Rod antennas are known in the art for the receipt of low frequency signals in the range typically of a few tens of kHz up to 1 to 2 MHz. A conventional low frequency rod antenna usually comprises a number of turns of wire on a former, the former usually being a ferrite rod. The coil forms an inductor and the antenna may be tuned to a particular frequency by adding a capacitor for example in parallel with the inductor so that the inductor and capacitor form a resonant circuit. However, the inductor-capacitor combination provides a frequency response with very high Q, which, whilst acceptable for the reception of narrow band signals, is not acceptable for the reception of wide band signals, where the bandwidth is a significant portion of the antenna centre frequency. For example, in the LORAN™ navigational system, the centre frequency is 100 kHz, but with a −20 dB bandwidth of 20 kHz. A rod antenna with high Q thanks to the inductor-capacitor resonant circuit may therefore not be suitable for receiving such a wideband signal.
It is known to reduce the Q in several ways. A conventional way is to add an additional resistor, but this has the effect of reducing the sensitivity of the antenna, and decreasing the signal to noise ratio. Another known technique is to apply negative feedback to the antenna magnetic circuit, by use of a secondary feedback coil. Such a technique is known from U.S. Pat. No. 2,787,704, but more particularly from U.S. Pat. No. 4,314,378, the arrangement of which is shown in FIG. 1.
As shown in FIG. 1, a parallel inductor-capacitor resonant tank circuit, wherein the inductor has a ferrite rod core M and provides an output via field effect transistor Q1 to the inverting input of an operational amplifier A1. Connected to the output of the amplifier A1 is inductor L2, which is magnetically coupled to the ferrite rod so that it responds to the same magnetic field as inductor L1. The turns of the inductor L2 are applied to the ferrite rod M in the same sense as inductor L1 so that the voltage applied to inductor L2 will produce a voltage on L1 that is in phase with the voltage on L2. Because of the inversion produced by operational amplifier A1, the magnetic field produced by the current in L2 will be 180° out of phase with the antenna detect signal 1 output from L1. The magnitude of the negative feedback can be controlled via a feedback resistor Rf.
The effect of the negative feedback on the Q of the rod antenna is to reduce the Q, but the sensitivity of the antenna is not reduced.
Whilst the prior art of record reduces sensitivity of the rod antenna to proximal components, it would be beneficial if such insensitivity could be increased. In addition, sensitivity of the antenna can also be increased by improvements aimed at reducing noise in the circuit.