Parasitic array elements_ antennas and wave prpoagation.pptx

On parasitic antennas

1. ANTENNAS AND WAVE PROPAGATION PARASITIC ARRAY ELEMENTS

2. Parasitic elements are the elements which depend on other elements for feed as it doesn’t have its own feed. Parasitic elements are not directly connected to feed.

3. Construction & Working of Parasitic Array The main parts are − Driven element Parasitic elements Reflector Director Boom

4. Driven element The antennas radiate individually and while in array, the radiation of all the elements sum up to form the radiation beam. All the elements of the array need not be connected to the feed. The dipole that is connected to the feed is known as a  driven element . Parasitic Elements The elements, which are added do not possess an electrical connection between them to the driven element or the feed. They are positioned so that they lie in the induction field of the driven element. Hence, they are known as  parasitic elements .

5. Reflector If one of the parasitic element, which is 5% longer than driven element, is placed close to the driven element is longer, then it acts as a concave mirror, which reflects the energy in the direction of the radiation pattern rather than its own direction and hence is known as a  reflector . Director A parasitic element, which is 5% shorter than the driven element, from which it receives energy, tends to increase radiation in its own direction and therefore, behaves like convergent convex lens. This element is called as a  director . A number of directors are placed to increase the directivity.

6. Boom The element on which all these are placed is called a  boom . It is a non-metallic structure which provides insulation, so that there will not be any short circuit between the other elements of the array.

7. These arrays are used at frequencies ranging from  2MHz  to  several GHz . These are especially used to get high directivity, and better forward gain with a  uni-directional radiation pattern. The most common example of this type of array is the  Yagi-Uda antenna . Quad antenna, Log periodic antennas are other examples. Parasitic array antennas have a narrow bandwidth, which limits their efficiency to a small frequency range. Their directivity is fixed, so the radiation pattern cannot be easily changed once designed. They also demand precise spacing and tuning of elements, making their design more complex.

8. Examples of parasitic array antennas YAGI-UDA ANTENNA Yagi-Uda antenna  is the most commonly used type of antenna for TV reception over the last few decades. It is the most popular and easy-to-use type of antenna with better performance, which is famous for its high gain and directivity. For commercial purpose and to tune over a range of frequencies, we need to have another antenna known as the  Log-periodic antenna . A Log-periodic antenna is that whose impedance is a logarithmically periodic function of frequency. LOG–PERIODIC ANTENNA

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