Active antenna

• US 5,311,198 A
• Filed: 08/23/1990
• Issued: 05/10/1994
• Est. Priority Date: 08/23/1990
• Status: Expired due to Fees

• Current Assignee: The United States of America As Represented By The Secretary of Agriculture
• Original Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Inventors: Sutton, John F.
• Primary Examiner(s): Wimer, Michael C.
• Application Number:
  US07/571,060
• Time in Patent Office: 1,356 Days
• Field of Search: 343/701, 343/850, 343/860, 455/291
• US Class Current: 343/701
• CPC Class Codes: H01Q 1/242 specially adapted for hand-held use
• CPC Class Codes: H01Q 23/00 Antennas with active circuits or circuit elements integrated within them or attached to them

Abstract

An antenna, which may be a search coil, is connected to an operational amplifier circuit which provides negative impedances, each of which is in the order of magnitude of the positive impedances which characterize the antenna. The antenna is connected to the inverting input of the operational amplifier, a resistor is connected between the inverting input and the output of the operational amplifier, a capacitor-resistor network, in parallel, is connected between the output and the noninverting input of the operational amplifier, and a resistor is connected from the noninverting input and the circuit common. While this circuit provides a negative resistance and a negative inductance, in series, which appear, looking into the noninverting input of the operational amplifier, in parallel with the antenna, these negative impedances appear in a series loop with the antenna positive impedances, so as to algebraically add. This circuit is tuned by varying the various circuit components so that the negative impedances are very close, but somewhat less, in magnitude, to the antenna impedances. The result is to increase the sensitivity of the antenna by lowering its effective impedance. This, in turn, increases the effective area of the antenna, which may be broadband.

Claims

1. An active antenna including an antenna and circuit means:

said antenna having positive inductance and positive resistance, including any impedances coupled into said antenna from its environment;

said circuit means, including a signal common and an operational amplifier, coupled to, and driven by, said antenna, for providing a negative inductance and a negative resistance, the magnitude of said negative inductance being approximately equal to the magnitude of said antenna positive inductance but somewhat less, and the magnitude of said negative resistance being approximately equal to the magnitude of said antenna positive resistance but somewhat less;

said antenna being connected to the inverting input of said operational amplifier, a resistor being connected from said inverting input to the output of said operational amplifier, a resistor-capacitor parallel network being connected from said output to the noninverting input of said operational amplifier, and a resistor being connected from said noninverting input to the signal common; and

said antenna and said circuit means being connected in a configuration which provides an algebraic addition of said positive and negative inductances and resistances, with the total active antenna impedance being a very small, but positive, series inductance and resistance, sufficient in magnitude to maintain the stability of said active antenna in its operation environment.