Measuring loss on coax

Measurement of loss on coax is probably misunderstood by many but is basically simple.
Let’s take a 100 foot length of RG-8X as an example. In the first case let’s assume that it is simply open at the far end. No connection at all. This is an “open circuit” condition. 
If we apply a very small radio frequency voltage at the near or input end this voltage will travel down the coax at somewhat less than the speed of light. It could actually be 80% or 90% of the speed of light. When this signal reaches the far or open end of the coax it will be reflected. All of it will be reflected back towards the input end.
If we have a reasonably good instrument it can measure both the magnitude of input voltage we started with and the magnitude of the voltage that has returned after being reflected.
The magnitudes of these two voltages gives us a lot of information. 
First of all we can calculate the standing wave ratio that exists at the input to the coax. We can also calculate the loss in the cable at the frequency of the applied voltage. I say we can calculate these things but most of the time the instrument does the calculations and you just read the answer on the display!
What actually happens when we apply a RF voltage or RF power at the input is that as the signal travels to the far end it is attenuated due to loss in the cable and the forward voltage that is applied by the instrument will be somewhat less after traveling 100 feet in the coax. In our case of an open ended piece of coax however much that turns out to be will all be reflected. This reflected voltage or wave will again be attenuated on the way back to the input end and when it arrives there the instrument will measure a significantly lower voltage than we started with. When doing any calculations we need to be careful about what voltages we use. 
SWR is defined as being the ratio of maximum to minimum voltage on the line and that ratio changes depending on where you measure the forward and reflected voltage. 
SWR is simply the sum of the forward and reflected voltages divided by the difference in those two voltages. If we do our calculation at the input, using voltages measured there, we will end up with a lower SWR than if we do our calculation using the voltages measures at the far end of the coax. The reason for this is simply because the losses in the coax reduce the voltages as they travel down the line.  
As far as coax loss is concerned, by simply knowing the starting forward voltage and the final reflected voltage at the input end we can calculate the total loss of power in the wave as it travels out to the open end and back to the input. This will be the loss for two 100 foot trips so we need to divide that total loss by 2 to get the one way loss for our 100 foot of coax. 
Of course by knowing the Voltage and the fact that we are using 50 ohm coax lets us easily calculate the currents that correspond to the voltages we measure. As soon as we know the voltage and current we can calculate the power. Once we know the power we can easily convert the power ratio into dB. Once we have the total loss in dB we divide by 2 and get the loss for our 100 foot length of coax. Depending on the instrument and exactly how you have it set you may or may not have to device by two. The MFJ 259B will give coax loss directly but if measuring Return Loss you have to divide by 2.
If you have a MFJ 259B or most any other antenna analyzer you can simply connect your coax to the analyzer and read the answer directly on the display, but now you know what’s behind the answer!