Paul,
I have a question as to how you are mounting the antenna. If you are not top supporting the antenna and mounting it on top of the tower that would explain why as to you getting noise in your transmit signal. Same goes for DB antennas especially the DB224 being so long and not top supported you will eventually get noise in the signal as well. Mike Mike Mullarkey K7PFJ 6886 Sage Ave Firestone, Co 80504 303-954-9695 Home 303-954-9693 Home Office & Fax 303-718-8052 Cellular _____ From: [email protected] [mailto:[email protected]] On Behalf Of Paul Kelley N1BUG Sent: Monday, May 18, 2009 1:01 PM To: [email protected] Subject: [Repeater-Builder] Sinclair dipole array premature failure (noisy) Several weeks ago I posted about my ongoing battle with "duplex noise" on a 2 meter repeater. I have now found a big piece of the problem (maybe all of it) but I'm a little surprised. I am wondering if others have had similar experiences. Two years ago I put up a new (well... NOS, actually) Sinclair SD2352 antenna (8 dipoles, bidirectional pattern). I had no noise for several months after that, but then it started coming back. By this Spring the repeater had become all but unusable. Recently I took down the Sinclair and installed a temporary antenna. Noise gone! Huh? I subsequently disassembled the Sinclair to check for problems. Every piece of hardware was tight. I found no evidence of water in any of the N connectors on the harness, which I had wrapped with Scotch 23 rubber tape followed by Super 88 vinyl tape. The impedance of the complete array and of each individual dipole was still nominal, as it had been prior to being installed. I have now put one dipole from the array on the tower and it is running absolutely noise free. Moving it around on the tower doesn't have any affect... it is noise free wherever I put it. Lacking any other explanation it would seem something in the array became noisy after a short time. I don't know if it is a problem with one or more of the dipoles or perhaps something in the factory assembled portion of the harness. I have not yet attempted to do a post mortem on the factory harness assemblies. I am wondering if this is a unique experience or if this is a common failure mode in exposed dipole arrays? I don't recall hearing much about such arrays becoming noisy, at least in such a short time. Since these dipoles are 50 ohms, I think it would be easy enough to build two 4-dipole cardioid arrays from it, *if* the problem lies in the harness and not in one or more of the dipoles. I wonder if anyone knows what (if any) gimmick Sinclair used to get such broad SWR bandwidth on these dipoles? The exposed portion of the coax on each dipole is RG-213, 50 ohms... but I'm wondering if they may use some quarter wavelength (or ???) of some other impedance on the part hidden inside the dipole, especially since these things exhibit a clear double dip SWR curve (one dip near the low end of the design range, 138 MHz, and another dip near the upper end, 174 MHz, with a somewhat reactive bump in between). 73, Paul N1BUG
