| Antennas |
GeneralitiesIt exists a countless antenna types with quite different characteristics and each antenna type fits with a defined need. More, their cost is often proportional to their performances.The main characteristics are : Their choice depends on large factors and remains a specialist job. The vertical polarization in interesting in town and for buildings because it favour reflection on surrounding structures. The horizontal polarization is preferable for long range link. As for the circular polarization, it is used mainly for link with mobile stations (vehicule, plane, etc.) InstallationWhatever the antenna type, it's important to realize the installation carefully. The site where it will be installed will be chosen for its clearance relatively to other stations of the same network, but also for its distance from all perturbation sources or other surrounding networks.The antenna must be far away from all mass or structure capable to modify its characteristics and its functioning. The recommanded average distance is at least two or three wave lengths. It's often preferable to loose some dBs in a longer cable than to neglecte the antenna's installation itself. Importance of antennaIn a HF product, the antenna plays an essential role: it's the most important link of the chain.A quarter wave antenna, without its ground plan, will behave practically as a wire, but when its ground plan is sufficient, its efficiency will be maximal. The antenna must be placed always in high position and clear from all metallic obstacles. The antenna cable is another important element not to be neglected. Depending on the type of cable used, the loss in dB will be more or less high. Referencing frame E-visibilité |
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