Distance to PIM
There is a lot of buzz recently about Distance to PIM. There are discussions on blogs, websites, and even on a few sales materials. In a recent OnPoint/Summitek training class, minutes after the instructor began, a student asked, "when will we have Distance to PIM?" This was before he even knew what PIM actually is, or even how a PIM test set works. “What about Distance to PIM” is a common question posed by installers and new technicians alike, especially those whose troubleshooting training is limited to a Site Master. These questions seem to be driving the demand for a distance to PIM feature. Be careful what you wish for.
Although distance to PIM (DTP) is still in the developmental stages, the demand for the feature seems to be increasing. That demand appears to be centered around the ideas that the Distance to PIM feature will reduce the time it takes to PIM test a new installation, make PIM testing easier, and make passing a PIM test easier. DTP is cool, really cool. But, will it actually save time? Or, will it lead to an unnecessary test requirement and an overall decline in the integrity and validity of the test results?
There are potentially great advantages to having a DTP option available, but there are serious doubts that quantifiable time savings is one of them. In theory, when a tech comes across a line that has a static failure of, say, a solid -120 dBc, the DTP would tell the technician where to start troubleshooting. This would be a legitimate use of DTP as a troubleshooting aid. For Site Technicians working for a carrier there is potential for even greater value in DTP. A site technician could use DTP on an existing site to determine if a tower crew needs to be brought in to do repairs or if he could do the repairs himself. It seems pretty cut and dry that DTP will save site technicians time. How can that not have value? But, there is a difference between isolating a PIM source at an existing site and certifying a new installation or upgrade.
On a new installation or site upgrade, given the same solid -120 dBc failure, times savings reaped through the use of DTP would be marginal. Assuming there were no fluctuations in PIM levels during dynamic testing (dynamic testing is the first step in PIM source isolation), DTP could save the technician the time it takes to jump the load to isolate the failure. However, if the technician has to take the time to calculate and input the electrical lengths (which could vary from sector to sector, if not line to line), it is probably a wash, time-wise. In fact, DTP could drastically increase the amount of time required to PIM test a site, and consequently ultimately lead to an overall degradation of the integrity of the results.
PIM is a measure of installation quality. And, PIM testing is a dynamic test. Without dynamic testing, the site certification is invalid; the test results are invalid. The concern is that when DTP becomes a standard feature on PIM test sets, untrained techs and/or tower crews will use the DTP to cheat the dynamic test standard. It is well known that crews cheat the test when they become frustrated and can't isolate a PIM source and conveniently "forget" the dynamic standard when facing deadlines. DTP will give those crews and technicians inclined to cheat justification to skip all that tapping and wiggling. "Nothing showed up on the DTP. The line is good." So much for test validity. So long to quality control.
Once the RF and Performance Engineers know that DTP is available, if they don’t understand that DTP is only a troubleshooting aid, they may require another "mandatory test." That means they will soon require a DTP test on each line, just like it happened with the VSWR Distance to Fault test requirement. Over time, carries, engineers, and even the techs will forget that DTP is a troubleshooting aid, not a test standard. Once that happens, test teams will have to unnecessarily break the connection between the antenna and jumper to run the DTP test. It's never a good idea to break connections unnecessarily during PIM testing. Worse, there would be no real value added by the additional test. By leading to a new test requirement, DTP could ultimately increase the amount of time needed to test a site, for what would amount to be no real reason.
There is little doubt that DTP will be available soon. The demand is too great for test equipment manufacturers to ignore. Unfortunately, the potential misuses of DTP outweigh any time savings that might be gained. And, it is doubtful that DTP will significantly decrease PIM testing time. There is one way to significantly reduce the time it takes to perform and pass a valid PIM test, however.
A well-trained technician and tower crew can dynamically test, troubleshoot, and make necessary repairs on an 18-line site in 2 to 4 hours. We do it regularly. 18 months ago we allowed 6-8 hours for testing a new site. Now, we estimate no more than 4 hours for those crews that have embraced the "new" standard of quality brought on by PIM testing. The best way to reduce testing time is to improve the quality of workmanship and to use well-trained techs.
