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Dipoles, why height matters
For those that do not have a firm understanding of what the chart of dipole height over ground shows, I offer the following explanation. The charts show, for a dipole antenna at various heights in wavelengths over perfect, very good, average, and extremely poor ground, the gain and elevation angle of the antenna main lobe. The main lobe is where the majority of the energy is radiated. To understand what the charts mean in the real world, first you have to understand a little bit about propagation of RF. For a dipole antenna, there are two modes of propagation that are relevant, and those are NVIS (Near Vertical Incidence Skywave) and skywave which is sometimes called skip. Both modes depend on the RF being reflected or refracted back toward Earth by the ionosphere. For NVIS mode, the RF is directed straight up, that is an elevation angle close to 90 degrees is desired. The range of NVIS communications is on the order of 50 - 650 km, depending on the state of the ionosphere. The amateur bands where this is effective is limited primarily to the 160M to 40M band, again depending on the state of the ionosphere. It is not impossible to have NVIS communications on the higher bands, just much less probable to happen. For skywave mode, a low elevation mode is desired. Most of the literature recommends angles of 30 degees or less. In this mode the RF "bounces" at more obtuse angles, and with good conditions in the ionosphere, more than once, providing communication over global distances. Skywave depends heavily on the condition of the ionosphere and during sunspot peaks often occurs well past 10M. Now since a dipole with a main lobe at 90 degrees still has some gain at low angles, though it can be 20 to 60 dB down from the main lobe, when conditions are very good some stations can still be heard by skywave mode, though it is a rarity and can not be depended on. Conversely a dipole with a low elevation angle of the main lobe has some gain at very high angles and can occasionly hear stations by NVIS mode, but again it is a rarity. The bottom line of all this is that if you desire NVIS communications, you should mount your dipole at a height where the elevation angle is close to 90 degrees while if you desire long distance communications you should mount your dipole at a height where the elevation angle is less than 30 degrees, or higher if possible. If the required height is impractical at your location, then the alternative is a ground mounted vertical or a close to ground mounted ground plane antenna, which will have an elevation angle in the 20 degree range. -- Jim Pennino |
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