The Many Different Types Of Antenna For Your System
The receiving aerial intercepts electromagnetic waves. The voltage induced in the receiving aerial is fed to the receiver input circuit through the antenna coupling circuit. An ideal receiving aerial must receive efficiently all the desired signals without wave band switching. For broadcast reception it normally has omni directional characteristics on long, medium and short wave ranges. Directional aerials are generally required for very high frequency and ultra high receptions and also in communication receivers operating on short waves. It has small variations of the resistance and reactance components of its terminal impedance with change of signal frequency. It should minimize fading. Thus for long and medium waves only the direct ray should be selected where as for short waves, the aerial should be located for optimum reception of the ionosphere wave. It should minimize interference effects from house wiring etc. it may be achieved by placing the aerial out of the interference zone and connecting it to the receiver by shielded cable. It should be resistant to corrosion or damage by weather and should be easily installed. For broadcast receivers the aerial should be cheap and good looking
The receiving aerial may be extremely varied in shapes and sizes ranging from single long wire to a series of closed loops or a wire mesh or even just a small opening in a metal sheet. The receiving aerials for use on medium and short waves can be of different types like indoor aerials that include frame aerials, mesh aerials; ferrite's rod aerial, metallic rod aerials and outdoor aerials are vertical aerials, inverted aerials, tee aerials and dipole aerials. An indoor aerial has relatively poor pick up and its terminal impedance that is the impedance between the aerial and ground looking into the aerial from the receiver, has large capacitance and resistive components. However indoor aerials are more handy and occupy less space and hence they are popularly used in preference to outdoor aerials in domestic receivers.
Outdoor aerials, in general, provide better pick up and may be designed for terminal impedance which does not vary excessively with frequency. How ever outdoor aerials occupy more space and are costly hence out door aerials are no longer popularly used. They are principally used in communication receivers or in table model broadcast receivers practically when excellent performance is required. If the terminal impedance of an aerial is known over the tuning frequency range, then for the purpose of design of the aerial coupling circuit the aerial may be replaced by a generator having internal impedance equal to the terminal impedance of the aerial and open circuit voltage equal to the effective pick up voltage of the aerial. The aerial coupling network may then be designed for either maximum signal voltage transfer or for maximum signal to noise ratio.
Tymon Hytem has worked in the electronics feild for the past 15 years. He enjoys helping people decide on electronic gadgets from telephones to XM Radio and choosing the perfect XM Satellite Radio system for their needs.
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