GB2117182A - Aerial arrangements - Google Patents

Abstract

An aerial arrangement, in particular for a pocket paging receiver, comprises an inductive limb and a capacitive limb (C, CV) connected in parallel with one another so as to form a parallel resonant circuit at a desired frequency (e.g. 150 MHz), the inductive limb comprising an inductor (10) formed from a conductor wound on a ferrite core. The above- mentioned conductor is interrupted at a position between its ends and the interruption is bridged by a capacitor (C'), which renders the arrangement less sensitive to "hand effect". <IMAGE>

Description

SPECIFICATION Aerial arrangements This invention relates to aerial arrangements.

It is well known for a radio receiver to include an aerial arrangement that comprises an inductive limb and a capacitive limb connected in parallel with one another so as to form a parallel resonant circuit at a desired frequency. (As is well known to those skilled in the art, a parallel resonant circuit is of such a nature that it has a maximum impedence at the desired (resonant) frequency, in contrast to a series resonant circuit which has a minimum impedence at the resonant frequency.) It is well known in such aerial arrangements for the inductive limb to comprise an inductor formed from a conductor wound on a ferrite core, typically a ferrite rod, Typicaily, the resonant circuit will be connected directly or through a tapping arrangement to the input of a radio frequency (RF) amplifier.

A disadvantage of such ferrite core aerial arrangements, at least at higher frequencies (VHF and UHF) and in certain applications, is the socalled "hand" effect. According to this, the presence of the hand or other parts of the human body adjacent the ferrite rod can, because of its small but perceptible capacitance, substantially de-tune the resonant circuit and thereby reduce the sensitivity of the receiver. The hand effect can in particular cause problems in connection with so-called paging receivers, i.e. small radio receivers designed to be worn on the person.

A technique of modifying ferrite core aerial arrangements that is said to alleviate the problem of hand effect is disclosed in UK Patent No.

1 507 864. This involves the use of a series resonant arrangement rather than a parallel resonant arrangement. More specifically, a conductor wound round a ferrite rod is interrupted at numerous positions to define a plurality of individual conductor segments, each capable of resonance at a frequency of interest, and the adjacent pairs of conductor segments are connected by a multiplicity of series-resonating capacitors to form a plurality of series-resonant tuned circuits. Naturally, such an arrangement cannot be used when a parallel-resonant arrangement is required. Further, the need to make many interuptions in the conductor and to bridge the interruptions by numerous capacitors makes the arrangement difficult and complex to manufacture.

According to the present invention there is provided an aerial arrangement comprising an inductive limb and a capacitive limb connected in parallel with one another so as to form a parallel resonant circuit at a desired frequency, the inductive limb comprising an inductor formed from an inductor wound on a ferrite core, wherein said conductor is interrupted at a position between its ends and the interruption is bridged by a capacitor.

It has been found, somewhat suprisingly, that modifying a parallel resonant circuit in this simple way can lead to the arrangement being substantially less sensitive to hand effect and can also lead to the circuit being somewhat easier to tune.

Preferably, the conductor is separated as set forth above at one position only, which position may be at least approximately half way along the length of the conductor.

The value of said capacitor is preferably such that, at the resonant frequency, its reactance is at least of the same order of magnitude as the reactance the inductor would have at the same frequency if the capacitor were not present and the interruption were joined, though the capacitor's reactance will of course be less than the inductor's reactance to preserve a net inductance in the inductive limb.

The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawing, in which like references indicate like items in the different figures, and in which: Figure 1 is a circuit diagram of a conventional parallel resonant aerial arrangement, shown connected to an RF amplifier stage; and Figure 2 shows the arrangement of Figure 1 modified to form an aerial arrangement embodying the present invention.

Figure 1 shows a parallel-resonant ferrite rod aerial arrangement which can be used, for example, in a pocket paging receiver operating at a frequency of 1 50 MHz. The parallel resonant circuit comprises an inductive limb constituted by an inductor 10 formed by a conductor wound round a ferrite core in the form of a ferrite rod.

Connected in parallel with the inductive limb constituted by the indicator 10 is a capacitive limb comprising a fixed capacitor C and a varible or trimmer capacitor CV. The voltage developed across the parallel resonant circuit in use, upon receipt of a radio signal, is tapped down by virtue of the capacitors C and CV, that act as a capacitive potential divider, and applied to an RF amplifier 12.

In a circuit tuned to resonate at 1 50 MHz, the inductor 10 typically has an inductance of 100 nH, the capacitor C typically has a value of 1 8 pF and the variable capacitor CV is variable between 2 and 8 pF. The variable capacitor CV is used as a trimmer to adjust or tune the tuned circuit to resonate at the frequency of interest (e.g. 1 50 MHz) thereby to maximise the voltage applied to the RF amplifier 12 for a received signal of this frequency.

The circuit as described with reference to Figure 1 is somewhat susceptible to the hand effect described above. That is to say, the disposition of a hand or other portion of the human body in the vicinity of the tuned circuit can substantially de-tune it and therefore reduce the input voltage to the RF amplifier 12.

