Return Loss Bridge Basics

1.0 Introduction

Return loss bridges have many useful applications for the two-way radio technician

These bridges are particularly helpful when used with the tracking generator feature of many service monitors.

A return loss bridge may have a built-in reference termination, or it may require an external reference termination. The reference termination determines the system impedance. If the bridge is used in a 75 ohm system, then the reference termination must be 75 ohms. Similarly, a 50 ohm system would require a 50 ohm reference termination.

Some bridges have a built-in RF detector and provide a dc output from the measurement port. The return loss bridge referenced in this column provides an RF output at the measurement port. Now, let's back up a moment and take a closer look at a typical return loss bridge, as shown in Fig 1.1 below. This bridge is designed for use with 50 ohm systems. It has a built-in 50 ohm reference termination. There are three ports labeled as follows:

SOURCE (or RF input Port).
DUT (device under test or Load Port).
REFLECTED (RF output or measurement Port).

Fig 1.1 picture of a typical return loss bridge

2.0 Theory of Operation

Figure 2.1, at the top of the next column, represents a typical return loss bridge and illustrates the signal paths to the various ports. This bridge contains an internal reference and is designed for 50 ohm systems.

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2.0 Theory of Operation-continued

Theoretically, the loss from the source port to the DUT port will be 6 dB, and the loss between the DUT and the reflected port will be 6 dB. This assumes that the DUT port is either open or shorted because in these two conditions all of the power that reaches this port is reflected back. If there is a termination at the DUT some of the power will be absorbed by it and therefor the loss from the DUT to reflected port could be much higher. Now, if a 0 dBM signal is applied to the source port with no load connected to the DUT port(this port being open), the signal level at the reflected port will be 12 dB below the level at the source port, or -12 dBM.

Normally, when the return loss bridge is used to make a measurement, the signal at the reflected port is used to calibrate, or to set, the 0 dB reference level on the instrument used to make the return loss measurements. If you are using a spectrum analyzer with the top of the screen set at -10 dBM then you would adjust the generator output to give -10 dBM at the reflected port. This would be about +3 dBM depending on the exact bridge and connecting cable loss.

3.0 Calibration

Refer to Figure 3.1 on page 2, the calibration is done as follows:

With the DUT port left open, the signal level at the source port is adjusted to the desired level on the analyzer screen-this is usually about +3 dBM if you are going to set analyzer for -10 dBM. Normally, the exact signal level at the bridge input is not critical if a usable output is available at the reflected port.

Return Loss Bridge Basics

3.0 Calibration-continued

Next, adjust the spectrum analyzer display to -10dB at the top of the screen. This will serve as the reference level. In other words this is 0dB return loss. The reason being that with the DUT port open or shorted all of the power must be reflected to the reflected port.

Refer now to Figure 3.1 below. With a precision termination connected to the DUT port, the signal level at the spectrum analyzer has dropped to -40 dB. This indicates that the bridge and termination are in good agreement with each other. The better the match the lower this number will be. The curve may vary somewhat, usually lower than 40 dB, over the frequency range. This is normal as a very small disturbance will greatly effect the return loss reading at these low levels.

The Analyzer and bridge are now calibrated and ready for use

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4.0 Using the Bridge

A major advantage of the return loss bridge method of VSWR is the ability to measure a band of frequencies simultaneously. Figure 4.1 shows the connection.

Figure 4.2 is a typical curve of an antenna. As you can see the exact resonant frequency is easily determined as well as the bandwidth of the antenna at a given VSWR (or return loss).

As shown in the diagram there is other information that is available about the antenna and transmission line. The loss, and line match are also present in this curve.

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