Scott, Eric, and others,
Scott, I followed your recommended procedure a few weeks ago. It told me that the problem is not enough isolation in the duplexer. Adding a cavity on the RX side improved things a lot. Now I plan to switch the cavity (now re-tuned to TX freq) to the TX side. I'll let you know how this works. Eric, I tried to wiggle the loops to reduce the insertion loss. The best I can do is still 1.1 dB. This cavity is made with the coupling loops diametrically opposite. They are copper rectangular loops with no provision for adjustment. I plan to install the cavity tomorrow. Does anyone have a six can Celwave or other good duplexer that can give me 100 dB of isolation? I won an auction for one but then the seller said it was listed by mistake - it was already sold!!! John T. -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Scott Overstreet Sent: Wednesday, August 20, 2008 10:44 PM To: [email protected] Cc: Scott Overstreet Subject: Re: [Repeater-Builder] De-Sense and Circulator Question John--- I'm repeating my two previous messages to you on 8/3/08----they again seem appropriate. Scott John--- I don't know if you have done the following yet but if not you should. Put an iso-T in front of your receiver and do a simple desense test using your signal generator into the iso-T. Set the generator for a couple of kc, deviation at 1 kc so that you can easily recognize it. Run the signal level down to where you can just hear it in the receiver with the transmitter off. Turn your transmitter on. Do you still hear your generator signal? Lets assume you don't as if you do you don't have desense. Now, remove your antenna feedline from your duplexer and put a good screw on dummy load in it's place and repeat the above test. Do you still have desense? If yes, you have insufficient duplexer performance to support your transmitter (power and sideband noise spectra) or leaky interconnect cabling. If no, you are getting desensing from either your feedline or your antenna or something your antenna radiated signal is exciting like a rusty tower joint or joints which is or are producing wideband noise which your antenna is hearing and feeding back to your receiver as a desensing signal. If this looks to be the case, put the dummy at the end of your feedline in place of your antenna and repeat the testing. This should leave you with either a feedline or connectors to replace or a possibly bad antenna. I've been this far and found a corroded Hustler. Took it down, cleaned it up and put it back as a replacement was not immediately available----it is still up and working as well as it ever did with absolutely no desense. Hello (Again) John---- Yes--It is necessary that you put the iso-T immediately in front of your receiver----you want to be testing against the signal that is presented to your receiver after all of the processing before it. A couple more thoughts---- Don't overlook the possibility that your duplexer is noisy or maybe I should say fried. I have two 2 meter Phelps-Dodge duplexers here that on the instruments show 110 db. Tx freq. attenuation into the Rx port as they should but badly desense with transmitter power applied----they are apparently internally bad connectivity wise----possibly adjusted with power applied or just corroded internal finger stock. In any case, they make wideband noise with transmitter power applied that is visible as a "hump" of noise at the receive frequency when the Rx port on the duplexer is observed by spectrum analyzer. And, one interesting fact is that this wideband (desensing) noise is not easily detectable by listening to the audio from the repeater receiver. In other words----If this sort of thing is your problem----you may not hear any change just turning the transmitter on and off----you must work against a modulated weak signal either into the antenna or via an iso-T. ----- Original Message ----- ----- Original Message ----- From: John Transue <mailto:[EMAIL PROTECTED]> To: Repeater-Builder@ <mailto:[email protected]> yahoogroups.com Sent: Wednesday, August 20, 2008 10:54 AM Subject: RE: [Repeater-Builder] De-Sense and Circulator Question Yes, the de-sense is present when transmitting into a dummy load and when transmitting into the antenna. -----Original Message----- From: Repeater-Builder@ <mailto:[email protected]> yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of DCFluX Sent: Wednesday, August 20, 2008 1:06 PM To: Repeater-Builder@ <mailto:[email protected]> yahoogroups.com Subject: Re: [Repeater-Builder] De-Sense and Circulator Question <snip> Also have you measured the desense into a dummy load? Does it only appear when the antenna is attached? If so, Take a look at the FM broadcast band and write down the stations you hear, then look for a pair that is spaced 5 MHz apart. On Wed, Aug 20, 2008 at 8:49 AM, John Transue <[EMAIL PROTECTED] <mailto:jtransue%40cox.net> net> wrote: > The RG-214 is new. The cable was built by The RF Connection. I didn't > see the shield, but RG-214 is supposed to have two silver-plated copper > shields according to everything I have read. Has this been changed? > > At any rate, the problem of de-sense predated the installation of the > new cable. > > John > >>-----Original Message----- >>From: Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com [mailto:Repeater- >>[EMAIL PROTECTED] <mailto:Builder%40yahoogroups.com> .com] On Behalf Of DCFluX >>Sent: Wednesday, August 20, 2008 11:33 AM >>To: Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com >>Subject: Re: [Repeater-Builder] De-Sense and Circulator Question >> >>Is this new RG-214 that has the foil and braid? >> >>On Tue, Aug 19, 2008 at 5:15 PM, John Transue <[EMAIL PROTECTED] <mailto:jtransue%40cox.net> net> >>wrote: >>> >>> >>> >>> >>> -----Original Message----- >>> From: Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com >>> [mailto:Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com] On Behalf Of DCFluX >>> Sent: Tuesday, August 19, 2008 4:13 PM >>> To: Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com >>> Subject: Re: [Repeater-Builder] De-Sense and Circulator Question >>> >>> >>> >>> Are you using LMR series or Belden 9913 for a feedline? >>> >>> >>> >>> No, I have RG-400 in the cabinet and RG-214 leading to the Heliax >>feed line. >>> >>> On Tue, Aug 19, 2008 at 1:08 PM, John Transue <[EMAIL PROTECTED] <mailto:jtransue%40cox.net> net> >>wrote: >>>> Members of Repeater Builder have been very helpful .<snip> Now I >>am >>>> wondering whether a circulator would provide additional isolation >>and >>>> further decrease the de-sense problem. Your advice would be much >>>> appreciated. >>>> >>> >>> >>> __________ NOD32 3192 (20080616) Information __________ >>> >>> This message was checked by NOD32 antivirus system. >>> http://www.eset. <http://www.eset.com> com >>> >>> >> >>------------------------------------ >> >> >> >>Yahoo! Groups Links >> >> >> >> >>__________ NOD32 3192 (20080616) Information __________ >> >>This message was checked by NOD32 antivirus system. >>http://www.eset. <http://www.eset.com> com > > > > ------------------------------------ > > > > Yahoo! Groups Links > > > > __________ NOD32 3192 (20080616) Information __________ This message was checked by NOD32 antivirus system. http://www.eset. <http://www.eset.com> com __________ NOD32 3192 (20080616) Information __________ This message was checked by NOD32 antivirus system. http://www.eset.com
