Radar Antenna Mount

Abstract

In a short range radar detection device for burglar and like intrusion protection, operating on the doppler principle of detecting a frequency shift caused by a moving object, the mounting of the antenna provides structural rigidity and at the same time automatically furnishes the capacitances between the antenna, ground and an inductor, that are required for tuning the antenna system.

Description

This invention relates to a manner of mounting an antenna in a radar device.

More specifically, it relates to a manner of mounting an antenna in which the structural elements serve simultaneously to provide the capacitances between the antenna, ground and an inductor that are required for tuning the antenna system.

In the description that follows, the invention is exemplified as applied to a short range radar detection device for detecting the movement of objects in its vicinity. The device operates on the doppler principle of detecting signals returned by moving objects at a slightly different frequency from that of the transmitted signal. Stationary objects are not detected.

While the device can be employed for any type of surveillance duty, the particular purpose for which it has been developed is as an alarm device for the detection of burglars or other intruders into the area under surveillance, e.g. a shop, warehouse or private home.

Devices of this general type are already known, but they suffer from the disadvantage that they are complex and expensive, and consequently beyond the financial reach of many persons, especially private citizens.

An object of the present invention is to provide an antenna mount that will render a device of this character so simple in construction that it can be mass produced and sold at a price well within a range that would willingly be paid by individual house and apartment dwellers in return for improved security, e.g. a cost of up to about $100.

A further object of the invention is to achieve a device that can be made for this low price without sacrifice of efficiency and reliability of operation.

Further objects and features of the invention will become apparent from the following description which, together with the accompanying drawings, illustrates one embodiment of the invention.

While the antenna mount is herein shown and described in association with a specific form of radar device, it is to be understood that the mount may be employed with devices that differ in other aspects of their circuits and constructions. Thus the illustrated embodiment is shown by way of example only, and not by way of limitation of the broad scope of the invention, which latter is defined in the appended claims.

In the drawings:

FIG. 1 is a small scale, perspective view of a radar device;

FIG. 2 is the electrical circuit of the device of FIG. 1, shown diagrammatically;

FIG. 3 is a larger scale, broken away side view of an antenna mount employed in the device of FIG. 1;

FIG. 4 is a section on IV--IV in FIG. 3;

FIG. 5 is a further enlarged fragment of FIG. 3; and

FIG 6 is an exploded version of FIG. 5.

As shown in FIG. 2, an oscillator 10, e.g. 400MHZ, drives a tank circuit L1, C1, the inductor L1 of which is loosely inductively coupled with another loop inductor L2, the two ends 16, 17 of which are connected to respective arms 11 of a "rabbit ears," half wave dipole antenna. The arms 11 are mounted in an antenna mounting member 12 secured to the top of a casing 13 in which the other components including a key operated switch 14 are housed. Due to the simplicity of the device, the casing 13 can be small, e.g. about 6 inches long, 3 inches deep and 2 inches high, i.e. about the size of a small radio receiver. It can readily be located in the area to be surveyed without occupying any appreciate space. The components are operated from a battery 9, so that the device is independent of any power supply, although a battery charger can be included as an extra component. Since the device operates with radar frequency waves, its signals penetrate wood and most other materials including thin walls; it can thus be mounted out of sight in a cupboard or other concealed location.

The center point 15 of the inductor L2 is connected to ground (casing) and hence to the member 12 which is of metal, preferably aluminum. The connections of the inductor ends 16, 17 to the antenna arms 11 are not direct, but through capacitances C2 and C3. These series capacitances, which, at the frequency employed, exhibit very low impedances, are not separate electrical components, but are built into the structure of the antenna mount, as is described in detail below.

In a similar fashion, the antenna arms 11 are physically mounted so as to have capacitances to ground represented by C4 and C5, these shunt capacitances exhibiting relatively high impedances at the frequency employed. For example, capacitances C2, C3 could each be about 20 pico farad; and capacitances C4, C5 each about 0.1 pico farad. The capacitances C2 to C5 together with the inductor L2 form a network tuned to the oscillator frequency. While the tuning is not so sharp that the doppler shifted frequencies are eliminated, this network does serve to filter out extraneous signals of other frequencies, as may arise from adjacent electrical equipment, e.g. motors, or from lightning.

Signals at 400MHZ reflected back to the antenna and shifted on frequency by a moving object, e.g. a shift of the order of 0.1 to 10HZ, will beat with the base frequency to produce the low difference frequency. This low frequency is recovered by demodulating the high frequency, using diodes D1 and D2 connected between points 16, 17 and a point 18, and providing full wave rectification. Since the tank circuit L1, C1 presents a low impedance to the beat frequency, the loose coupling between inductors L1 and L2 is necessary. A capacitor C6 filters out high frequency signals, the beat frequency entering a low frequency amplifier 19. The gain of the amplifier 19 can be varied to determine the range of the device. Typically, with full gain, there has been found to be no difficulty in obtaining a range radius of 20 feet around the device, using an oscillator of 10 milliwatts at low gain this radius can be reduced to the order of about 5 feet. The height of the space effectively covered by the radar field will be typically about half the range.

The output of the amplifier 19 energizes a trigger and timer circuit 20 which causes continuous operation of a horn 21 or other alarm mechanism for a predetermined period, e.g. 2 minutes, whereupon the circuit resets itself. When the device is turned on by the owner using his key in switch 14, his own subsequent movement away from the device will actuate the trigger and timer circuit 20 to sound the horn 21 for 2 minutes. This action will provide a test that the device is functioning properly. After the 2 minutes have elapsed, the device will again become silent, when the trigger and timer circuit 20 resets itself, assuming that the owner has by now moved out of range and that no other moving objects are within range to reenergize the amplifier 19.

