Antenna theory often bases itself on a single dipole antenna, which in literature is termed a"Hertzian dipole"antenna.

This type of antenna is very short relative to the wavelength of the electromagnetic field. The electromagnetic radiation of the dipole antenna is largely dependent on the direction in question, relative to the principal axis of the antenna.

Thus the dipole antenna is a direction-sensitive antenna.

Seen in relation to an imaginary antenna with equal radiation in all directions, the dipole antenna will for the same power input, not taking losses into account, in some directions have greater radiation than the imaginary antenna, and in other directions less radiation. The relationship between the maximum radiation intensity of the directional antenna and the uniform radiation intensity of the imaginary antenna is

termed gain, and is an expression of the directional sensitivity of an antenna.

However a real antenna does not radiate all input. It is customary to view an antenna as a circuit in which an antenna resistance representing the radiated power, an ohmic resistance representing the power lost e. g. through heating of the antenna, and a reflection impedance representing the potential of the antenna to return part of the input to the transmitter connected to the antenna, are connected in series. The ohmic losses in an antenna places considerable restrictions e. g. on the use of ferrite in transmitting antennae, as overheating changes the magnetic property of the ferrite. Due to its magnetic property, ferrite is extensively used in receiving antennae.

Ever since the electromagnet field was discovered, the development of antennae has centred around improving the ratio between the types of resistance in an antenna, remedying and/or adapting its impedance to the transmitter, and adapting the antenna to the frequency range in which it is intended to operate.

An electromagnetic field comprises an electric and a magnetic field. Most known antenna are virtually pure electrical antennae in the sense that they generate/are sensitive to electrical fields. One type of antenna, the magnetic loop antenna, generates/is in principle only sensitive to the magnetic part of the electromagnetic field. Several fundamentally different versions of this type of antenna are known. One variety comprises an antenna in which many turns of the antenna conductor have been wound around a magnetic

rod. Upon transmission, a magnetic field is formed, which is directed along the central axis of the winding. However this solution, which is very good per se, is not suitable for transmission due to the ohmic losses as described above, but is extensively used as an AM antenna in radio receivers, where its main disadvantage is its great directional dependency.

Antennae that are chiefly sensitive to the electrical part of the electromagnetic field are influenced by the multitude of electrical fields that surround the antenna. These fields may cause serious disturbance, e. g. to a radio circuit. A magnetic antenna is not subject to the same degree of this type of disturbance.

The object of the invention is to remedy the negative aspects of prior art.

The object is achieved in accordance with the invention by the characteristics stated in the undermentioned description and in the appended claims.

In its basic configuration, the antenna comprises a coil in which one conductor of a connecting cable is connected to one end portion of the coil, and where the other conductor of the connecting cable is connected to the coil at a point between the two end portions of the coil. The number of coil windings between the two connection points must be adapted to the frequency range in which the antenna is to operate. The part of the coil which is located between the connection points constitutes the feeder part of the antenna. The remainder-of the windings of the antenna, the resonant part, which forms

an extension of the feeder windings, requires a number of windings sufficient to make the antenna resonant without the use of a capacitor or other tuning devices. The resonant winding is terminated in a free end; i. e. the end of the antenna wire in the basic configuration is not electrically coupled. Experiments have shown that the first windings of the resonant coil, counted from the connecting point, must have a certain mutual spacing in order to avoid heating the coil. The remainder of the resonant windings may be closely wound.

A fixed or travelling ferrite rod, or alternatively a ferrite tube, may be positioned inside the coil in parallel with the central axis of the coil. The purpose of this is to increase the antenna resistance of the antenna. By using a travelling ferrite rod, the resonant range of the antenna may be changed and matched to the frequency of the relevant electromagnetic field.

It is necessary to adapt the ferrite material to the frequency range to be covered by the antenna. In the case of relatively low frequencies, use may be made of ferrite rods such as used in medium wave receivers. In the case of higher frequencies, a ferrite rod having a lower permeability should be used, preferably one manufactured through use of powder technology. For antennae that are to operate at the highest frequencies, it has proven difficult to obtain ferrite materials of the desired permeability, probably because such materials are not in great demand. A general rule is that a higher frequency range requires the ferrite rod to have a lower magnetic permeability. When the antenna is to be used only as a receiving antenna, using the same materials as

those found in a conventional ferrite rod antenna will be sufficient.

