Low PIM RF Safety Signs

As stated in previous posts, loosely touching metal surfaces above, below, and behind base station antennas are a common source of external Passive intermodulation (PIM) at cell sites.  While the metal brackets used to support cables are the most common source of PIM found near antennas, other external PIM sources are also present.  One such source is radio frequency (RF) safety signs.

RF safety signs are required in accessible areas near antennas to identify locations where the RF energy level might exceed the FCC’s Maximum Permissible Exposure (MPE). Safety warning signs are crucial on rooftop sites where many types of workers with different levels of RF awareness have access to the roof.  Wireless operators typically install RF safety signs during construction and send crews to inspect the signs at least once per year.

A convenient, highly visible location to mount RF safety signs is the mounting pipe directly behind base station antennas. The signs are often made of metal and are secured to the pipe using steel bailing wire passing through small holes in the sign’s corners.  While inexpensive and straightforward, this mounting method generates a huge PIM problem due to the loose metal-to-metal contacts between the sign, the wire, and the pipe.

Teams performing PIM Hygiene at rooftop cell sites often fix the PIM generated by RF safety signs by moving the signs farther away from the antenna. Alternatively, removing the bailing wire, insulating the pipe with vinyl tape, and re-securing the sign using double-sided tape can help lower PIM. While effective, the fix is often short-lived. Teams performing the annual inspections often re-install the signs in their original position using, you guessed it, bailing wire!

ConcealFab has developed a secure, low PIM method to mount RF safety signs at cell sites to solve this problem. The solution involves drilling or punching two 0.375-inch diameter holes in the sign and attaching Low PIM plastic supports using plastic bolts. The sign is then secured to the antenna mounting pipe using weather-resistant Acetal straps. The plastic straps and plastic support hardware insulate the sign from the pipe, eliminating all metal-to-metal contact. Tests conducted by ConcealFab show that this new, low PIM support system reduces PIM by 60 dB compared to the standard bailing wire attachment method!

ConcealFab offers customers two different ways to purchase RF safety sign support hardware. The first option, PN 901034, includes everything needed to support one RF safety sign, including two Acetal straps.  The second option, PN 901031-10, is for customers who already have Acetal straps and only need the plastic support hardware.  This kit includes ten plastic supports and ten plastic attachment bolts; enough hardware to secure five RF safety signs.

For additional information, please reference ConcealFab’s PIM Shield RF Safety Sign Mounting application note.