Loop antenna parasitics reduction technique

• US 6,359,594 B1
• Filed: 12/01/1999
• Issued: 03/19/2002
• Est. Priority Date: 12/01/1999
• Status: Expired due to Fees

• Current Assignee: Logitech Europe S A (Logitech International SA)
• Original Assignee: Logitech Europe S A (Logitech International SA)
• Inventors: Junod, Philippe
• Primary Examiner(s): Wong, Don
• Assistant Examiner(s): TRAN, CHUC
• Application Number:
  US09/452,567
• Time in Patent Office: 839 Days
• Field of Search: 343/741, 343/743, 343/744, 343/745, 343/748, 343/859, 343/866, 343/702, 343/718, 343/713, 343/821
• US Class Current: 343/744
• CPC Class Codes: H01Q 7/005 with variable reactance for tuning the antenna

Abstract

An antenna circuit and matching technique that cancels the inductive reactance of an antenna and thereby reduces the reactive voltage of the antenna are provided. Serial tuning capacitors are inserted along the conductor of the loop antenna as often as necessary to achieve a negligible instantaneous level of reactance on the antenna. The loop antenna is broken up into loop segments, where each segment may or may not have a serial capacitor depending on the desired performance criteria. Each capacitor is selected so as to have a reactance that effectively cancels the inductive reactance of a portion of the loop segment preceding the corresponding serial capacitor. The advantage is that the instantaneous level of reactance on antenna stays nulled, and thus any reactive voltage difference between loop segments remains negligible, even with high current flowing inside the antenna. Parasitics such as ohmic losses, internal capacitive loss and capacitive loss to the external world are all reduced. Moreover, the selected serial tuning capacitors are placed along the antenna wire to effect an average reactive voltage of substantially 0 volts across the antenna. The antenna is thus balanced about GND. Principles of reciprocity regarding passive antennas apply, so both transmitting and receiving antenna configurations are applicable.

Claims

1. A method for optimizing the performance of a loop antenna that is functioning at its operating frequency, the loop antenna having an inductive reactance, and a conductor having a first loop segment and a second loop segment, and where a total capacitive reactance needed to match the inductive reactance of the loop antenna is predetermined, the method comprising:

distributing a portion of the total capacitive reactance serially between the first loop segment and the second loop segment of the conductor of the loop antenna, thereby leaving a remaining portion of the total capacitive reactance; and

distributing the remaining portion of the capacitive reactance across the conductor of the loop antenna.

2. The method of claim 1 wherein distributing the portion of the total capacitive reactance serially between the loop segments of the antenna further comprises:

selecting a capacitor that provides the portion of the total capacitive reactance;

determining a position of the capacitor on the conductor; and

connecting the capacitor at the determined position.

3. The methods of claim 2 wherein determining the position of the capacitor on the comductor further comprises:

placing the capacitor along the conductor of the antenna at a distance from one end of the conductor, the distance determined by the formula

 

x=[1−

(w²*L_a*C_x)/2]*L,

where x is the distance, L is a total length of the conductor, w is 2*PIE*operating frequency, L_a is a inductor value associated with the conductor, and C_x is a value of the capacitor.

4. The method of claim 1 wherein the conductor of the loop antenna has a reactive voltage across it, and the distributing steps further comprise:

balancing the loop antenna about ground such that an average reactive voltage across the antenna is substantially zero volts.

5. The method of claim 4 wherein balancing the antenna about ground comprises:

providing a polarity change that causes substantially one half of the reactive voltage across the conductor of the loop antenna to be positive, and substantially one half of the reactive voltage across the conductor of the loop antenna to be negative.

6. A method for optimizing the performance of a loop antenna that is functioning at its operating frequency, the loop antenna having an inductive reactance, and a conductor having a first loop segment and a second loop segment, each loop segment having an inner end and an outer end, where a total capacitive reactance needed to match the inductive reactance of the loop antenna is predetermined, the method comprising:

distributing a portion of the total capacitive reactance serially between the inner end of the first loop segment and the inner end of the second loop segment, thereby leaving a remaining portion of the total capacitive reactance, wherein the reactance of the portion between the loop segments is substantially equal to one half of the inductive reactance;

dividing the remaining portion of the capacitive reactance into a first sub-portion, a second sub-portion, and a third sub-portion, wherein the reactance of the third sub-portion is substantially equal to one quarter of the inductive reactance;

connecting the second sub-portion serially along the outer end of the first loop segment of the conductor;

connecting the third sub-portion serially along the outer end of the second loop segment of the conductor; and

connecting the first sub-portion across the serial connection of the second sub-portion, the first loop segment, the portion of the capacitive reactance between the inner ends of the first and second loop segments, the second loop segment, and the third sub-portion.

7. A method for optimizing the performance of a loop antenna having a first loop turn, and second loop turn that is adjacent to the first loop turn, the method comprising:

adjusting a reactive voltage at a point of the first loop turn to match a reactive voltage at a corresponding adjacent point of the second loop turn so that a reactive voltage difference between the two points is substantially zero.

8. The method of claim 7 wherein adjusting the reactive voltage further comprises:

providing a polarity charge between the first and second loop turns so that the first loop turn has a starting voltage that is substantially equal to the starting voltage of the second loop turn.