Antennae according to the invention distinguish themselves by the basic configuration exhibiting little gain; in terms of radiation pattern they are approximately isotopic, which means that they are not very direction-oriented. Low ohmic equivalent resistance allows an antenna containing a ferrite rod to be used as a transmitting antenna, also at considerable transmission power. Further, it is a great advantage that the antenna may readily be tuned without the use of special tuning circuits. Tests that have been carried out indicate that the antenna is principally a magnetic antenna. Compared with other magnetic transmitting antennae, the antenna according to the invention has a considerably smaller physical size and weight.

The basic configuration of the antenna may be modified in a number of ways in order to adapt it for special purposes.

Some examples of this have been described in the specification, in which reference is made to the accompanying drawings.

The following describes a non-limiting example of a preferred embodiment of the basic configuration of the antenna, along with several examples of possible modifications of the antenna. The embodiments are illustrated in the accompanying drawings, in which: Figure l schematically shows the basic configuration of the antenna;

Figure 2 schematically shows the antenna of Figure 1 with a connected tuning capacitor; Figure 3 schematically shows the antenna of Figure 1 with a tuning capacitor and a separate coil wound by the resonant part of the antenna; Figure 4 schematically shows the antenna of Figure 1 with a tuning capacitor and a separate coil wound by the feeder part of the antenna; Figure 5 schematically shows the antenna of Figure 1 with a tuning capacitor and a separate coil wound next to the antenna coil; Figure 6 schematically shows the antenna of Figure 1 with a tuning capacitor connected to the two end portions of the coil conductor; Figure 7 schematically shows the antenna of Figure 1 with a conductor connected to the free end portion of the coil conductor; Figure 8 schematically shows the antenna of Figure 1 with a capacitance cap connected to the free end portion of the coil conductor; Figure 9 schematically shows the antenna of Figure 1, where the pitch of the coil windings varies; and

Figure 10 shows an embodiment of the ferrite rod of the antenna in which the different sections of the ferrite rod have different permeability.

In the drawings, reference number 1 denotes an antenna according to the invention, comprising a coil conductor 2 surrounding a fixed or travelling ferrite rod 4. One conductor 12 of a connection line 10 connected to a transmitter or receiver (not shown) is electrically coupled to one end portion 2a of the coil 2. The other conductor 14 of the connection line 10 is electrically coupled to a point 2b on coil conductor 2, the point 2b being located somewhere between the two end portions 2a and 2c of the coil conductor.

In this basic configuration, the end portion 2c is not electrically coupled. The coil portion located between the points 2a and 2b constitutes the feeder part of the antenna 1, while the coil portion located between points 2b and 2c constitutes the resonant part of the antenna 1. The antenna 1 will also function without using the ferrite rod 4. The ferrite rod 4 may comprise one or more ferrite sections Xa, Xb, Xc and Xd, possibly with different shapes and permeabilities, see figure 10, and possibly with intermediate or connected-up sections made from one or more other materials.

By displacing the ferrite rod 4 along the central axis 3 of the coil 1 in the direction of the feeder point 2a, part of the coil conductor 2 falls outside the ferrite rod 4. Thus the resonant frequency of the antenna is changed, allowing the antenna to be adapted to a different frequency range.

In an embodiment with a fixed ferrite rod 4 it is possible to tune the antenna by means of a capacitor 5 connected to the points 2b and 2c, see figure 2. Figures 2 to 8 all show alternative embodiments designed to tune the antenna 1. In figure 3, the capacitor 5 is inductively coupled to the antenna 1 by means of a coil 6. The coil 6 may be wound between or over the coil conductor 2. It is important to the operation of the circuit that the coils 2 and 6 be wound in the same direction. The advantage of the circuit as shown in figure 3 is that the capacitor voltage is relatively low, allowing the use of a capacitor 5 with small spacing between the plates. In figure 4, the coil 6 is positioned by the feeder part of the antenna 1. In this embodiment it is also important that the coils 2 and 6 be wound in the same direction. In figure 5, the coil 6 is wound to encircle the ferrite rod next to the coil conductor 2. In figure 6, the capacitor is connected between the end portions 2a and 2c of the coil.

Figure 7 shows an embodiment in which a conventional conductor 7 is connected to the end portion 2c of the coil conductor 2, and where the length of the conductor 7 may be used to tune the antenna 1, either by merely changing the length of the conductor 7 or in combination with making the coil 2 resonate, either by means of a capacitor 5 as shown in the preceding drawings, or by moving the ferrite rod 4 in or out of the coil 2.

In figure 8, the end portion 2c of the coil conductor 2 is connected to a capacitance cap 8. This embodiment is particularly suitable when it is desirable for the antenna

not to take up a lot of space. Resonance may be produced as described for figure 7.

Two or more of the embodiments shown may be combined in order to adapt the antenna for special purposes.