Microstrip Directional Coupler Phase Velocity Mismatch
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Solution Overview
Problem
Conventional microstrip directional couplers suffer from poor directivity and large size, leading to noise in power measurements and reduced dynamic range, especially at frequencies below 1 GHz, due to significant coupling variations and the presence of undesired power at isolated ports.
Innovation Solution
A microstrip directional coupler design featuring microstrips with lengths less than one sixteenth of a wavelength and a gap between them, optimized to reduce phase velocity differences between even and odd modes, achieving coupling and directivity exceeding 25 dB without additional circuit elements, thereby enhancing frequency flatness and reducing size.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional quarter wavelength microstrip directional coupler is used, then coupling is achieved, but directivity is poor (7 to 15 dB) and size is large
Solution Approach 1:
The patent changes the operating parameters by using lengths less than one sixteenth of a wavelength (rather than quarter wavelength) and optimizing the gap between microstrips to less than a predetermined amount. This parameter change enables high directivity (exceeding 20 dB) while maintaining a compact size, resolving the contradiction between measurement precision and device length.
2Measurement precision
If microstrip directional coupler length is reduced to improve directivity, then directivity improves, but coupling varies significantly over frequency
Solution Approach 1:
The patent optimizes multiple parameters simultaneously: microstrip length (less than 1/16 wavelength), gap dimension (less than predetermined amount), and microstrip width. This multi-parameter optimization achieves both high directivity and flat coupling characteristics over frequency, resolving the contradiction between directivity improvement and frequency stability.
3Measurement precision
If compensating elements are added to improve directivity, then directivity improves, but device complexity and cost increase
Solution Approach 1:
The patent extracts and eliminates the compensating elements (inductors, capacitors, resistors) from the directional coupler design. By using optimized microstrip geometry alone (length < 1/16 wavelength, controlled gap), the design achieves high directivity without additional circuit elements, thereby reducing device complexity and cost while maintaining measurement precision.
4Area of stationary object
If conventional microstrip directional coupler is used, then compact size is achieved, but undesired power at isolated port creates noise and reduces dynamic range
Solution Approach 1:
The patent changes critical geometric parameters: microstrip length (less than 1/16 wavelength) and gap dimension (less than predetermined amount). These parameter changes achieve high directivity that prevents undesired power at the isolated port, eliminating noise and improving dynamic range while maintaining compact size suitable for printed circuit board integration.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The design achieves high directivity and frequency stability, preventing energy leakage to isolated ports, with coupling variance less than 0.5 dB over a wide frequency range, and eliminates the need for compensating elements, making it more cost-effective and compact.
Implementation Method 1
The first and second microstrips are positioned to exhibit a gap between the first portion and the second portion... achieving coupling and directivity exceeding 25 dB
Implementation Method 2
The electromagnetic fields of the microstrip directional coupler exist in the dielectric and in the air... The gap is less than a predetermined amount to reduce a difference in phase velocity of even and odd modes
Data Source
AI summary
Microstrip directional couplers and methods of their design are disclosed. According to one aspect, a microstrip directional coupler has a substrate of a first thickness. Disposed upon the substrate is a first microstrip having a first portion of a first length and a second microstrip having a second portion of a second length. The first and second microstrips are positioned to exhibit a gap between the first portion and the second portion. The first and second lengths are less than one sixteenth of a wavelength at the lowest frequency of operation of the directional coupler. The gap is less than a predetermined amount to reduce a difference in phase velocity of even and odd modes of the microstrip directional coupler.