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]> >Date: 2008/07/11 Fri PM 07:04:01 EDT >To: [email protected] >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:[EMAIL PROTECTED] On Behalf Of >Ron Wright >Sent: Thursday, July 10, 2008 9:04AM >To:[email protected] >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]> >>Date: 2008/07/10 Thu AM 03:55:04 EDT >>To: [email protected] >>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> >> >>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 > Ron Wright, N9EE 727-376-6575 MICRO COMPUTER CONCEPTS Owner 146.64 repeater Tampa Bay, FL No tone, all are welcome.
