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-APRIL NEWS LETTER
Isotron 20/15/10 HOW HIGH SHOULD AN ANTENNA BE.2 Last month mentioned HF reflection, ground absorption and a possible way of determining absorption. The ground and height are not the only factors that affect HF radiation. Also the style of antenna. A dipole design is known to have a gain of radiation broadside to the antenna and a loss toward the ends compared to the broadside. The predicted values may turn out to be quite different. If the dipole does not have sufficient height, then the predicted values can change. The antenna may become more omni-directional. In other words the expected pattern will radiate more uniformly as you check pattern around the antenna. This is noticeable with a yagi beam. It is built on a dipole design, however if not given sufficient height, will loose some of its gain and directivity. The inverted V tends to be prone towards more of an omni-directional pattern. If set up properly, it will work well and evenly in all directions. For both these antennas, the higher the better. This style is quite forgiving. At heights of 60 feet plus, it can be used for very short skip and moderately long skip, depending on time of day and season. On reception, noise factors may reduce with more height above ground. None of this is an exact science. HF radiation is a very sloppy radiation. It tends to do what it wants to do. It will go through objects that you think it would not. Like metal, concrete, wire mesh and so on. At other times plywood will absorb the radiation. The above mentioned objects can do the same. You do not really know until the antenna is setup and tried. One thing you can count on, HF rarely reflects. It will bend and it can induce in various objects. When it skips in the upper atmosphere, it is a refraction or a bending of a normally straight wave pattern. Reaching the ground it can skip again, but again, it is a refraction or bending of the wave. Getting a good take off angle and efficient antenna system can be a challenge if you are cramping the antenna. This is one advantage to the Isotron. I have to get a sales pitch in somewhere. It will allow you to have an efficient radiator, but giving you more distance from absorbing objects. It is easier to give it height when lateral space is limited. Hopefully this information will give you an idea of why your station is working the way it is. Whether it is working great or not so good, hopefully it will help you to make changes that will enhance your performance. 73, Ralph WD0EJA 04-15 OUR EMAIL LIST IS ACQUIRED BY PERSONS WHO HAVE CONTACTED US IN THE PAST ONLY IF YOU DESIRE NOT BE CONTACTED PLEASE EMAIL YOUR REQUEST. http://wd0eja@isotronantennas. |
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