GB2206243A - Dual-frequency helical antenna - Google Patents

Abstract

A radio antenna which is resonant at harmonically related upper and lower frequencies comprises a helical conductor 3 wound on an electrically insulating hollow core 4 and provided with an electrically insulating sheathing 5. A linear conductor 6 is inserted into one end of the core 4 and electrically connected to the conductor 3 before application of an end cap 7. The dimensions of the conductors 3 and 6 are selected to give desired upper and lower resonant frequencies. <IMAGE>

Description

DUAL-FREQUENCY HELICAL ANTENNA This invention concerns a dual-frequency helical antenna.

An antenna of the kind comprising a helically wound coil is a known form of an antenna conventionally utilised, among other applications, as the transmission antenna of a distress beacon. Such distress beacons currently operate on two frequencies respectively allocated for military and civil use. One frequency is a harmonic of the other, the lower frequency being 121.5 MHZ and the higher 243 MHZ.

Although it is desirable to be able to use a single construction of aerial in relation to both frequencies, it is a known characteristic of the helical antenna that the resonant frequencies of the antenna are not directly harmonically related, so that a helical antenna with a natural resonance at a lower frequency, e.g. 121.5 HHZ, will not resonate harmonically at 243 MHZ. Accordingly, various constructions have previously been proposed in order to modify a helical antenna to enable adjustment of the relationship between lower and higher resonant frequencies. Such constructions include providing transformer coupling to the antenna via a tapping on the wire helix, and the division of the antenna into sections that are isolated via a high pass filter, e.g. a capacitor, in order to vary the effective length of the antenna at different frequencies.

Such prior proposals, however, suffer from the disadvantage that they increase the complexity and the corresponding manufacturing costs of the antenna.

It is thus an object of the present invention to provide a dual-frequency helical antenna that is capable of being matched to two desired operating frequencies, whilst being of simple and economical construction.

The invention accordingly provides an antenna comprising a helical conductor, one end of which is adapted to be coupled to the circuit of a radio frequency transmitter and/or receiver, and the other end of which is electrically connected to a conductor extending linearly and coaxially within the helical conductor, over a portion of its length, the relative dimensions of the said helical and linear conductors being so selected that the combination thereof is resonant at predetermined lower and upper frequencies.

Preferably, the upper frequency is a harmonic of the lower frequency.

An arrangement in accordance with the invention provides a construction of antenna that is very simple to manufacture, since the body of the antenna can be massproduced in continuous lengths, in known manner, by helically winding an insulated wire upon a hollow core and then applying an electrically insulating outer sheathing. The sheathed helical conductor can then be cut to appropriate lengths, a connecting base being fitted at one end of the length, and the linear conductor being inserted into the hollow core at the other, electrically connected to the helical conductor and covered with an appropriate end cap.Manufacture of the antenna does not therefore require access to any intermediate part of the sheathed helical conductor, and, moreover, adjustment of the linear conductor to trim the antenna to resonate at the upper frequency can be effected without any need to disturb the base connection of the antenna.

The invention is illustrated by way of example in the accompanying drawings, in which; Fig. 1 is a longitudinal cross-section through one embodiment of antenna in accordance with the invention, and Figs. 2A and 2B are respectively graphs illustrating the performance of an antenna of known construction, and an antenna incorporating the features of the present invention.

Referring to Fig. 1, the reference numeral 1 indicates a conventional coaxial screw-connector, the central pole 2 of which is coupled to one end of a helical conductor 3 wound on a hollow electrically insulating core 4, and provided with an outer sheathing 5.

The construction of the antenna thus far described is of known type.

In accordance with the invention, however, the free end of the conductor 3 remote from the connector 1 is electrically connected to a linear conductor 6 inserted into the central core 4 for a predetermined length. The electrical connection between the conductors 3 and 6 is covered by means of a known end cap 7.

It has surprisingly been found that the simple presence of the linear conductor 6 is sufficient to reduce the upper resonance frequency or frequencies of the antenna in such a manner as to enable operation of the antenna at lower and upper resonant frequencies that are harmon ically related. The performance of such an antenna in accordance with the invention in relation to that of the prior art can be seen by comparison of Figs.

2A and 2B, each of which is a graph illustrating the magnitude of reflected radio frequency energy over the radio frequency spectrum from 115 MHZ to 250 MHZ.

Thus, the lower the magnitude of the reflected energy shown in the graph, the higher is the efficiency of the antenna in transmitting radiated energy at the corresponding frequency. Whereas the graph in Fig.

2A relating to the known antenna illustrates a relatively high radiation efficiency at point A corresponding to a frequency of 121.5 MHZ, the efficiency of the antenna is substantially lower up to the illustrated limit of 250 MHZ. In Fig. 2B, on the other hand, the antenna in accordance with the invention is shown to have a relatively high radiating effiency both at point A corresponding to the frequency of 121.5 MHZ, and at point B corresponding to the frequency of 143 MHZ.

Typical constructional paramaters for an antenna as illustrated in Fig. 1 and having the performance shown in Fig. 2B are as follows: Diameter of wire conductor 3 - 1.21 mm Material of wire conductor 3 - steel copper plated Core diameter of helix - 6.7 mm Pitch of helix - 2.5 mm Overall length of helix - 210 mum Effective length of linear conductor 6 - 57 mm Diameter of linear conductor and - 7/0.2 covered 0.3 mm construction PVC

Claims (6)

1. CLAIMS 1. An antenna comprising a helical conductor, one end of which is adapted to be coupled to the circuit of a radio frequency transmitter and/or receiver, and the other end of which is electrically connected to a conductor extending linearly and coaxially within the helical conductor, over a portion of its length, the relative dimensions of the said helical and linear conductors being so selected that the combination thereof is resonant at predetermined lower and upper frequencies.
2. 2. An antenna as claimed in Claim 1, wherein the said upper frequency is a harmonic of the said lower frequency.
3. 3. An antenna as claimed in Claim 2, wherein the said upper frequency is substantially twice the said lower frequency.
4. 4. A method of manufacturing an antenna as claimed in any one of Claims 1-3, comprising helically winding said helical conductor onto a hollow core to form a continuous length greater than the finished length of said antenna, applying an electrically insulating sheathing over said helical conductor, cutting a predetermined length from the sheathed assembly, inserting a linear electrical conductor into the hollow core at one end of said length and electrically connecting said linear electrical conductor to the helical conductor at said one end.
5. 5. A method as claimed in Claim 4, further comprising the step of applying an end cap over said one end.
6. 6. A method as claimed in Claim 4 or 5, further comprising the step of fitting a connecting base to the other end of the sheathed assembly.