You'll need a tuneable RF signal generator and several common attenuator pads to operate the return loss bridge.

A Dual-Pole, Dual-Throw (DPDT) power switch will be used to turn both the LCD meter and the return loss bridge on and off. Also, the PM-128 LCD meter requires a separate 9 volt battery power supply, as the PM-128 needs to be isolated from the voltage source it is monitoring. Try to use 1% metal-film resistors if you can. Resistor R A should be changed to 300 kohms and resistor R B should be changed to 2.7 megaohms. The modification involves adding two divider resistors, R A and R B. In its stock configuration, it can only display up to 200 millivolts. The PM-128 is a very common display which is both low-cost (under $10) and easily available from places like JDR Microdevices (The PM-128 LCD meter will need to be slightly modified to display a maximum of 2 volts. This signal is then fed to a common PM-128 LCD voltage meter.

The AD8307 will output a DC voltage level between around 200 millivolts and 1.5 volts. The RF input will be split between these two ports using a simple resistive divider network, and any 'unbalance' in the divider (when compared to 50 ohms) will be detected, amplified, and output by the AD8307. The AD8307 is a perfect choice for this project, as the AD8307's +INPUT and -INPUT differential inputs will do the majority of the work for us. It is designed to work from DC to over 500 MHz, and has a resolution of 92 dB. This particular return loss bridge will be based around an Analog Devices AD8307 logarithmic amplifier.

The bridges provide DC isolation to prevent undesired loading of the circuitry by associated test equipment. These devices develop a DC potential with respect to ground which is proportional to the degree of unbalance in the arms of the bridge circuit. Return loss (reflection coefficient) bridges are broadband RF comparators. The common Standing Wave Ratio (SWR) meter which is normally used to verify an antenna system has a very narrow RF bandwidth, usually only accurately covering 50 MHz or so at a time. The version documented here should be accurate from below 1 MHz to over 500 MHz. Return loss bridges are ideal for checking antenna systems as they are simple to build and have a very large RF bandwidth. Generally, the higher the DC voltage output, the worst the impedance mismatch is. A DC voltage signal is generated which corresponds to the level of impedance mismatch between the 'known' and 'unknown' impedances. It works by comparing an 'unknown' impedance to a 'known' impedance, which is usually 50 ohms in radio work. A return loss bridge is a wideband resistive bridge network which can be used to verify the impedance of coaxial cables, antennas, tuning stubs, filters, etc.

Return loss bridge rlb 251