ACTIVE ANTENNA

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INTRODUCTION way of implementing compact broadband antennas. based on the idea that drastically shortening the dipole length of an antenna will result in a corresponding reduction in output voltages for both the useful signal and the noise.

 Principle of an active antenna  electrically short radiation is compensated in rod and dipole antennas by feeding the voltage on the terminals of the antenna directly to a very high- impedance active component (usually a field effect transistor) which acts as an impedance transformer and also commonly amplifies at the same time

CONSTRUCTION AND WORKING  Possible versions of active antennas include all the previously mentioned radiator types – rod, dipole and loop antennas.  Active antennas are mainly but not exclusively used for low frequencies (up to about 200 MHz) at which the noise floor in the atmosphere is very high. Above diagram shows on the right a combination of active antennas in which the rod antenna covers the wide frequency range from 10 kHz to 80 MHz.  The horizontal dipoles are dimensioned for 600 kHz to 40 MHz.

 active turnstile antenna for omnidirectional reception of horizontally polarized waves in the wide frequency range from 20 MHz to 500 MHz.  It turns out to be an advantage that a single active dipole is often enough to cover the entire frequency band  Due to the extremely broadband characteristics, active antennas are being increasingly used even in the higher frequency ranges

 Active antennas for radio monitoring are oriented in the same direction as the expected signals.  This can be done very easily for the VHF/UHF range,  A turnstile antenna with a monopole and remote- controlled switching will be able to handle almost any imaginable reception scenario.

Active antennas...  are smaller than comparable passive antennas  are more broadband than comparable passive antennas cannot transmit  have little coupling with their environment.  are more susceptible to interference if incorrectly installed  are highly suitable for use as broadband test antennas  have a frequency-independent radiation pattern must have sufficient large-signal immunity  need to be very carefully balanced  must not be installed in the midst of interference  may be installed closely adjacent to one another  are actually not so bad as many believe.

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