GB2125225A - A loop antenna for radio communication - Google Patents

Abstract

A loop antenna for radio communication includes an antenna member in the form of a coil (L2) comprising several turns, a coupling loop (L1) inductively connected to the coil and adapted for turning inside it, a wideband transformer (2) connected between the connecting loop and a transmitter/receiver, a tuning capacitor (9) for coupling in parallel across the coil, and electrically insulating spacers (5, 22) for carrying the coil and coupling loop and for insulating these units from earth. The characteristic features of the invention is a switch 3 for coupling in an optional number of turns of the coil in parallel across the tuning capacitor and an unsymmetrical mounting of the wideband transformer 2 on the coupling loop. <IMAGE>

Description

SPECIFICATION A loop antenna for radio communication The present invention relates to a loop antenna for radio communication comprising an antenna member in the form of a coil consisting of several turns, a coupling loop mounted for turning inside the coil, a wideband transformer connected between the coupling loop and a radio communication unit, a tuning capacitor connected in parallel with the coil, and spacer members of electrically insulating material for carrying and insulating from earth, the coil, coupling loop and tuning capacitor.

A loop antenna of this kind is described in Swedish Patent Application 76093491and in Radio Communication, September 1981, pages 820-821, published by The Radio Society of Great Britain. This known loop antenna has a frequency range extending from about 1.9 MHz to about 1 6 MHz. The antenna is omnidirectional, but in its in-field, i.e. an area within a radius of about 20 m from the antenna, it has produced nulls in its horizontal radiation pattern. The nulls are situated at right-angles to the plan of the antenna loop, and each null has the form of a sector within which the signal strength is dB (1000 times less) compared with the signal strength in the remaining areas at a corresponding distance around the antenna.Outside the antenna in-field it is omnidirectional in the horizontal plane, and in the vertical plane it has a vertical radiation pattern from at least 300 up to zenith, and probably even from the horizon up to zenith.

The presence of nulls in the horizontal radiation pattern of the antenna makes the antenna attractive in applications where it is desired to protect electronic equipment against the radiated HF energy of the antenna. By placing. such electronic equipment in the region of said nulls, such equipment can be used at the same time as radio transmission takes place from the loop antenna.

The known loop antenna also has a number of other desirable properties such as the ability to withstand vibration, low standing wave ratio (better than 1:1.3), small size (only about 40 x 80 x 70 cm) and a high Q value (better than 1000), signifying that a pure signal free from harmonics leaves the antenna during transmission and that the antenna is suitable for reception, due to the narrow band width which enables undesirable signals from adjacent transmitters to be supressed.

However, a problem with this known loop antenna is the limited frequency range. This problem can be overcome by reducing the number of coil turns in parallel across the tuning capacitor.

In this way the utilizable frequency range of the antenna is increased upwards from about 1 6 MHz to about 30 MHz. Unfortunately, a reduction in the number of turns also results in the horizontal radiation pattern in the in-field changing, in that the nulls are not as pronounced as before, i.e., the field strength within those sectors at right-angles to the loop antenna tends to increase.

It is an object of the present invention to provide a loop antenna in which the abovementioned problems are obviated or mitigated.

According to the present invention there is provided a loop antenna for radio communication, comprising an antenna member in the form of a coil comprising several turns, a coupling loop mounted for turning inside the coil, a wideband transformer connected between the coupling loop and a radio communication unit, a tuning capacitor connected in parallel with the coil and spacer members of electrically insulating material for carrying and insulating from earth the coil, coupling loop and tuning capacitor, characterised in that a switch is connected between one contact of the tuning capacitor and at least one turn of the coil for coupling said at least one turn of the coil in parallel with the tuning capacitor to extend fhe utilizable frequency range of the antenna upwardly and in that the wideband transformer is mounted off-centre on the coupling loop.

The reason for the upper working range of the antenna increasing from about 1 6 MHz to about 30 MHz is not entirely understood. Stray capacitance and induction probably occur between the turns of the coil coupled in to the oscillation circuit of the antenna and the remaining turns of the coil, which are fully floating from the electrical aspect, thus resulting in the antenna radiating substantially the full supplied power at the highest frequencies.

