How Do You Test Coax Cable Assembly Characteristics Using a VNA and Smith Chart?

[dnyberg2]

I need to test a coax cable assy for loss, resistance (AC&DC?) capacitance and inductance? Using a VNA in smith chart mode, how is it done? Do I sweep the coax (S11) with the other end open, shorted and then terminated? I know sweeping S21 will give me the loss @ freq of interest. Its the capacitance and inductance I'm not sure about.

 

[the_emi_guy]

How long is your cable assembly, and at what frequencies do you need L and C?

 

[yungman]

For loss, assuming you know the characteristic impedance, you use the VNA and plot the S21 in rectangular form with the correct termination. You see the signal attenuate with frequency. From that, you find the attenuation constant $\alpha\;$ and back calculate.

 

[dnyberg2]

There are 5 diff lengths 3.5, 5.5, 6.5, 9 & 12" all at 49 MHz.

 

[yungman]

Are you asking about the inductance or capacitance at the input of the coax when the end is open or shorted? That's totally different from the inherent L and C. The characteristic L and C is governed by

$$Z_0=\sqrt{\frac L C}$$

Where Z0 is the characteristic impedance, L and C is the inductance and capacitance per unit length.

If you are asking whether the input is inductive or capacitive of the coax at different length at 49MHz when the end is open, then it's a different story. For 49MHz, let's assume εr=4, this mean the speed of the EM wave propagates through the coax is about half of light which is 1.5EE8 m/sec. For 49MHz, the wavelength λ≈118"

Your longest coax is 12", it is only 1/10 λ. This is way smaller than λ/4. The input is inductance if the end is open. You can find the impedance by using the Smith Chart and then use 49MHz to back calculate the inductance.

The number I used is assume εr=4, you have to do the calculation with the number for your coax. The speed of the EM propagation is

$$V_p= \frac 1 {\sqrt{ε_0 ε_r μ}} \; , \; λ= \frac {V_p}{49\times 10^6}$$