In simple terms, the return loss bridge is a device used to measure RF power reflected from a load or device under test (DUT) when power is sourced to the device through the RLB. The return loss bridge has 3 ports. An RF signal generator is used to generate a signal into the input port. The output port provides an RF signal to go to a spectrum analyzer or other device used to measure this signal. The DUT port connects to the device that you are testing (antenna, coax, etc.). A signal is generated into the input port. The RLB splits this input signal between the DUT and to an internal 50-Ohm reference. The DUT will reflect a portion of the signal that it receives (due to impedance mismatch) back to the DUT port. This RF signal appears at the output port. The closer your DUT impedance is to the RLB's internal reference impedance (50 Ohms), the lesser the signal will be at the output port. A perfect 50-Ohm DUT will have no reflected power so a perfect RLB will have no output signal at its output port (infinite return loss). If a non-50-Ohm load is at the DUT port, there will be an RF signal at the output port. The further from 50 Ohms that the DUT is, the more signal will occur at the output port. The extreme cases are for the DUT to be a short or an open connection. This will cause maximum reflected power and, therefore, maximum signal at the RLB output port. Return loss is the difference amount in dB of the reflected signal of your DUT compared to the worst case reflected signal. The further you go from "worse case" the better. The higher the return loss number, the closer the DUT is to 50 ohms. This is somewhat the opposite when compaired to SWR ( high SWR means high reflected power or poor DUT) For example: with an open or short connected to the DUT port and an RF signal source generating into the RLB input port, let's say we measure the RF power at the RLB output port to be -22 dBm. If the open or short were replaced with your load at the DUT port, the amount of power at the RLB's output port would then change. Let's say that the RLB output signal power is now measured to be -42 dBm. This would be a return loss of 20 dB. The amount of reflected power is 20 dB less than the worse case of all your power reflected. This is equal to an SWR of 1.22:1. If this were an antenna, it would be considered an excellent match with about 1% of the power to the antenna reflected. A return loss of 10 dB would correspond to an SWR of 1.91:1 with 10% of your power reflected. This is generally considered a poor match. Most commercial systems want to see a return loss of 18 dB or better.