John, Yes, it is possible! A case in point: I was bench-testing a brand-new Motorola GR1225 desktop VHF repeater that came equipped with a factory-tuned Celwave notch duplexer. The repeater was programmed to operate on one of the VHF "taxicab pairs" that have a 5.26 MHz split. Although I confirmed that the duplexer was properly tuned and the frequencies in the repeater were correct, I was measuring more than 10 dB of desense. After scratching my head for awhile, I took a good look at the factory-supplied internal cable harness that connected the duplexer to the R1225 transceiver, and found that it was entirely made of single-shield RG-58/U cable!
I fabricated a new cable harness of RG-400/U double-shielded cable and the appropriate silver-plated N and mini-UHF connectors, and ran another test. No desense at all. The only reason I can think of, why Motorola supplied such leaky cable with a VHF repeater, is because the GR1225 is (or was) marketed as a low-end, economy-grade, construction site repeater where poor performance may not be noticed due to the close proximity of the users. Once the cable is upgraded to RG-400, the performance is greatly improved. Any between-series adapters can also contribute to desense, and that's why I always make up new jumpers with the proper connectors on each end. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of John Transue Sent: Saturday, July 19, 2008 8:53 AM To: [email protected] Subject: RE: RE: Re: [Repeater-Builder] Measuring Desense I made the desense measurements, and I can't believe the results. I get about 12 to 14 dB of desense. Is that possible? The measurements are not precise because the signal generator is not perfectly stable, but I can't believe the measurement error is anywhere close to 12 dB. We definitely are getting desense because I had a ham in an outlying area call into the repeater with the repeater transmitter turned off. He got in with a good signal, just a little cracking in the background. Then I turned on the transmitter and he could not get in at all. I believe that re-tuning the duplexer is the next step. The "touch up" a couple weeks ago was not complete. I have installed new cables in the cabinet. These are RG 400 so I don't think the desense is coming from cable leakage. John -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of John Transue Sent: Saturday, July 12, 2008 6:51 PM To: [email protected] Subject: RE: RE: Re: [Repeater-Builder] Measuring Desense Ron, Don, Joe, Jeff, Nate, and others, Thanks for all the good ideas. And Ron, thanks for explaining that I won't be damaging the signal generator. I hope to make the desense measurements Monday. I'll use the isotee that I made. The signal generator has plenty of range of power so that I can get the power I need. Maybe later I can get one of the "store bought" adjustable isotees. John -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Ron Wright Sent: Saturday, July 12, 2008 11:48 AM To: [email protected] Subject: Re: RE: Re: [Repeater-Builder] Measuring Desense John, Even if the transmitter was at 100 W the 38 db loss in the isotee would give only 0.016 watts into the sig gen which believe me would not be a problem. Sure this is a major signal compared to a typical receiver input or sig gen output, but should not be a problem as far as the sig gen handling such on its output. Most crossband couplers have only 30 db isolation, even the expensive commercial ones. Of course the "other band" signals are far away in freq, but I would not have a problem connecting a sig gen to one of the ports with the "other band" transmitting. 