Tony, Our club ran some wideband plots years ago with a spectrum analyzer and tracking generator on a VHF db-4001 BP cavity and a VHF WP-641 BPBR duplexer. We were getting killed by the site's local TV station transmitter. Very discouraging results if you expect a BPBR duplexer to provide rejection beyond the tuned target frequencies. The band-pass db-4001 had a repeating pattern up the spectrum letting in several pass peaks, so you have to be careful there as well.
I have a PDF of the test results if anyone is interested. We've had success adding extra BP cavities and a DCI filter on the receiver for high RF ambient sites. Ralph W4XE -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Tony KT9AC Sent: Monday, November 23, 2009 12:36 PM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Has anyone measured out-of-band rejection for a duplexer? Hi Everyone, Without the benefit of a spectrum analyzer, I would like to find out how much rejection of out-of-band signals can be expected from a typical UHF duplexer. Have a MSF5000 on 452 that works fine with the T4084 duplexers (1500 style), but have a lot of VHF data and FM broadcast hash that is trying to make it in (the 45kW FM is about 400 yards away and the VHF data is almost 1/3 harmonic). Looking at the documentation, I can guess its about 20db per cavity (or can), but the graphs don't extend very far. So for a regular four-can duplexer I might be providing 40db of protection. I want to increase this, and plan on adding one or two more cans on the receive side, and a Sinclair preselector in-between the latter two to make up for the increased insertion loss. Just wondering if anyone ever tried/measured this, or had ideas about filtering out FM broadcast. Eric mentioned using a single 7" Sinclair cavity, but I'd like to see if I can use some spare 1500 bandpass/reject cavities first. I don't think a 1/4-wave stub will work with that much field strength prying open the MSF's front-end. Thanks, Tony ------------------------------------ Yahoo! Groups Links
