Mutual inductance / coupling factor in twisted wire transformer

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SUMMARY

This discussion focuses on the design of UHF quadrature couplers utilizing twisted wire transformers, specifically addressing the estimation of the coupling factor based on wire diameters, separation, and twist. The ideal coupling factor for these transformers is 1, with practical values above 0.7 being acceptable. The conversation highlights the limitations of using equivalent circuits for this estimation, emphasizing the necessity of employing a 3D solution to Maxwell's Equations, particularly through the use of Maxwell3D software. The references provided include academic papers and schematic diagrams relevant to the design process.

PREREQUISITES
  • Understanding of UHF quadrature couplers
  • Familiarity with twisted wire transformer design
  • Knowledge of characteristic impedance calculations
  • Proficiency in using Maxwell3D software for electromagnetic simulations
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  • Research the principles of UHF quadrature couplers and their applications
  • Study the calculations for characteristic impedance in twisted pairs
  • Explore the functionalities of Maxwell3D software for electromagnetic modeling
  • Investigate advanced techniques for estimating coupling factors in transformers
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Electrical engineers, RF designers, and anyone involved in the design and analysis of UHF quadrature couplers and twisted wire transformers will benefit from this discussion.

Swamp Thing
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I am trying to design UHF quadrature couplers from the following references:


[1] 'Broad-Band Twisted-Wire Quadrature Hybrids' , http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=1127996


[2] 'Twisted wire Quadrature Hybrid Directional Couplers' QST Vol 63 January 1978 pages 21-23


[3] https://awrcorp.com/download/kb.aspx?file=VHF_Lump_Coupler.pdf



Schematic diagram here :
http://www.seboldt.net/k0jd/TwPQH.gif
and here:
http://c15_manali.tripod.com/hybrid.htm

This circuit needs a 1:1 transformer that should ideally have a coupling factor of 1. In practice, coupling factors above 0.7 seem to work reasonably well.

My question is about estimating the coupling factor from information on the wire diameters, separation, and twist. Now, the characteristic impedance of the twisted pair can be estimated from equations, e.g.:

http://qucs.sourceforge.net/tech/node93.html


Is there a way to relate the characteristic impecance to the coupling factor, e.g. by using the basic equations on series / parallel coupled inductors with aiding / opposing connections ?
 
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Swamp Thing said:
Is there a way to relate the characteristic impecance to the coupling factor, e.g. by using the basic equations on series / parallel coupled inductors with aiding / opposing connections ?
I don't think so. You can't use equivalent circuits. You need a 3D solution to Maxwell's Equations. Look into the Maxwell3D software tool.
 
Wow, thanks for taking the time to reply to that eight-year old question!

Although I have retired from that design job, it's still good to know the answer.
 

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