Slinky-Coil Dipole (2021)

Summary

My website uses JavaScript for menus etc. It is currently disabled in your browser! Please (re-)enable it for full functionality. -2021 F. Dörenberg, unless stated otherwise. worldwide. No part of this publication may be used without permission from the author. Latest page update: 25 May 2021 (added ref. 22) Previous page update: 6 July 2019 INTRODUCTION A standard Slinky® is a toy made of a flexible 90-turn metal spring. It has diameter of 2¾ inch (≈7 cm). Slinkies have been popular since the 1940s and can still be bought today. Each Slinky contains about 67 feet (≈20.4 m) of flat steel wire (1/10 inch wide), and weighs approximately ½ pound (225 gram). Compressed, a Slinky coil is only 2¼ inch long, but it can be stretched into a helix as long as 15 feet (4.5 m) in length - without deforming it permanently. Note: some Slinky coils only have about 58 ft of wire (≈17.7 m). Figure 1: 1999 US commemorative stamp and 1946 advertising Antennas can be made from any piece of wire, so why not a Slinky? A Slinky is made of 67 feet of steel wire. However, a 15 ft long antenna made with a Slinky coil does not have an electrical length of 67 ft! The Slinky-wire is "helically" or "spirally" wound - basically a cylindrical coil. The diameter of the coil is small ( < 0.5%) compared to the wavelength that the antenna is used for. Likewise, when the coil is stretched, the pitch of the turns ( = axial distance between successive turns) is similarly small. In this case, the antenna is called a "normal-mode" helix. In this mode, the coiled wire has maximum radiation in directions perpendicular to the axis of the coil. The coiling of the wire adds inductance that is distributed all along the antenna. I.e., the antenna has built-in continuous inductive loading. As the loading is obtained by the helical shape of the antenna wire, it is also referred to as "helically loaded". Note that the inductance also adds resonance frequencies that a same-length straight wire antenna does not have. ...