Snap-Ins can create RF interference on a multi-use tower.

Discussion in 'Technical Discussions' started by Christopher Mika, Mar 23, 2008.

  1. Christopher Mika

    Christopher Mika Friend of the Community

    This past year we ran into an RF interference problem on one of our towers. An electric utility's low band repeater began to experience so much noise during periods of inclement weather (just when they needed it) that it would become unusable. We had 2 radio technicians on site with almost 60 years of experience between them on site trying to figure out the problem. As the problem sounded like one heck of a loose connection in the antenna to one of the techs, we proceeded to replace the antenna. No help whatsoever. I was given a radio on the repeater and sent up the stick to see if I could locate a loose connection on the antenna or mount that could be causing the RFI. Upon arriving at the location of the antenna, a signal was generated over the repeater that I could listen to as I tried moving various items on the tower. I immediately noticed that the noise was rather severe when the wind gusted (gusts of 30mph+). After physically shaking the tower and hearing the increase in the noise level, I determined that a physical connection was the source of the RFI. It must be noted that the antenna experiencing the RFI was located less than 30' above an FM broadcast antenna, one of two stations on the tower. My attention turned to a 12-line cellular installation that had been installed around the beginning of the RFI problem. Many of the snap-ins had been improperly installed and had slid down against the T-brackets below them. I could move one of these snap-ins and hear it crackle over the repeater. Several of the T-brackets were loose and kept from sliding down the tower by the snap-ins. After re-installing the snap-ins and tightening down all of the loose T- brackets, the problem was reduced somewhat but still present. I wiggled a couple of the lines and noticed a corresponding increase in noise. The snap-ins that were used were the snap-stack type. They have almost locking "hooks" on the tabs you insert into the bracket holes that make it almost impossible for the hanger to pull out. The only problem is that they fit a little loose in the hole, and are able to jiggle about a bit. This jiggle was the culprit of the RFI. We made a temporary fix by securing the feedlines to each other with super 88, which greatly reduced their capacity to jiggle the snap-ins. We are going to install mechanical hangers on the cellular installation to permanently fix the issue. The broadcast FM antennas were turned down to a lower power during the investigation.

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