73, ron, n9ee/r >From: John Transue <[EMAIL PROTECTED] <mailto:jtransue%40cox.net> > >Date: 2008/07/11 Fri PM 07:04:01 EDT >To: [email protected] <mailto:Repeater-Builder%40yahoogroups.com> >Subject: RE: Re: [Repeater-Builder] Measuring Desense > > >I madean isotee today by cutting off the central pin in a F-M-F tee. I measured theattenuation by comparing the power through the tee to the power that escapesfrom the isotee port. With the central pin cut off flush with the dielectricthe attenuation is about 38 dB. With the central pin removed entirely theattenuation is about 71 dB. The sampling port on my Bird can be adjusted fromabout 46 to 51 dB. > >I am concerned that the transmitter power,as attenuated by the isotee or Bird, will still be high enough to damage thesignal generator. Has anyone had such a problem? Is there a coupler that couldprevent the transmitter power from entering the line to the signal generator? > >If I use the isotee with the pin removed, thesignal generator can be operated between -36 and -56 dBm to give -107 to -127 dBmat the receiver. For the transmitter power amplifier putting out 60 watts, the71 dB attenuation reduces the signal seen by the signal generator to -23 dBm. Isthis low enough to be of no concern? > >Ideas and comments? > >John > >P.S. The idea proposed by William494 (billb)sounds right, i.e. let the signal generator see a 50-ohm impedance. > > >-----Original Message----- >From:[email protected] <mailto:Repeater-Builder%40yahoogroups.com> [mailto:[email protected] <mailto:Repeater-Builder%40yahoogroups.com> ] On Behalf Of Ron Wright >Sent: Thursday, July 10, 2008 9:04AM >To:[email protected] <mailto:Repeater-Builder%40yahoogroups.com> >Subject: Re: Re:[Repeater-Builder] Measuring Desense > >John, > >The procedure is primarily measuring the site noise in your system, notdesense. It simply measured first the receiver sensitivity then connected theantenna and did the same. > >For the desense test we are looking to see the noise caused by the repeatertransmitter although site noise can be part of this noise. > >In step 3 of the test one would key and unkey the transmitter to see thedefferent effects. I think the test suggest the transmitter is keyed at alltimes. Not unkeying/keying the transmitter would give you the noise results,but not tell you if the noise is from the transmitter or some other source. Thetest is effectively telling you the site noise with all connected which isimportant. > >However, to determine if you have desense from your repeater you need tokey/unkey the tx. > >Step 1 can be removed for would think you have done this before, know thereceiver sensitivity. Doing the same with the T on the duplexer output with thetx unkeyed would be your starting reference for the receiver, then keying itwould give tx noise level. > >If you connect all in and do steps 2 & 3, but keying and unkeying the tx instep 3 is what you want to do for tx desense. > >We are looking for desense, not site noise in your case. Site noise isimportant, but often one can do little about it for it comes from many sourcesinclusing 100 transmitters within 10 miles of you. > >73, ron, n9ee/r > >The test in the link is a must for repeaters and is a good one. > >>From: Mike Morris WA6ILQ <[EMAIL PROTECTED] <mailto:wa6ilq%40arrl.net> > >>Date: 2008/07/10 Thu AM 03:55:04 EDT >>To: [email protected] <mailto:Repeater-Builder%40yahoogroups.com> >>Subject: Re: [Repeater-Builder] Measuring Desense > >> >>At 12:18 PM 07/09/08, you wrote: >> >><<...>>Ron, Don, Mark, and others, >> >>The attachment shows how I think I should connect things tomeasure desense.I would use the Bird with sampling coupler in place ofthe iso tee shown. Doesthis appear to be a correct way to measuredesense? >> >>Also, I can replace the feed line and antenna with a dummyload as Ron hasexplained. >> >>John AF4PD >><http://www.repeater-builder.com/tech-info/effectivesens.html <http://www.repeater-builder.com/tech-info/effectivesens.html> > >> >>Mike WA6ILQ >> >> > >Ron Wright, N9EE >727-376-6575 >MICRO COMPUTER CONCEPTS >Owner 146.64 repeater Tampa Bay, FL >No tone, all are welcome. > >__________ NOD32 3192 (20080616) Information __________ > >This message was checked by NOD32 antivirus system. >http://www.eset.com <http://www.eset.com> Ron Wright, N9EE 727-376-6575 MICRO COMPUTER CONCEPTS Owner 146.64 repeater Tampa Bay, FL No tone, all are welcome. __________ NOD32 3192 (20080616) Information __________ This message was checked by NOD32 antivirus system. http://www.eset.com <http://www.eset.com> __________ NOD32 3192 (20080616) Information __________ This message was checked by NOD32 antivirus system. http://www.eset.com <http://www.eset.com>
