GB2310320A - Active loop antenna with constant output/frequency characteristic - Google Patents

Description

Active Loop Antenna This invention relates to loop antennas used for sensitive radio receiving purposes. Because external noise can be quite high relative to thermal noise in the low frequency bands, below 30 MHz, such antennas do not need to be as efficient as transmitting types and smaller dimensions can be used. Also wideband operation, over several frequency decades, is usual without tuning.

For a passive loop antenna, in a field of constant strength, the open circuit voltage across its terminals induced by the field is proportional to frequency. A better characteristic would be an output constant with frequency. Such a characteristic is useful in measuring applications or when combining the output of the loop antenna with a monopole for direction finding. This is one aim of the invention. Another aim of the invention is to maximise sensitivity by including an amplifier in the loop to produce a so called active loop.

According to the invention an active device having an input terminal an output terminal and a common terminal and having a non-inverting voltage gain just less than unity is connected to a loop made of transmission line and connected so that positive feedback coupled transversley through the line adds to the longitudinal input signal induced in the loop to produce an amplified output.

The loop is constructed of transmission line such that the longitudinal terminal voltage, proportional to field strength, is applied to a voltage amplifier. The output of the amplifer is connected to the transmission line so as to impress a transverse voltage across it at one end. The transverse voltage arising at the other end of the line, which is terminated substantially in the characteristic impedance of the line, is added to the longitudinal voltage applied to the amplifer input. Thus a positive feedback system is developed with stable gain.

Due to the time delay of the feedback signal travelling along the line there will be a phase difference between the input longitudinal signal and the feedback signal which causes the proportionality between field strength and output voltage to become approximately constant with frequency over a certain frequency range.

From feedback theory it can be derived that the output voltage will be equal to Vin.A/(1-A.e-iw3 where Vin is the induced input voltage across the loop terminals, w is the radian frequency, T is the delay time of the line and A is the amplifier voltage gain, typically between .9 and .999. Calculation shows that over a certain frequency range the characteristic of Vin proportional to frequency can be corrected to a voltage output nearly constant with frequency.

This characteristic is extended down in frequency range if the amplifier voltage gain is made closer to unity but it must not equal or exceed unity. A typical value of T would be 450/fmax nanoseconds for a single turn air cored loop, where fmax is the maximum useable operating frequency in MHz.

It so happens that this configuration also improves the sensitivity of the antenna when used with practical devices as compared with conventional configurations without feedback. The advantage tends to be inversely related to frequency.

Figure 1 shows the general arrangement of the invention.

Figure 2 shows a preferred embodiment using a transistor amplifier.

According to figure 1, the loop Is constructed of transmission line, TL, which is made up of two parallel conductors, W1 and W2, and wound in a loop with one or more turns. The area of the loop, the number of turns and the incident field strength are factors determining the loop terminal voltage. Conductor, W1, has terminals T1 and T3. Conductor, W2 has terminals T2 and T4. The transmission line, TL, may be a parallel wire line or a coaxial line. The loop may be air cored or wound on ferro or ferrimagnetic material. Terminal, T3, is connected to ground, G, and if using a coaxial line W1 would preferably be the outer conductor. The induced input signal voltage appears longitudinally between terminals T1 and T3, and similarly between terminals T2 and T4.

Terminating resistor, R1, connected between terminals T1 and T2 is substantially equal to the characteristic impedance of the transmission line, TL. Non-inverting amplifier, A, having a voltage gain just below unity, has its input connected to terminal T2. The common connection of amplifier, A, is grounded. The output of amplifier, A, is connected to terminal, T4, of the transmission line, TL. The output voltage, V, of amplifier, A, with respect to ground, provides the useful output of the antenna for receiving purposes.

Some lesser benefit may be obtained if the input of amplifier, A, is not connected to terminal, T2, but tapped down resistor, R1. Although this is a less than an optimum configuration it is to be understood that the same principle of positive feedback claimed in this invention is producing the benefit obtained.

Figure 2 shows a similar loop made of transmission line, TL, terminated in resistor, R1, and grounded at terminal, T3, as in figure 1. For the purposes of illustrating the ac operation of the circuit bias voltages and components are not shown. The base of NPN transistor, Q, is connected to terminal, T2 of transmission line, TL. The collector of transistor, Q, is grounded and the emitter is connected to terminal, T4, of transmission line, TL. The useful output voltage, V, is available from the emitter of transistor, Q, with respect to ground.

Practical antennas will generally embody additional design details which will be well known to those skilled in the art. These would include the use of complementary PNP/NPN transistors or push-pull amplifiers or amplifiers with other types or combinations of active devices. Amplifiers with a balanced output, for example, may have their outputs connected to terminals T3 and T4 so that the same positive feedback results without grounding T4, although that end of the line still remains on average grounded.

Loops of the type where one end is grounded are said to be unbalanced.

Balanced antennas of this invention may be constructed, by taking two grounded counterphased loops in close proximity and combining the outputs in a push-pull arrangement. This is a technique also known in conventional antenna design. A single unbalanced antenna may also be made nearly balanced by isolating the whole antenna from ground and increasing the inductance of the connections to it, as is known. An output interface consisting of a length of transmission line wound on a ferrimagnetic material is one method.

Claims (13)

Claims

1. An active loop antenna consisting of an active device having an input an output and a common terminal and having a non-inverting voltage gain of just less than unity connected to a loop made of transmission line so that positive feedback coupled transversely through the line adds to any longitudinal signal voltage induced in the line to produce an amplified output.

2. An active loop antenna as claimed in claim 1 where the transmission line is a pair of parallel wires.

3. An active loop antenna as claimed in claim 1 where the transmission line is a coaxial cable.

4. An active loop antenna as claimed in claim 1 where the active device is a transistor.

5. An active loop antenna as claimed in claim 1 where the active device is a complementary combination of transistors.

6. An active loop antenna as claimed in claim 1 where the the active device is a push-pull arrangement of transistors.

7. An active loop antenna as claimed in claim 1 where the output of the active device connected to the two terminals of one end of the transmission line is balanced with respect to ground.

8. An active loop antenna as claimed in claim 1 consisting of two counterphased grounded loops in close proximity with outputs combined in push-pull.

9. An active loop antenna as claimed in claim 1 where the loop has more than one turn.

10. An active loop antenna as claimed in claim 1 where the loop has a core of magnetic material.

11. An active loop antenna as claimed in claim 1 where the loop is combined with a monopole antenna for direction finding purposes.

12. An active loop antenna as claimed in claim 1 where the loop is used for measuring purposes.

13. An active loop antenna substantially as herein described with reference to figures 1 and 2.