. Bypass mode To place your AT-897 in bypass mode, press the Tune button on the front of the AT-897 tuner and quickly release before the red LED comes on (less than a half-second). The tuner will switch to bypass; RF from your FT-897 transceiver will go directly to the antenna with no matching. There are no LED indicators for the bypass mode. A word about tuning etiquette Be sure to pick a vacant frequency to tune. With today's crowded ham bands, this is often difficult. However, do your best to avoid interfering with other hams as you tune. Your AT-897's very short tuning cycle, usually only a few seconds or so, minimizes the impact of your tuning transmissions. 9 Theory of Operation Some basic ideas about impedance The theory underlying antennas and transmission lines is fairly complex, and in fact employs a mathematical notation called "complex numbers" that have "real" and "imaginary" parts1. It is beyond the scope of this manual to present a tutorial on this subject, but a little background will help you understand what your AT-897 is doing, and how it does it. In simple DC circuits, the wire resists the current flow, converting some of it into heat. The relationship between voltage, current and resistance is described by the elegant and well-known "Ohm's Law", named for Sir George Simon Ohm of England, who first described it in 1826. In RF circuits, an analogous but far more complicated relationship exists. RF circuits also resist the flow of electricity. However, the presence of capacitive and inductive elements cause the voltage in the circuit to lead or lag the current, respectively. In RF circuits this resistance to the flow of electricity is called "impedance", and can include all three elements: resistive, capacitive, and inductive. Capacitive Reactance Inductive Reactance The output circuit of your transmitter consists of inductors and capacitors, usually in a series/parallel configuration called a "pi network". The transmission line can be thought of as a long string of capacitors and inductors in series/parallel, and the antenna is a kind of resonant circuit. At any given RF frequency, each of these can exhibit resistance, and impedance in the form of capacitive or inductive "reactance". Transmitters, transmission lines, antennas and impedance The output circuit of your transmitter, the transmission line, and the antenna all have a characteristic impedance. For reasons too complicated to go into here, the standard impedance is about 50 ohms resistive, with zero capacitive and inductive components. When all three parts of the system have the same impedance, the system is said to be "matched", and maximum transfer of power from the transmitter to the antenna occurs. While the transmitter output circuit and transmission line are of fixed, carefully designed impedance, the antenna presents a 50-ohm, non-reactive load only at its natural resonant frequencies. [. . . ] The microprocessor runs a fine tune routine just after the tuner finds a match at an SWR of 1. 5 or less. This routine tries to get the SWR as low as possible (not just 1. 5); it takes about a half second to run. If the SWR is below 2. 0 when you press the tune button to start a tuning cycle, the tuner will first run the fine tune routine to see if it can achieve a low SWR without a complete re-tune. This also takes about a half second to run. [. . . ]