Apart from the frequency range of the inventive antenna being widened, which is per se an advantage, the field of use of the antenna is also increased, since so-called troposphere scattering can now be utilized. Troposphere scattering means that radio waves at frequencies within the upper utilizable frequency range of the antenna will propagate over very large distances without being appreciably attenuated due to scattering in the troposphere. Transmitter power can thus be kept low, which is an advantage.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an electrical circuit diagram for the loop antenna in accordance with the present invention; Figure 2 illustrates a loop antenna embodying the present invention in a perspective view from one side; Figure 3 illustrates the loop antenna of Fig. 2 in a perspective view from the other side; Figure 4 is a schematic diagram of a switch for use in the loop antenna of Figs. 2 and 3, and Figure 5 is a diagram of the radiation pattern of the loop antenna embodying the present invention.

The circuit diagram for the loop antenna in accordance with the invention is illustrated in Figure 1. A shielded coaxial cable 2 of optional length connects the antenna with a radio communication unit (not shown) comprising a transmitter and/or receiver. The receiver (not shown) is preferably connected via a wideband HF amplifier for improving the signal/noise ratio.

The loop antenna conventionally includes a coupling loop L1, heavily inductively coupled to an antenna member in the form of a main loop or multiturn coil L2, which is coupled in parallel with a variable tuning capacitor C. A wideband transformer 2 conventionally connects the coupling loop to the coaxial cable 1. In accordance with the invention, a switch 3 is arranged such that one or more turns of the coil may be coupled in parallel with the tuning capacitor. The switch 3 has two positions, in one of which only one turn of the coil is connected in parallel with the tuning capacitor, and in the other one of which all the turns of the coil are connected in parallel with the capacitor. However, it will be appreciated that several switching positions can be provided, as illustrated in Figure 1 by chain-dotted line.

The physical embodiment of the loop antenna in accordance with the invention will be apparent from Figures 2 and 3. The coil L2 has a square shape and comprises three turns, which are fixed at a given distance from each other by means of spacers in the form of electrically insulating plastic rods 4. By means of other spacers 5 in the form of electrically insulating plastic rods the coil L2 is attached to, and electrically insulated from a substructure 6. The coil is made from copper tubing with a diameter of 22 mm and a wall thickness of 1 mm. The square has a side length of 55 cm and the centre-to-centre distance between the turns is 10 cm.

One end of the coil 7 is connected by means of an electrically conductive plate 8, e.g. of copper, to a first plate (not shown) of a ceramic vacuum capacitor 9, which corresponds to the capacitor C of Figure 1. Its capacitance is adjustable between 5 and 1000 pF by turning a rod 10, which is mechanically connected to the shaft of the capacitor but is electrically insulated from it. The other end of the coil 11 is connected by means of an electrically conductive plate 12, e.g. of copper, to a first contact 1 3 (Figure 4) of switch 3. This switch has a second contact 14 (Figure 4) connected by an electrically conductive plate 15, e.g. of copper, to the second plate (not shown) of the capacitor 9.The switch further includes a third contact 1 6 (Figure 4) which, by means of an electrically conductive contact clip 17, e.g., of copper, is connected to a point on the coil situated such that one turn of the coil is connected in parallel with the tuning capacitor 9, when the movable slide 1 8 of the switch (Figure 4) connects the second and third contacts of the switch to each other. When said movable slide 18 connects the first and second contacts 13 and 14 of the switch to each other, all the turns of the coil are connected in parallel with the capacitor 9. The movable slide 1 8 is displaced by means of an operating rod 19, which is electrically insulated from the coil and substructure 6.When all three turns of the coil are connected in parallel across the capacitor 9 the working range of the antenna extends from about 2 MHz to about 7 MHz. When only one turn of the coil is connected in parallel with the capacitor 9 and the remaining two turns are freely floating from the electrical aspect, the utilizable operational range of the antenna is from about 7 MHz to about 30 MHz.

The plates 8 and 1 5 also serve as holders for mechanical retention of the tuning capacitor 9.

