This is a follow up to my original post.
The tower crew showed up yesterday to swap out the antenna. Once at the top of the water tank, we had them connect a dummy load to the top of the feedline and discovered that the feedline itself had a poor VSWR. They disassembled the top connecter, which showed evidence of corrosion but not water intrusion (at least not recently anyhow, as the foam in the hardline was not discolored and it was totally dry). They installed a new connector and the VSWR test was repeated with the same poor results. At this point, we tested the feedline with an ohmmeter (already had tested it with a Megger and a TDR. both with good results) and discovered that we had about 100k between the center conductor and the shield. Needless to say. we were concerned. Someone decided to pull the hardline out of the shack - I think the idea was to remove the bottom 35 feet, which had been spliced on using a hardline splice) to test the two sections individually. As we uncoiled it from where it passed behind the shack, next to the chainlink fence, the problem became quite apparent. Staring me directly in the face, was a spot on the hardline which was blown out by a lightning strike. I cut out about an 8 inch section around the blow-out and measured it with an ohmmeter - 100k! The two remaining halves from either side of the cut measured completely open. While all this was going on, the old antenna was brought down and the new antenna installed. The old antenna showed minor evidence of a lighting strike. VERY small pieces of metal melted from around the rivets at the top and the pointy tip of the antenna was somewhat melted away. You had to look pretty closely to see what had happened. We cut back the feedline about 6 inches from where it had been spliced (it looked surprisingly pristine) an spliced in some new stuff to get back to the shack. VSWR check confirms 30 watts forward, 0 reflected. I can already hear the purists saying that we needed to change out the entire run of hardline, but please don't remind me of what I already know. We did the best we could with what we had scraped together out of the pockets of the club members and have a working machine again. Additional protection and improvements to the ground system are forthcoming. We also have a case to take to the insurance company for some possible reimbursement. (They're already replaced 2 machines that got zapped through the phone line. but that was before my time in the club.) Bottom line: Almost all the advice I got here was 100% on-the-mark. Thanks to all who contributed. and please don't shoot me for not replacing the hardline. I don't get to make those decisions! 73 es tnx, Mike WM4B _____ From: Mike Besemer (WM4B) [mailto:[EMAIL PROTECTED] Sent: Saturday, May 26, 2007 10:02 AM To: [email protected] Cc: [EMAIL PROTECTED] Subject: VSWR Issues Greetings all, My club repeater system consists of a KRP-5000 feeding (what we think is) a DB-224 mounted on a watertower at about 140'. The feedline is about 170 feet of one-inch helix. We'd been getting some comments from users about decreased coverage, but had attributed most of them to the heavy foliage we're seeing now (we have the same issue every spring), but nonetheless last night the two of us on the technical committee went to the site to do some checking. (As a sidebar, my background is 24 years USAF as an avionics tech and my buddy works as a radio/radar tech for the FAA.) At the site, a quick power check revealed the problem. 9 watts forward, 3 watts back (checked from both sides of the cans). The transmitter output should be 30 watts. Checked the transmitter into a Comm Systems Analyzer and verified 8.6 watts from the transmitter. So. we know that the transmitter is toasted, no doubt from excessive reflected power. The next step was to put a TDR on the line. Everything appeared to be okay up to the antenna, but the termination point (antenna) looked pretty ragged. What does a DB-224 (or similar) antenna look like on a TDR? (We tried to print the TDR image so I could share it, but the printer was not cooperating.) I need to add that we DO NOT have access to the water tower. Previous club leadership pulled some fast ones on the county and we are forbidden from climbing the tower. Whatever needs to be done, we must use the same crew the county uses and pay all costs. Also, we're the only ones on the tower, so there is no routine maintenance done on the tower that we can piggy-back on. Bottom line. we need to figure out what we need to do and have everything in place if we need to hire a crew to come out and do the work for us. If I had access to the tower, I'd terminate the top of the hardline with a 50 ohm load and recheck the reflected power to verify that the feedline is good. I'd also check power at the top to see how much loss we had. (The antenna has been up there for more years than anyone can remember. we believe that the last time it was inspected was 1994.) The hardline itself looks very good, and we inspected the antenna as best we could from ground-level with a sighting scope and could not see anything obviously mucked up. I'm guessing that the feedline is okay but that the antenna is going to need either overhauled or replaced. and since we're going to have to pay for the rigging crew, replacing the antenna would probably be the cheapest route. Thoughts, suggestions? Did we miss anything obvious? Suggestions for a new antenna that will last as long as this one did? I hate to go to the membership with a 'we think this is what is wrong and we need to spend money', but without access to the tower we're pretty much hamstrung. Anybody got a TDR image of a DB-224 (or similar) that we can use for comparison? Thanks in advance, Mike WM4B