When an intruder or other moving object appears, the horn will sound for 2 minutes, which may in itself be sufficient time to achieve the desired result, i.e. to scare away the intruder and/or to alert the householder, superintendent or night watchman. By switching off its horn after 2 minutes, the device again becomes ready to detect a new intrusion. Of course, if any moving intrusion persists, the horn will continue, the 2 minutes duration of the horn being timed not from its initial actuation but from the last energization of the circuit 20 by the amplifier 19.

If desired, as an optional feature, one or more other alarm mechanisms may be connected to the device, e.g. a siren or bell external to the premises, floodlights, or a police switchboard indicator. This other alarm mechanism has been shown diagrammatically in FIG. 2 at 22 and can be actuated by a trigger 23 energized through a delay device 24 from the amplifier 19. A short delay, e.g. 30 seconds or perhaps 1 minute, furnished by the device 24 will enable the owner to approach the alarm device to turn it on and off. Under these conditions he will operate the horn 21, but he will not trigger the other alarm mechanism 22. In the case of turning off, the key switch 14 will be opened, before the delay period has expired; in the case of turning on, the owner will have moved beyond range, so that the amplifier 19 will no longer be ready to energize the trigger 23 when the delay period expires.

The manner of securing each of the antenna arms 11 in the mounting member 12 to achieve simultaneously the necessary structural and electrical characteristics, is best seen from FIGS. 5 and 6. A metal bolt 30 extends through a number of parts and finally into a tapped cavity 31 in the end of the arm 11. In sequence, starting at its head, the threaded shaft of the bolt 30 passes through a dielectric insulating washer 32, having a boss portion 33; a metallic conductor in the form of a lug 34 mounted on an end of the inductor L2 and having a hole 35 sized to receive the boss portion 33; a dielectric insulating washer 36; a metallic washer 37; a nut 38; a dielectric insulating washer 39; the wall of the member 12; a further dielectric insulating washer 40; a second nut 41; and finally the arm 11. The sides of the washers 39, 40 facing the member 12 are recessed to form boss portions 42, 43 that enter the hole in the member 12 and establish a firm structural connection therewith.

The first or series capacitances C2, C3 are each formed between the lug 34 on the one hand, and the bolt 30 and metallic washer 37 on the other hand, these latter parts being in direct metallic connection with the arm 11. These capacitances are comparatively large by virtue of the thinness of the dimension A of the dielectric washer 36, e.g. 4 thousandths of an inch, and the thinness of the radial dimension B of the boss 33 of the dielectric washer 32, also about 4 thousandths.

The second or shunt capacitances C4, C5 are each formed between the nuts 38, 41 and bolt 30 on the one hand, and the grounded mounting member 12 on the other hand. These capacitances are comparatively small by virtue of the much greater values chosen for the dimensions C and D.

An important attribute of the system as described is an absence of elements in the tuning circuits that are dependent on temperature and other variables such as humidity. This feature contributes significantly to the operating stability of the system.

As an alternative to using a bolt 30 with a head, a head-less bolt can be used, threaded at least at both ends. The head will then be replaced by a nut that can be screwed onto one end of the bolt, the other end entering the tapped cavity 31.

Claims

1. An antenna mount comprising a. an antenna in the form of at least one metallic arm, b. a grounded metallic mounting member for supporting said arm, c. an electrical conductor for capacitive coupling to said arm, d. and means mechanically connecting said arm to the mounting member and to the conductor, e. said connecting means comprising i. a metallic bolt having a head or nut at one end and a threaded shaft extending therefrom through a hole in said conductor and then through a hole in said mounting member into a tapped cavity in said arm, ii. a pair of metallic nuts engaging the threaded shaft of the bolt, one on each side of the mounting member, iii. a metallic washer on the bolt between said conductor and the nut nearer the bolt head, iv. first dielectric insulating means located between the bolt and the washer on the one hand, and said conductor on the other hand, to form a first capacitance between the antenna arm and said conductor, v. second dielectric insulating means located between the two nuts and the bolt shaft on the one hand, and the mounting member on the other hand, to form a second capacitance between the antenna arm and the mounting member, vi. said first and second dielectric means being so dimensioned that the

2. An antenna mount as claimed in claim 1, wherein said antenna comprises a pair of metallic arms arranged to form a half wave dipole, and wherein each said arm is mounted on said mounting member by a respective said

3. The combination of an antenna mount according to claim 2, and radar transmission and receiving means comprising f. a first loop inductor having two ends each connected to a respective said electrical conductor, the center of said loop being grounded, g. a second loop inductor loosely inductively coupled with said first inductor, said second inductor having two ends, h. a capacitor connected across the ends of the second inductor to form a tank circuit therewith, and i. a radar frequency oscillator connected to said tank circuit, j. the first and second dielectric means associated with each respective antenna arm being so dimensioned that said first and second capacitances form with the antenna arms and with said first inductor a system tuned to

4. The combination of claim 3, further including k. means connected to said first inductor for detecting a low frequency beat between said radar frequency and a return frequency received by the antenna with doppler shift, and l. means connected to said means (k) for triggering an alarm means upon

5. The combination of claim 4, including timing means for interrupting said

6. The combination of claim 5, including further alarm means and delay means connected thereto and to said means (k) whereby to trigger said further alarm means upon receipt of a signal at said low frequency sustained for a predetermined time interval.