RE: [Repeater-Builder] Re: 2m repeater IMD issue
> -Original Message- > From: skipp025 > > I've seen some of the other posts. You should be running > a dual port circulator in addition to the proper duplexer. Well, I don't necessarily disagree with using an isolator/harmonic filter. However, the isolator will not keep 800MHz signals from getting to the transmitter of the 2m repeater. I'm sure the harmonic filter may reduce the 800 levels some but the isolator isn't going to stop it. Of course, an isolator will keep other VHF signals from getting back through it but I don't have any VHF signals that are a problem. The duplexer is a new TX/RX 4 can vari-notch. I've added bandpass cavities on the rx & tx side between the duplexer & repeater. > You mention the 7/8 line is used. If you remove the > connector from any end and the line is not shinny, clean > the oxide off with a brass brush and replace the connector > back on the cable. If the oxide color is really dark, you > have moisture leakage in the cable and that's really bad > news. I'd don't know exactly how old it is but it is in "good shape" according to the tower crew. It was previously used by a law enforcement agency before they switched to the 800 system. > If the PD220-2 antenna is over 10 years old, consider > trying a swap out. I have replaced a number of aged PD > antennas, which (along with station masters) can become > IMD generators when placed in modest proximity to other > antennas (and metal objects). I purchased the PD220-2 antenna new late last year. It was drop shipped directly from Celwave. I installed a new polyphaser at the entry port but I haven't tried taking it out of the circuit to see if it's contributing to the problem. Has anyone seen a polyphaser do bad things? > You can measure the system desense of the terminated > feed, which will provide additional clues. Good idea. > You should try for min 1/4 wave spacing (at vhf) from > the 800Mhz antennas, where possible. The closest antenna to the PD220 is about 4 to 5 feet away. > cheers > skipp > > www.radiowrench.com > 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/
RE: [Repeater-Builder] Re: 2m repeater IMD issue
At 12:43 PM 5/5/2004 -0400, you wrote: >However, the isolator will not keep 800MHz signals from getting to the >transmitter of the 2m repeater. 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/
Re: [Repeater-Builder] Re: 2m repeater IMD issue
At 5/5/2004 08:56 AM, you wrote: >I've seen some of the other posts. You should be running >a dual port circulator in addition to the proper duplexer. As Richard has correctly indicated in his previous posts, an isolator will not have an effect on his IMD problem. Having said that, however, it still would be a good idea to put a single-port isolator on the TX. The reason is so that the RFPA "sees" 50 ohms over a broad bandwidth. Some RFPAs (like the high-power GEs) don't like to see pure reactance anywhere within their operating range, even if it's not on the frequency where power is being generated. My guess is that the IMD is in fact being generated on the tower. 95% of the on-site mixes I discover these days are found in tower joints, electrical conduit, air conditioning ducts, etc. One just popped up on my input after changing a 420 link channel (2 watt TX 100 ft. below main RX antenna). I didn't even bother to look for the sources; I just decided to swap link channels again. Bob NO6B 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/
RE: [Repeater-Builder] Re: 2m repeater IMD issue
Skipp has some good ideas, here's another one. Try adding an LPF between your duplexer and hard line. A harmonic filter out of a high band Micor or something similiar. This shouldn't affect your signal at 2M much, but it should drop the 800 MHz stuff down 80 dB or more, depending on filter. If there is no change in IM, then it is probably occuring at the antenna or tower. A cracked solder joint in your antenna, or one of the 800M antennas could be causing the problem. This could act as a mixing point when your 2M TX is on. Of course anything in close proximity to your TX antenna could be causing this as well, if it is acting as a junction. Joe --- "Richard Sharp, KQ4KX" <[EMAIL PROTECTED]> wrote: > > -Original Message- > > From: skipp025 > > > > I've seen some of the other posts. You should be > running > > a dual port circulator in addition to the proper > duplexer. > > Well, I don't necessarily disagree with using an > isolator/harmonic filter. > However, the isolator will not keep 800MHz signals > from getting to the > transmitter of the 2m repeater. I'm sure the > harmonic filter may reduce the > 800 levels some but the isolator isn't going to stop > it. Of course, an > isolator will keep other VHF signals from getting > back through it but I > don't have any VHF signals that are a problem. The > duplexer is a new TX/RX > 4 can vari-notch. I've added bandpass cavities on > the rx & tx side between > the duplexer & repeater. > > > You mention the 7/8 line is used. If you remove > the > > connector from any end and the line is not shinny, > clean > > the oxide off with a brass brush and replace the > connector > > back on the cable. If the oxide color is really > dark, you > > have moisture leakage in the cable and that's > really bad > > news. > > I'd don't know exactly how old it is but it is in > "good shape" according to > the tower crew. It was previously used by a law > enforcement agency before > they switched to the 800 system. > > > If the PD220-2 antenna is over 10 years old, > consider > > trying a swap out. I have replaced a number of > aged PD > > antennas, which (along with station masters) can > become > > IMD generators when placed in modest proximity to > other > > antennas (and metal objects). > > I purchased the PD220-2 antenna new late last year. > It was drop shipped > directly from Celwave. > > I installed a new polyphaser at the entry port but I > haven't tried taking > it out of the circuit to see if it's contributing to > the problem. Has > anyone seen a polyphaser do bad things? > > > You can measure the system desense of the > terminated > > feed, which will provide additional clues. > > Good idea. > > > You should try for min 1/4 wave spacing (at vhf) > from > > the 800Mhz antennas, where possible. > > The closest antenna to the PD220 is about 4 to 5 > feet away. > > > cheers > > skipp > > > > www.radiowrench.com > __ Do you Yahoo!? Win a $20,000 Career Makeover at Yahoo! HotJobs http://hotjobs.sweepstakes.yahoo.com/careermakeover 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/