The coupling loop L1 has a square shape with a side length of 27 cm. The loop has two holders 20, 21 arranged at either end thereof by means of which the loop is rigidly connected to a cylindrical rod 22. The rod 22 can be rotated so as to allow the plane of the coupling loop relative the plane of the coil L2 to be altered. When the coupling loop plane is arranged at right-angles to the plane of the loop 12, the antenna is basically tuned for operational frequencies between about 2 MHz to about 1 6 MHz. Fine tuninq takes olace by adjusting the capacitor 9. When the plane of the coupling loop coincides with the plane of the loop L2, the antenna is basically tuned for frequencies between about 16 MHz to about 30 MHz Fine adjustment takes place by tuning the tuning capacitor 9.

The wideband transformer 2 is for adjusting the impedance of the coupling loop L1 to the impedance of the radio communication unit, said transformer having a torus-shaped core, on which the windings (see Figure 1) are mounted. The torus core is attached between holders 20 and 21 by means of a rod (not shown) of electrically insulating material, extending between said holders and through the center of the torus.

Preferably the transformer should be connected to the underside of the coupling loop at a place situated approximately 1/3 of the length of the coupling loop. The impedance ratio of the wideband transformer is 50 ohms:200 ohms. By this method the antenna is given the characteristic nulls 23, 24, as shown in Figure 5, in the horizontal radiation pattern of the loop antenna. It should be noted that these nulls are only present in the in-field of the antenna. For areas further away the antenna is omnidirectional in the horizontal plane.

If the wideband transformer is placed centrally on the underside instead, nulls are indeed obtained, but they are not at all as heavy as down to 30 dB.

The radiation diagram illustrated in Figure 5 has been plotted for the frequencies 2.5, 4.0. 6.0, 8.0 and 14 MHz. The distance between the loop antenna and the transmitter antenna was 3.5 mm and the couplingioop was parallel to the loop L2.

The concentric rings denote the signal attenuation expressed in decibels and counted relative to a reference signal level measured in the direction 0 (parallel to the plan of the loop antenna). The coaxial cable 1 illustrated in Figure 1 is taken through a central bore (not shown) in the rod 22.

Below the substructure 6 there are circuits for automatically tuning the antenna. Since these circuits do not constitute any part of the present invention they will not be described in detail here.

Finally it should be pointed out that the given dimensions for the antenna can be reduced and that a corresponding increase of the operational range of the antenna thus obtained.

The illustrated embodiment of the invention can be modified in many ways and varied within the scope of the inventive concept.

Claims (6)

1. A loop antenna for radio communication, comprising an antenna member in the form of a coil comprising several turns, a coupling loop mounted for turning inside the coil, a wideband transformer connected between the coupling loop and a radio communication unit, a tuning capacitor connected in parallel with the coil and spacer members of electrically insulating material for carrying and insulating from earth the coil, coupling loop and tuning capacitor charaterised in that a switch is connected between one contact of the tuning capacitor and at least one turn of the coil for coupling said at least one turn of the coil in parallel with the tuning capacitor to extend the utilizable frequency range of the antenna upwardly and in that the wideband transformer is mounted off-centre on the coupling loop.

2. An antenna as claimed in claim 1, characterised in that the coupling loop is substantially quadratic, and that the wideband transformer is situated at a point one third of the length of the lower horizontal side of the loop.

3. An antenna as claimed in claim 2, characterised in that the coil is quadratic and has three turns, all of which are coupled in when the operational frequence of the radio communication unit is between approximately 2 MHz and 7 MHz, and that only one turn of the coil is coupled in when said operational frequency is between approximately 7 MHz and 30 MHz; in the latter case stray capacitances and inductance between the coupled-in turn and the remaining turns, which are freely floating from the electrical aspect, result in extension of the utilizable frequency range of the antenna upwardly.

4. An antenna as claimed in claim 1, characterised in that the switch includes a first contact which is connected by a first plate to one end of the coil, a second contact connected by means of a second plate to one plate of the tuning capacitor, a third contact connected by a clip to the first turn of the coil and a movable contact slide for connecting the second contact either to the first contact or to the third contact, the second plate of the capacitor being connected to the other end of the coil by means of a third plate.

5. An antenna as claimed in claim 4, wherein the coil is made from copper tubing, characterised in that spacers of insulating material are arranged between the individual turns of the coil and that the radial extension of the coil is large relative to its axial extension, the Q factor being better than 1000.

6. A loop antenna substantially as hereinbefore described with reference to the accompanying drawings.