How to Reduce VSWR of Coaxial Connectors

Reducing VSWR of RF connectors is a consecutive processing that must be carried out throughout the entire process of product formation, that is, from the design, production, assembly, and testing and inspection of the product. Careful design is the structural foundation for low VSWR, and qualified processing and assembly are the guarantee for achieving structural design. Meanwhile, with aid of qualified testing equipment, we can see if such it works. This article will introduce a comprehensive measures to reduce VSWR.

1 Theoretical Design

The theoretical design determines the product structure, and the product structure determines the VSWR performance of the product. Whether the product structure can meet the low VSWR requirements depends on the product's structural design.

1.1. Scheme

Choose a proper scheme from varied structural schemes. It shall

theoretically considered to be low VSWR. In other words, the factors affecting VSWR are eliminated as much as possible

1.2. Structural Design

Significant advances in the performance of precision coaxial standards and components have resulted from rigid adherence to three basic design principles: 1) incremental constancy of characteristic impedance, 2) coplanar compensation of discontinuities, and 3) control of mechanical tolerance sensitivity.

1)One principal is the conductor resistance and contact resistance are maintained as small as possible on the RF current channel.

Once product structure was determined, the conductor resistance and contact resistance are the key factors affecting the electrical performance. 

Conductor materials and plating with the smallest possible conductivity should be employed;

2)the fewer contacts on the current channel, the better.

3)When contacts are inevitable, the contact should be expanded as much as possible, therefore contact pressure increased and maintain smooth surface.

and the surface finish should be improved:

4)Steps should be reduced as much as possible. If it has to be, the smaller the step, the better, give the condition that mechanical properties can be fulfilled.

1.3. Compensation Design

Compensation design when capacitance discontinuity happens. Carry out calculation under relevant formulas. Verify the key dimension to meet with standard interface size dimension.

1.4.Machining Dimension Tolerance

For the key dimensions that affect VSWR, refer to relevant tolerances of the standard connectors.

Considering milling precision and operator capability, select a feasible dimensional tolerances that is suitable for mass production dimension. At same time, reduce VSWR as much as you can.

1.5. Select Applicable Materials and Coatings

In order to ensure low contact resistance, good conductivity, and low loss , materials of inner and outer conductor and surface plating with low conductivity and good elasticity is the right choice;

As to insulator, dielectric with stable performance at high frequency and small linear expansivity, such as polytetrafluoroethylene or PTFE in short.

2.1. Case Study

check the structural parameters and try to find the reasons resulting the deviation.

For those with unexpected VSWR, check compliance of each components in terms of dimensions , assemble sequence, and quantity etc. Typical deviations are found:

a. deviation of the size of a certain part leads to dimension nonconformity, furthermore mechanical and electrical reference plane is affected, such as improper pin, socket, plane distance,  shrinkage, etc.;

b. the insulation support is inverted, and the part size is out of tolerance;

c. disordered parts,wrong sequence , and the missing or excessive parts

d. wrong usage of similar parts, guidance etc. :

e. cable stripping size is out of dimension.

2.2.Possible Mistakes Spotting

The RF connector can be deemed as a section of non-uniform coaxial line with a connection structure and a cable bonding device. Taking the straight connector as an example, compared with the uniform coaxial line, the insulation support structure area, the conductor diameter  transition and the termination to cable often is observed with obvious impedance uniformity. Common out-of-tolerance mistakes include: 

Insulation support area:

a. The insulation support size is beyond tolerance, the coplanar compensation groove depth is excessive; an obvious air gap between the metal conductor and the dielectric contact surface; the support parts are of uneven thickness, causing the interface size out of tolerance, deviation etc.;

b. residual in cavity affects the position and size of the parts;