Communications: radio wave antennas – Antennas – With ice clearer or preventer
Patent
1992-01-28
1994-03-08
Hille, Rolf
Communications: radio wave antennas
Antennas
With ice clearer or preventer
343713, H01Q 1020, H01Q 1320
Patent
active
052931730
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a signal separating device for separating a radio signal from the heating element of an electrically heated motor vehicle window to enable such window to be used as a radio antenna.
BACKGROUND ART
Our earlier U.K. patent 1520030 describes a device of this kind which uses a bifilar choke connected between the heating element and the vehicle d.c. power supply. The choke presents low impedance paths to the relatively large current flow required to power the heating element, and high impedance paths to radio signals.
In practice, when used with a heated window typically consuming 17 A at 12 V for heating purposes, two bifilar chokes may be used, one of relatively small inductance, about 1 .mu.H, which is effective in the vhf range (from 50 MHz to 400 MHz), and a second of much larger value, ideally greater than 10 mH, which is effective over the lowest frequency range of operation, typically down to 150 kHz. Around this lowest frequency, the impedance of the usual heater antenna with respect to the vehicle bodywork closely approaches that of its capacitance, in the region of 80 pF, with a reactance of about 12K.OMEGA. at 150 kHz. The use of the double-wound bifilar configuration is of key importance for this latter choke: since the d.c. magnetisation of the two windings will cancel, a closed magnetic core, such as a ferrite pot-core, may be used without saturation occurring. The required inductance can therefore be achieved with a relatively small number of turns of the thick conductor required to carry the d.c. operating current (say 1.6 mm diameter for 17 A) with a pot-core of acceptable bulk.
With this arrangement, the impedance of the choke is high compared with that of the antenna at the signal frequency whereby a strong radio signal having good signal to noise ratio can be separated.
However, even with the advantage of the double-wound bifilar configuration the low frequency range choke remains a relatively large and expensive component. In practice therefore a compromise value of inductance, say about 1 mH, is employed, for example with 91/2 double turns on a 30 mm diameter core, and this results in some loss of performance at the lowest frequencies. Also, there can be cases where the size or cost of even such a compromise choke may be disadvantageous.
An object of the present invention is to provide an effective signal separating device with which it is possible to achieve good performance at low frequencies with a choke configuration of relatively small size.
DISCLOSURE OF THE INVENTION
According to the invention therefore there is provided a signal separating device for separating a radio signal from the heating element of an electrically heated motor vehicle window to enable such window to be used as a radio antenna, said device comprising first terminals for connection to the heating element, second terminals for connection to a power supply for the heating element, a radio signal terminal connected to at least one of the first terminals, and a LF choke arranged between the first and second terminals for blocking passage of radio signals at the radio signal terminal to the power supply whilst permitting flow of current from the power supply to the heating element, said LF choke comprising two mutually inductively coupled coils wound in a common direction, characterized in that there is provided a radio signal amplifier connected to the radio signal terminal connection, the choke being of relatively low impedance and the LF amplifier being of low input impedance. The input impedance of the amplifier may be similar or lower than that of the choke.
With this arrangement due to the use of the low input impedance amplifier it is possible to achieve optimum coupling with the antenna circuit comprising the window heating element and the relatively low impedance choke whereby, surprisingly, despite this low impedance, excellent performance can be obtained, even at low frequencies, with relatively small dimensions for the choke. This gives rise to two possibi
REFERENCES:
patent: 4654669 (1987-03-01), Kropielnicki et al.
Easter Brian
Kropielnicki Jerzy J.
Brown Peter Toby
Glass Antennas Technology Limited
Hille Rolf
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