Okay, so you have a 5 MHz split instead of a 600 kHz split- that makes it much easier. I like to use two 8" bandpass cavities, simply because they will fit in a 19" rack and provide maximum performance. I have three commercial UHF MTR2000 repeaters on the air now, each with two Telewave 8" bandpass cavities mounted on a panel (TWPC-4508-2) and an Angle Linear preamp upstream of them. One of the repeaters is less than 100 feet away from a 5 MW Air Route Surveillance Radar, and there is absolutely no interference from it. The other two repeaters have nearby in-band stations belonging to others.
Two 8" bandpass cavities each set to 0.5 dB insertion loss are significantly more effective than one 8" bandpass cavity set to 1.0 dB insertion loss. Likewise, two 8" bandpass cavities have steeper cutoff responses than two 5" cavities. As in many things in life, you get what you pay for. I also have an Amateur 2m repeater that is co-located with a 10 kW broadcast FM station, and I found that one Sinclair 7" bandpass cavity filter was sufficient to eliminate any desense. When properly applied, a bandpass cavity can perform miracles. Although the relatively inexpensive DCI helical filters are somewhat effective in reducing desense, they cannot perform as well as a bandpass cavity filter- especially when a preamp is in the receive chain. As others have pointed out, a preamp is seldom a cure-all, and may cause more problems to occur than fewer. To answer your question, I do not suggest that you get a bandpass/reject cavity. I suggest that you get a "pure" bandpass cavity. Its pass response is very narrow when compared to a pass/reject cavity. 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of Pointman Sent: Thursday, November 26, 2009 9:56 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Re: pre-amp placement So your suggestion is to get a Band pass/reject cavity instead? Or should I get 2 for the added isolation? keep in mind I am on UHF.... de KM3W ________________________________ From: Eric Lemmon <wb6...@verizon.net> To: Repeater-Builder@yahoogroups.com Sent: Thu, November 26, 2009 2:48:54 PM Subject: RE: [Repeater-Builder] Re: pre-amp placement A total insertion loss of about 1.0 dB works well, in my experience. With two 8" bandpass cavities in series, this gives at least 25 dB of isolation from the transmitter carrier at a 600 kHz split. Bear in mind that your notch cavity has the same deficiency as the typical BpBr duplexer- there is relatively little bandpass effect. A pass-notch cavity is a poor substitute for a bandpass cavity. 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@ yahoogroups. com <mailto:Repeater-Builder%40yahoogroups.com> [mailto:Repeater-Builder@ yahoogroups. com <mailto:Repeater-Builder%40yahoogroups.com> ] On Behalf Of Pointman Sent: Thursday, November 26, 2009 10:09 AM To: Repeater-Builder@ yahoogroups. com <mailto:Repeater-Builder%40yahoogroups.com> Subject: Re: [Repeater-Builder] Re: pre-amp placement So what is the recommendation to set the loss of the BP cavity? I have a setting as to 3 dB, 1 dB, .5 dB etc. Running the ARR preamp on a UHF repeater, it seems the preamp is a little too much and we get a little desense. I am only running a 4 cavity duplexer and a notch cavity with the preamp. de KM3W
