How to test a coaxial cable

Power loss in relation to standing waves

When performing a test, it is essential to first consider the difference in attenuation between the cables being tested.

Cable attenuation must also be taken into account.

It is well known that a cable with very low insertion loss typically has a higher RL, because the reflected signal undergoes the same attenuation as the direct signal.

Example: Suppose we have two transmission lines with the same impedance mismatch, resulting in a RL of 12 dB. The first has an insertion loss of 5 dB, the second of 10 dB. In the first case, we will measure an RL of 17 dB (RL-Att) at the transmitter, and in the second case, 22 dB. The losses caused by return loss are the same, only we see them "masked." The same applies to the intrinsic standing waves of the cable itself; the higher the insertion loss, the lower the SWR.

As an example of a VSWR of 1.2 (about -20 dB),

The Return Loss (RL) expressed in decibels represents the logarithmic ratio between the incident power (Pinc) and the reflected power ().

The formula for determining the linear ratio of the reflected power is:

Step 2: Calculate the linear ratio

Substituting the value of 20 dB into the formula:

Step 3: Convert to a percentage.

To obtain the percentage loss, we multiply the linear ratio by 100:   0.01 x 100 = 1%

This means that 99% of the power is transmitted to the load, while 1% returns to the source due to impedance mismatch.

Simpler example:

100W power, I have an RL of -20 dB, so part of the signal returns attenuated by -20 dB.

Att/10 = 20/10 = 2

 10² = 100

P/100 = 100/100 = 1W = 1%

with -10 dB attenuation

10/10 = 1

10¹=10

100/10 = 10W (lost) = 10%