Application Notes and Articles Miniature （应用微型笔记和文章）.pdf

Application Notes and Articles Miniature Magnetic Loops By David Posthuma, WD8PUO A General Overview After several years of curiosity and several months of research, I recently built two magnetic loops. The fist loop is semi- portable and cost about $100 to construct. The second loop is designed for camping/back-packing, and is still in an experimental stage. The “back-pack” loop weighs only about 3 lbs, can be completely disassembled and cost only about $15 to construct. Both antennas cover at least three bands: 15, 17, 20 meters. Additional bands can be covered, based upon the inductance/capacitance ratio determined by the rough-tune coaxial capacitor. The loop design SWR ranges from 1.1:1 at the lowest frequency to 1.6:1 at the highest frequency. No traditional antenna tuner is required since the antenna contains its own tuning network. All part components (other than the optional motor) can be readily found in any ham shack and the local hardware store. The magnetic loop antenna is a high-Q tuned antenna with a very narrow pass band…typical range is 10-25 KHz…and so must be retuned as the operator significantly changes frequency. Re-tuning is not a problem since the antenna is usually positioned only a few feet off the ground. The high-Q performance has an additional benefit of significantly reducing off- frequency noise, acting much like a pre-selector for receivers with a poor front-end. The performance of the magnetic-loop antenna is similar to a ½ wave dipole. When positioned vertically at only a couple feet off the ground, the antenna will exhibit bi-directional characteristics. The magnetic loop antenna does not like being near metal of any kind. In fact, so sensitive is the loop to metal, that the best SWR is achieved by running the coax directly down to ground level and then away from the antenna, perpendicular to