Overview of theTest Set’s Built-In Tools

Using the RF Tools Program

Swept Return Loss

 

Measurement Description

 

Swept return loss is a measure of reflection characteristics. The

 

purpose of the swept return loss test is to detect problems in the

 

antenna feedline system and the antenna itself. A portion of the

 

incident power will be reflected back to the source from each

 

transmission line fault as well as the antenna. The ratio of the reflected

 

voltages to the incident voltage is called the reflection coefficient. The

 

reflection coefficient is a complex number, meaning it has both

 

magnitude and phase information.

 

The return loss is defined as the magnitude part of the reflection

 

coefficient and is expressed in decibels (dB). Therefore, the return loss

 

is a measure of how large the reflected wave is to the original incident

 

wave. Remember that this measurement is in terms of loss, therefore a

 

large number means that very little signal was reflected back. A loss of

 

0 dB indicates that all of the incident wave is reflected, whereas a

 

return loss of 40 dB, for example, would indicate that very little of the

 

signal is reflected. When a device is frequency swept, a graph similar to

 

Figure 4-14 on page 95 is obtained. This is an example of a radio

 

transmitting at a particular frequency. It can be seen that the return

 

loss at that frequency is a very large number which tapers off to small

 

return losses at all other frequencies.

 

Another way of looking at the same information is the SWR (also

 

known as VSWR: Voltage Standing Wave Ratio). VSWR is stated as a

 

ratio. For example: 1.2:1 or “one point two to one” VSWR. The first

 

number in the ratio is a value between 1 and infinity. 1 indicates that

 

none of the incident wave is reflected. Infinity implies that all of the

 

incident wave is reflected. Therefore, the closer this number is to 1, the

 

better the feedline system performance. The second number in the ratio

 

is always one.

 

Making the Measurement

 

When testing antennas or cables with antennas attached to them, a test

NOTE

 

signal is radiated. Verify that the level and frequency span used for the

 

test cannot result in interference to other nearby systems.To minimize

 

interference when running the program, set the power level at the

 

DUPLEX OUT port to the minimum value needed for good

 

measurement resolution. Set the frequency range carefully.

 

1. Select Swept Return Loss from the Select Test (Main) Menu. The

 

Swept Return Loss Information menu will appear.

92

Chapter 4

C:\Spk\Ref\RefGuideRevE\Tools.fm

Page 90
Image 90
Agilent Technologies 8935 series e6380a manual Swept Return Loss