HK1084777B - Am antenna noise reduction - Google Patents

Description

The present invention relates in general to radio antenna noise reducing and more particularly concerns novel apparatus and techniques for reducing interfering noise in the AM band with an AM antenna.

Operation of electronic power controllers, such as a triac light dimmer, can create severe interfering noise in the AM radio band. The interfering noise may enter the radio through any of the mechanisms of capacitive coupling to the antenna, conduction through the AC mains, or magnetic coupling to the antenna. In home use, a major mode is through the AC mains.

Typical antennas for AM radios are external loop or internal loop types, such as ferrite rod loop AM antennas. External loop antennas typically use twisted pairiead-ins connected to a balanced input. Internal ferrite rod loop antennas are typically unbalanced, with one side of the loop at RF ground while the other side is connected to a varactor diode. An unbalanced pickup coil is typically used to drive the detector integrated circuit (IC).

US 3209358 relates to tuning an antenna and more particularly to reducing the size of the antenna.

It is an important object of the invention to provide an AM radio antenna to reduce electrical interference in an AM radio.

According to the invention, there is provided a tunable AM radio antenna in the form of a ferrite bar loop antenna, including a ferrite bar having a resonating structure forming a balanced antenna circuit, wherein said resonating structure has a winding structure, an antenna tuning structure comprising a varactor diode tuning structure presenting a controllable capacitance to said winding structure, a DC path including said winding structure coupled to said varactor diode constructed and arranged to deliver a tuning signal to said varactor diode, wherein said winding structure has two ends, and an intermediate tap coupled to RF ground is constructed and arranged to carry said tuning signal via said DC path; and means for connecting the tunable antenna to the input of an external detector circuit.

Other features, objects and advantages will become apparent from the following description when read in connection with the accompanying drawing in which:

• FIG 1 is a schematic circuit diagram of a center grounded ferrite bar loop antenna according to the invention.

Referring to FIG 1, there is shown a schematic circuit diagram of an embodiment of the invention incorporating a center grounded ferrite bar loop antenna. The circuit includes a ferrite bar 11 having a resonant circuit winding 12 and a pickup winding 13. One end of resonant circuit winding 12 is directly coupled to varactor tuning diode 14, the other end of winding 12 is coupled to varactor diode 14 through low impedance coupling capacitor 15. An intermediate tap 16 of resonant circuit winding 12 is coupled to a reference potential through a low impedance coupling capacitor 17. The reference potential is assumed to be ground for the rest of this disclosure, but it should be noted that the reference can be set to be any desired potential. Intermediate tap 16 also receives a tuning voltage through resistor 18 for controlling the effective capacity of varactor diode 14 to tune the resonant circuitry to the frequency of the desired AM carrier. The junction of varactor diode 14 and low impedance capacitoris connected to ground through resistor 21. Representative parameter values are set forth in FIG 1. A low impedance bypass capacitor 22 couples the end of pickup winding 13 that receives a bias voltage for the detector integrated circuit input to ground. The other end of pickup winding 13 is connected to the input of the detector integrated circuit.

The embodiment of FIG. 1 balances the antenna circuit by placing the RF ground near the center of the resonant circuit winding 12. The intermediate tap 16 is preferably displaced from the physical center of winding 12 to account for the effects of unbalanced pickup coil 13 and the capacitance to the external environment of the conductors attached to the detector integrated circuit input. The position of intermediate tap 16 should be offset from the center of the winding coil and may be experimentally determined for maximum interference reduction. In this example, intermediate tap 16 was located 16 turns from the capacitor end and 31 turns from the varactor end of winding 12 in a 220 microhenry inductance with winding 13 having 24 turns and an inductance of 55 microhenries to provide at least 27dB improvement in line conducted interference rejection.

It is possible to eliminate coil 13 of Fig. 1. In this case, an appropriate intermediate point along coil 12 is located where an RF signal can be tapped off. This point is chosen such that the coil impedance matches the input impedance requirements of the circuitry coupled to this intermediate tap, which would typically be the RF input of the detector IC.

There has been described novel apparatus and techniques for significantly reducing undesired noise entering the antenna circuit of an AM radio. It is evident that those skilled in the art may now make numerous uses and modifications of and departures from the specific apparatus and techniques herein disclosed without departing from the inventive concepts. Consequently, the invention is to be limited solely by the scope of the appended claims.

Claims (8)

1. A tunable AM radio antenna in the form of a ferrite bar loop antenna, including a ferrite bar (11) having a resonating structure forming a balanced antenna circuit, wherein said resonating structure has a winding structure (12), an antenna tuning structure comprising a varactor diode (14) tuning structure presenting a controllable capacitance to said winding structure (12), a DC path including said winding structure coupled to said varactor diode constructed and arranged to deliver a tuning signal to said varactor diode, wherein said winding structure (12) has two ends, and an intermediate tap (16) coupled to RF ground is constructed and arranged to carry said tuning signal via said DC path; and means for connecting the tunable antenna to the input of an external detector circuit.
2. An antenna in accordance with claim 1, wherein said means for connecting the antenna further includes a second winding (13) having two ends with a first end of said second winding constructed to receive an external signal and a second end of said second winding connected to the external detector circuit.
3. An antenna in accordance with claim 1 or claim 2, wherein said intermediate tap (16) is maintained at a reference potential at radio frequencies.
4. An antenna in accordance with any of claims 1 to 3, wherein the location at which said intermediate tap (16) connects to said winding structure (12) is offset from the center of said winding to account for interferences.
5. An antenna in accordance with claim 1, wherein said means for connecting the antenna further includes a structure having a second intermediate tap on said winding (12), said second intermediate tap is connected to the input of said external detector circuit.
6. An antenna in accordance with claim 5, wherein the location at which said second tap connects to said winding (12) is chosen such that the coil impedance of said winding matches the impedance requirement of said external detector circuit.
7. An antenna in accordance with any of claims 1 to 6, wherein the first end of said winding structure (12) is directly coupled to said varactor diode (14) and the second end of said winding structure is coupled to said varactor diode via a capacitor (15).
8. An antenna in accordance with claim 2 and any claim dependent thereon, wherein the external signal is a bias voltage.