How to choose the best rf test cable(1)?

How to find the best rf test cable among many choices? The following content will explain the mechanical and electrical performance of microwave cables and cable assemblies, and how to choose your ideal rf test cable.

In May 2004, the test engineers made the following overview of the requirements for 50 ohm test cables:

1. Loss: 0.09db/ft@1Ghz for one foot of cable (not including SMA and N-type connector)

2. Minimum bending radius: 0.75’’ (1.5’’ bending diameter)

3. Excellent flexibility (good bending resistance)

4. 25000 cycles of 180 degree bending life

5. Good voltage standing wave ratio

6. Maximum operating frequency: 4GHz

7. High shielding rate

8. Low cost

Next, the customer may ask: "I don't need things that are too difficult to get, because I want to use them right away." However, they did not realize that some parts of the above requirements conflict with each other, and they will be used in different fields of use. Apply different frequencies. Therefore, manufacturers must work hard to make high-performance cables to help us identify which are practical and low loss test cables. After understanding the mechanical properties and radio frequency performance, it is more conducive to help us choose the most suitable cable for the application.

The above will be a good start for our next discussion. With the development of various production technologies that can reduce attenuation without increasing the diameter, cable manufacturers tell us that the lower the loss, the better the cable. Perhaps this may not be completely correct, but we should be aware that some applications do need to achieve loss levels that are difficult to achieve with flexible RG coaxial cables. In addition, to rule out demand issues, many customers seeking high-quality test cables begin to regard low loss as a must.

The cable attenuation gradually increases with the increase in frequency, but the larger the diameter, the smaller the attenuation. Attenuation will also decrease as the propagation velocity (Vp) increases. Many flexible RG coaxial cables use solid polytetrafluoroethylene as the dielectric. With a propagation rate of about 70% (that is, the signal propagation speed is equivalent to 70% in a vacuum), the solid PTFE medium can make the cable stronger and relatively low-cost.

We need to increase the connector loss to achieve balance, and try to use RG coaxial cable to keep the cost low. Only cables like RG393 can meet customers' needs for attenuation.

As a result, some problems were immediately obvious. First of all, cables of this size are very stiff. Secondly, in the case of continuous bending, the bending radius will be limited to 10 times the outer diameter. According to the structure of the braided belt, working frequency and performance requirements, the minimum bending radius can occasionally be reduced to 5 times the outer diameter. In addition, it will permanently change (stretch, wrap) the braided tape and change its performance. 3.9 inches (equivalent to 7.8 inches in diameter) has far exceeded the maximum limit proposed by the customer.

Second, RG393 is not suitable for SMA connectors. It makes the installation process more complicated and may create unacceptable return loss. Finally, the characteristics of RG make it impossible to achieve the best shielding performance, so the supplier reduces the braiding angle (that is, using less braided wire). The less braided wire, the stronger the flexibility, but the bending life will be shortened. Because the braid is easier to move or loosen. Excessive research on RG cable specifications makes us also ignore other requirements.