I made an isotee today by cutting off the central pin in a F-M-F tee. I measured the attenuation by comparing the power through the tee to the power that escapes from the isotee port. With the central pin cut off flush with the dielectric the attenuation is about 38 dB. With the central pin removed entirely the attenuation is about 71 dB. The sampling port on my Bird can be adjusted from about 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 the signal generator. Has anyone had such a problem? Is there a coupler that could prevent the transmitter power from entering the line to the signal generator? If I use the isotee with the pin removed, the signal generator can be operated between -36 and -56 dBm to give -107 to -127 dBm at the receiver. For the transmitter power amplifier putting out 60 watts, the 71 dB attenuation reduces the signal seen by the signal generator to -23 dBm. Is this 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: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Ron Wright Sent: Thursday, July 10, 2008 9:04 AM To: Repeater-Builder@yahoogroups.com Subject: Re: Re: [Repeater-Builder] Measuring Desense John, The procedure is primarily measuring the site noise in your system, not desense. It simply measured first the receiver sensitivity then connected the antenna and did the same. For the desense test we are looking to see the noise caused by the repeater transmitter although site noise can be part of this noise. In step 3 of the test one would key and unkey the transmitter to see the defferent effects. I think the test suggest the transmitter is keyed at all times. 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. The test is effectively telling you the site noise with all connected which is important. However, to determine if you have desense from your repeater you need to key/unkey the tx. Step 1 can be removed for would think you have done this before, know the receiver sensitivity. Doing the same with the T on the duplexer output with the tx unkeyed would be your starting reference for the receiver, then keying it would give tx noise level. If you connect all in and do steps 2 & 3, but keying and unkeying the tx in step 3 is what you want to do for tx desense. We are looking for desense, not site noise in your case. Site noise is important, but often one can do little about it for it comes from many sources inclusing 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> net> >Date: 2008/07/10 Thu AM 03:55:04 EDT >To: Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.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. Does this appear to be a correct way to measuredesense? > >Also, I can replace the feed line and antenna with a dummyload as Ron has explained. > >John AF4PD ><http://www.repeater <http://www.repeater-builder.com/tech-info/effectivesens.html> -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