The embodiment of the invention described hereinbelow with reference to Figure 2 is in many respects similar to that described with reference to Figure 1 and will only be described in so far as it differs therefrom. The circuit of Figure 2 is in substance the same as that of Figure 1 except that the conductor of the inductor 10 is interrupted at a position at least approximately half way along its length and the gap is bridged by another capacitor C'. It has been found, somewhat suprisingly, that simply doing this substantially reduces the sensitivity of the circuit to hand effect without, on the other hand, substantially detracting from the performance of the circuit. Further, another advantage has been found to arise.This is that, in the modified circuit of Figure 2, the adjustment of the trimmer capacitor CV to tune the circuit for maximum impedance at the frequency of interest is considerably less critical. That is to say, tuning of the circuit is somewhat easier and the sensitivity of the circuit is therefore not reduced to the same extent as Figure 1 in the event that it is slightly off-tune, e.g. if it is not accurately tuned or if it drifts off-tune.

The values of the components in Figure 2 may be substantially the same as in Figure 1, though it may be expedient to increase slightly the value of the capacitor C, for instance from 1 8 pF to 22 pF.

The value of the capacitor C' is preferably such that its reactance Xc, at the resonant frequency is of the same order of magnitude as the reactance X,O at the same frequency of the inductor 10 in the absence of the capacitor C', through the reactance X1O will of course be greater than the resistance Xc, to ensure that the inductive limb is in fact inductive at the resonant frequency, i.e. at the frequency at which it is in parallel resonance with the capacitive limb constituted by the capacitors C and CV.For example, if the inductance of the inductor 10 is, say, 1 60 nH, whereby its reactance at 1 50 MHz is 1 50 ohms, the capacitor C' may have a value of, say, 10 pF whereby its reactance at 1 50 MHz is about 106 ohms, producing a net inductive reactance of (150-1 06)=44 ohms.

Claims (Filed on 1 6th March 1983 1. An aerial arrangement comprising an inductive limb and a capacitive limb connected in parallel with one another so as to form a parallel resonant circuit at a desired frequency, the inductive limb comprising an inductor formed from a conductor wound on a ferrite core, wherein said conductor is interrupted at a position between its ends and the interruption is bridged by a capacitor.

2. An aerial arrangement according to claim 1, wherein the conductor is separated as set forth above at one position only.

3. An aerial arrangement according to claim 2, wherein said one position is at least approximately half way along the length of the conductor.

4. An aerial arrangement according to claim 1, claim 2 or claim 3, wherein the value of said capacitor is such that, at the resonant frequency, its reactance is at least of the same order of magnitude as the reactance the inductor would have at the same frequency if the capacitor were not present and the interruption were joined.

5. An aerial arrangement substantially as herein described with reference to Figure 2 of the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. it differs therefrom. The circuit of Figure 2 is in substance the same as that of Figure 1 except that the conductor of the inductor 10 is interrupted at a position at least approximately half way along its length and the gap is bridged by another capacitor C'. It has been found, somewhat suprisingly, that simply doing this substantially reduces the sensitivity of the circuit to hand effect without, on the other hand, substantially detracting from the performance of the circuit. Further, another advantage has been found to arise. This is that, in the modified circuit of Figure 2, the adjustment of the trimmer capacitor CV to tune the circuit for maximum impedance at the frequency of interest is considerably less critical.That is to say, tuning of the circuit is somewhat easier and the sensitivity of the circuit is therefore not reduced to the same extent as Figure 1 in the event that it is slightly off-tune, e.g. if it is not accurately tuned or if it drifts off-tune. The values of the components in Figure 2 may be substantially the same as in Figure 1, though it may be expedient to increase slightly the value of the capacitor C, for instance from 1 8 pF to 22 pF. The value of the capacitor C' is preferably such that its reactance Xc, at the resonant frequency is of the same order of magnitude as the reactance X,O at the same frequency of the inductor 10 in the absence of the capacitor C', through the reactance X1O will of course be greater than the resistance Xc, to ensure that the inductive limb is in fact inductive at the resonant frequency, i.e. at the frequency at which it is in parallel resonance with the capacitive limb constituted by the capacitors C and CV.For example, if the inductance of the inductor 10 is, say, 1 60 nH, whereby its reactance at 1 50 MHz is 1 50 ohms, the capacitor C' may have a value of, say, 10 pF whereby its reactance at 1 50 MHz is about 106 ohms, producing a net inductive reactance of (150-1 06)=44 ohms. Claims (Filed on 1 6th March 1983

1. An aerial arrangement comprising an inductive limb and a capacitive limb connected in parallel with one another so as to form a parallel resonant circuit at a desired frequency, the inductive limb comprising an inductor formed from a conductor wound on a ferrite core, wherein said conductor is interrupted at a position between its ends and the interruption is bridged by a capacitor.

2. An aerial arrangement according to claim 1, wherein the conductor is separated as set forth above at one position only.

3. An aerial arrangement according to claim 2, wherein said one position is at least approximately half way along the length of the conductor.

4. An aerial arrangement according to claim 1, claim 2 or claim 3, wherein the value of said capacitor is such that, at the resonant frequency, its reactance is at least of the same order of magnitude as the reactance the inductor would have at the same frequency if the capacitor were not present and the interruption were joined.

5. An aerial arrangement substantially as herein described with reference to Figure 2 of the accompanying drawing.