Automatic and Manual Distance to Fault Measurements
The test set provides two ways of measuring distance to fault:
Automatic Frequency Range. You select the measurement distance and the test set automatically selects the Start Frequency and the Stop Frequency. The measurement distance is set using the [Start Distance] and the [Stop Distance] menu keys on the [Freq/Dist/Calibrate] menu. In this mode, the displayed and measured distances are the same. There are always 256 measurement points across the distance you set, so adjusting the distance settings allows you to display the maximum resolution for the portion of the cable you are testing. The disadvantage is that the start and stop frequencies are automatically set and may limit the test set's ability to sweep through filters or lightning protectors. This mode is best used for checking a cable that has no frequency limiting devices.
Manual Frequency Range. When set to Manual, you must specify the Start Frequency and the Stop Frequency, and the measured distance is worked out from these frequencies. Generally, the typical start and stop frequencies you use will result in a measured distance that will be larger than the distance over which you want to look for faults.
- Example 1: If you set [Start Distance] to 0 m (0 ft) and the [Stop Distance] to 60 m (197 ft), and you specify 256 Data Points, the instrument automatically selects a [Start Freq] of 375 MHz and a [Stop Freq] of 659.8 MHz.
- Example 2: If you again set [Start Distance] to 0 m (0 ft) and the [Stop Distance] to 60 m (197 ft), and but this time you specify 1024 Data Points to give you greater resolution, the instrument automatically selects a [Start Freq] of 375 MHz and a [Stop Freq] of 1.5142 GHz.
NOTE The Measured Distance and the Displayed Distance can be different. The distance over which the instrument has made its measurements, and which has been derived from the frequencies you specified, is called the Measured Distance. This is displayed at the top right corner of the measurement screen.
The Displayed Distance refers to that part of the entire Measured Distance that you choose to display on your measurement screen. You set the Displayed Distance manually by pressing the [Start Distance] and the [Stop Distance] menu keys on the [Freq/Dist/Calibrate] menu.
- To help isolate faults over the length of interest, you can set a displayed distance less than the measured distance. The displayed distance is set using the [Start Distance] and the [Stop Distance] menu keys on the [Freq/Dist/Calibrate] menu.
- Keep in mind that there are 256, 512, or 1024 measurement points across the measured distance. The exact number of measurement points is set using the [Data Points] key on the [Setup] menu. Therefore, the measurement points across the chosen displayed distance will be a ratio of displayed distance to measured distance times the number of points you have specified. The higher the number of data points, the greater the measurement resolution.
- In most cases, the default resolution using 256 data points will be adequate to locate the faults, but if more resolution is needed you can increase the span between the start and stop frequencies (which will decrease the measured distance) or use the other approach, automatic frequency range. If the measurement distance is not long enough for the cable you are testing, reduce the span between the start and stop frequencies (which will increase the measurement distance) or use automatic frequency range.
NOTE: When testing cables attached to antennas, test signals are radiated from the test antenna. Verify that the signal used for the test, and therefore being radiated from the test antenna, cannot interfere with other radiated signals from other antennas.