9. The method of claim 7, wherein the first loop turn and the second loop turn have substantially similar lengths, the adjusting comprising:

adjusting a plurality of reactive voltages, each one associated with a point along the length of the first loop turn so that each reactive voltage of the first loop turn is substantially equal to a reactive voltage associated with a corresponding adjacent point along the second loop turn resulting in a reactive voltage difference between the corresponding points of the first and second loop turns of substantially zero.

10. A method for optimizing the performance of a loop antenna that is functioning at its operating frequency, the loop antenna having and inductive reactance, and a conductor having a first loop segment and a second loop segment, and where a capacitor reactance needed to cancel the inductive reactance of the loop antenna is predetermined, the method comprising:

distributing the capacitive reactance in the form of a first capacitor, a second capacitor, and a third capacitor, where the reactance of the third capacitor is substantially equal to one half of the inductive reactance;

connecting the second capacitor serially along an outer end of the first loop segment of the conductor;

connecting the third capacitor serially along an outer end of the second loop segment of the conductor; and

connecting the first capacitor across the serial connection of the second capacitor, the first loop segment, the second loop segment, and the third capacitor.

11. A loop antenna circuit comprising:

a conductor having a first loop segment and a second segment, each loop segment having an inner end and an outer end, the conductor having an inductive reactance and a reactive voltage across it, the conductor for either receiving or generating radiation information;

a first capacitive reactance connected serially between the inner ends of the first and second loop segments of the conductor, the first capacitive reactance for providing a first reactive voltage that is substantially equal in magnitude to, and substantially 180 degrees out-of-phase with, a first component of the reactive voltage across the conductor thereby leaving a remaining component of the reactive voltage across the conductor; and

a second capacitive reactance connected across the outer ends of the first and second loop segments, the second capacitive reactance for providing a second reactive voltage that is substantially equal in magnitude to, and substantially 180 degrees out-of-phase with, the remaining component of the reactive voltage across the conductor.

12. A loop antenna circuit comprising:

a conductor having a first loop segment and a second loop segment, each loop segment having an inner end and an outer end, the conductor having an inductive reactance and a reactive voltage across it, the conductor for either receiving or generating radiation information;

a first capacitive reactance connected serially between the inner ends of the first and second loop segments, the first capacitive reactance for providing a first reactive voltage that is substantially equal in magnitude to, and substantially 180 degrees out-of-phase with, a first component of the reactive voltage across the conductor thereby leaving a remaining component of the reactive voltage across the conductor;

a second capacitive reactance connected serially along the outer ends of the first loop segment of the conductor;

a third capacitive reactance connected serially along the outer end of the second loop segment of the conductor; and

a fourth capacitive reactance connected across the serial combination of the second capacitive reactance, the first loop segment, the first capacitive reactance, the second loop segment and the third capacitive reactance.

13. The antenna matching circuit of claim 12 wherein the first capacitive reactance is substantially equal to one half of the inductive reactance, and the third capacitive reactance is substantially equal to one quarter of the inductive reactance.

14. A loop antenna circuit comprising:

a conductor having a reactive voltage across it, and a plurality of loop segments, where a first loop segment and a second loop segment are adjacent to each other, the conductor being for either receiving of generating radiation information; and

a capacitive reactance that has a reactive voltage equal to, and 180 degrees out of phase with, a portion of the reactive voltage across the conductor, the capacitive reactance being serially connected between the first loop segment and the second loop segment.

15. The loop antenna circuit of claim 14 wherein the reactive voltage of the capacitive reactance is substantially equal to, and 180 degrees out of phase with, a reactive voltage across either of the first or second loop segments.

16. A method for optimizing the performance of a loop antenna that is functioning at its operating frequency, the loop antenna having an inductive reactance, and a conductor having a plurality loop segments that comprise a length of the conductor, the method comprising:

connecting each one of a number of capacitors serially between a corresponding pair of adjacent loop segments of the plurality of loop segments, each capacitor having a reactive voltage that is substantially equal to a portion of a reactive voltage on the conductor.

17. The method of claim 16 wherein the capacitors have a combined capacitive reactance equal to a portion of a total capacitive reactance needed to cancel the inductive reactance of the loop antenna thereby leaving a remaining portion.

18. The method of claim 17 further comprising:

distributing the remaining portion of the capacitive reactance across the conductor of the loop antenna.

19. The method of claim 17 wherein the conductor has a first end and a second end, and the combined capacitive reactance of the capacitors is substantially equal to one half of the inductive reactance, the method further comprising:

dividing the remaining portion of the capacitive reactance into a first sub-portion, a second sub-portion, and a third sub-portion, wherein the reactance of the third sub-portion is substantially equal to one quarter of the inductive reactance;

connecting the second sub-portion serially along the first end of the conductor;

connecting the third sub-portion serially along the second end of the conductor; and

connecting the first sub-portion across the serial connection of the second sub-portion, the conductor, and the third sub-portion.

20. A method for optimizing the performance of a loop antenna having a conductor having a first loop segment and a second loop segment, each loop segment having an inner end and an outer end, the method comprising:

providing a polarity change between the inner ends of the first and second loop segments.