Bob, A bandpass filter should follow the duplexer, so that it does not affect the impedance matches between the transmit and receive cavities of the duplexer, and to the antenna.
In each of the cases where I added bandpass cavities on the receive side, it was to limit the bandwidth going to an RF preamplifier, and I used a random length of RG-400/U coax. Maybe it was an incredible coincidence, but the sensitivity numbers showed that everything was closely matched. Although I have used the famous Motorola MICOR preamps in several installations, I prefer to go first class with Angle Linear GaAsFET preamps preceded by two large bandpass cavities set for a total insertion loss of 1.0 dB, and followed by a 6 or 10 dB attenuator before going to the receiver. While others may get excellent results with much less complicated (and expensive!) installations, I like to over-design when the system is critical and may be compromised by other nearby radios. YMMV. 73, Eric Lemmon WB6FLY rtoplus wrote: > > --- In [email protected], Eric Lemmon <[EMAIL PROTECTED]> > wrote: > > Paul, > > ...snip... > > A 100W repeater normally requires 100 > > dB of isolation for zero desense, and that usually calls for six > > cavities. However, I have used a Q-202G duplexer with two added > > bandpass cavities on the receive side, and it had zero desense > with a > > 100W PA. > > > > 73, Eric Lemmon WB6FLY > > > > > > > > Hi Eric and others > > Question...I have the same duplexer and was thinking of adding a > couple of bandpass filters as well. Does the 14" cable length apply > to the added on cavities as well? And should they be installed > between the antenna port and first RX cavity or between the last RX > cavity and receiver or does it matter? I know the cabling for this > duplexer is kinda funky so I'm wondering just where in the cabling > scheme of things any add on filters should be added and their cable > lengths. > > I have a DB Products 4 can bandpass duplexer 150/160 mHz or so > that's just sitting around doing nothing and I figured I'd put some > of the cans to some use. The model is a DB 4044 but I don't know if > the rods are long enough to tune to 146.775/.175 mHz (my 2 meter > frequency)...never tried to tune it down low. Its currently on > 152.915/159.900. > > If they can be used, should the add on cans be tuned independently > and then the whole shebang re-tuned? All I have is a couple of > simple service monitors with no spectrum analyzers or network > analyzers...would I be able to get "close enough for government > work" do you suppose? I'm not sure that there are any "qualified > filter tuners" in my area (piedmont of NC). > > Thanks and Happy Holidays! > Bob, GMRS WPVV845, Amateur KG4WAD, LMRS WPXC892 > > > Yahoo! Groups Links > > > > Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
