Antenna for direction finders

Abstract

This invention relates to an antenna for direction finders presenting a high sensitivity in a narrow band by making a loop antenna and a shielding tube resonate at any two desired frequencies.

Description

This invention relates to antennas for direction finders.

Loop antennas are generally used as directional antennas for direction finders. It is the general practice to fit such loop antenna with a shielding tube. However, conventional antennas of this kind have a high sensitivity only at a comparatively low frequency. Further, by inserting a condenser between both ends of a loop antenna and making its electrostatic capacitance and the inductance of the antenna resonate with each other, the sensitivity can be increased near the resonating frequency.

An object of the present invention is to provide an antenna for direction finders wherein a suitable electrostatic capacitance is formed also between both ends of another auxiliary conductor arranged near and in parallel with a conductor forming a loop antenna as in the above mentioned shielding tube and said electrostatic capacitance and the inductance of the auxiliary conductor are made to resonate with each other so that a high sensitivity may be again presented at any desired frequency higher than the above mentioned resonating frequency.

The drawing is a view showing a wiring of an embodiment of the present invention.

In the drawing, a conductor forming a loop antenna 1 is fitted with a shielding tube 2 provided with a cut 3 in the top part. A proper insulator 4 is inserted between the inside surface of the shielding tube 2 and the antenna 1. A condenser 5 is inserted between both ends of such loop antenna 1 and a condenser 6 is connected also between both ends of the cut 3 of the shielding tube 2 so as to respectively form proper electrostatic capacitances. Both terminals 7 and 8 of the loop antenna 1 can be connected with a cable 10 through a transformer. Further, a resistance 11 connects to the secondary side or primary side of the transformer 9. In a direction finder, such two loop antennas as are described above are so arranged as to intersect at right angles with each other and are led to a goniometer or the like through respective cables. Further, the shielding tube 2 can be used not only as directly grounded but also without being grounded.

As described above, in the present invention, suitable electrostatic capacitances 6 and 5 are connected respectively in a cut in the top part, for example, of a shielding tube arranged near and in parallel with a conductor forming a loop antenna and between both ends of the loop antenna. The inductance of the loop antenna 1 and the electrostatic capacitance 5 between both ends of the loop antenna are resonated with each other at any desired first frequency and the inductance of the shielding tube 2 and the electrostatic capacitance 6 between both ends of the shielding tube are resonated with each other at any higher suitable second frequency so that all members 1 through 9 may resonate at a still higher third frequency. Therefore, for arriving electric waves of such low frequency at which the condenser 5 inserted between both ends of the loop antenna presents an impedance high enough, the antenna will have the same sensitivity as of a conventional loop antenna of this kind and the impedance as seen from the terminals 7 and 8 will show an inductivity. When the frequency of the electric waves increases to a value of said first frequency and the electrostatic capacitance of the condenser 5 and the inductance of the above mentioned loop antenna resonate in parallel with each other, the impedance as seen toward the antenna side from the terminals 7 and 8 will become so high that the sensitivity will quickly rise. When the frequency increases to be so high as to exceed the above mentioned resonating point, the impedance will vary to be capacitive from being inductive. However, if the frequency of the arriving waves further increases to a third frequency which is higher than said second frequency at which the electrostatic capacitance by the condenser 6 or the like and the inductance of the shielding tube 2 resonate with each other, the impedance as seen from the terminals 7 and 8 will vary again to be inductive and will resonate with the condenser 5, therefore a large electric current will be induced in said antenna 1 and the receiving sensitivity will increase again. Therefore, when the diameter of the loop antenna 1, thickness of the shielding tube 2, material of the insulator 4 and capacitances of the condensers 5 and 6 are properly selected in the design so that the loop antenna 1 and the shielding tube 2 may respectively resonate with each other at two desired frequencies, there will be obtained an antenna for direction finders having a high sensitivity at the above mentioned first and third frequencies.

In the above has been explained an embodiment wherein the shielding tube is utilized as an auxiliary conductor. In order to increase the sensitivity at the lower frequency by reducing the inductance of a loop antenna, it is necessary to enlarge the diameter of the conductor. Therefore, in such case, the loop antenna may be formed of a tubular conductor and an auxiliary conductor may be coaxially arranged within said tubular conductor.

Claims

What is claimed is:

1. An antenna for direction finders comprising a first conductor forming a loop antenna having opposite ends, an auxiliary conductor having a cut at the top thereof arranged near and in parallel with said first conductor, first capacitor means connected between both ends of said loop antenna for providing a resonant circuit with said loop antenna which is resonant at a first frequency and second capacitor means connected between both ends of said cut of said auxiliary conductor at the top part of said antenna for providing a resonant circuit with said auxiliary conductor which is resonant at a second frequency higher than said first frequency, the values of inductance of said auxiliary conductor and of capacitance of said second capacitor means being selected with respect to the values of inductance of said loop antenna and of capacitance of said first capacitor means so that at a third frequency higher than said second frequency the combination of said auxiliary conductor and second capacitor means provides inductive impedance of such value as to cooperate with said first capacitor means and cause said loop antenna to be resonant at said third frequency permitting a high sensitivity in each narrow band of said first and third frequencies.

2. An antenna according to claim 1 wherein said auxiliary conductor is a shielding tube.

3. An antenna according to claim 1 wherein said loop antenna is formed to be tubular and said auxiliary conductor is coaxially arranged within this tubular conductor so that the sensitivity at said first frequency is increased.