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. ----- Original Message ----- From: John Transue To: [email protected] Sent: Sunday, August 03, 2008 8:35 AM Subject: RE: [Repeater-Builder] Measuring Desense Skipp, Mike, and others, A ham club member has a tracking signal generator/spectrum analyzer that he said has recently been calibrated. He re-tuned the duplexer for me. Now the receiver hears better than before, but the de-sense has not changed much. I have tried the repeater with and without the receiver pre-amp and with and without the power amplifier. The de-sense changes magnitude as you might expect, but it is still significant. And without the power amplifier we are down to only 5 watts, not enough to cover our intended area. I am suspecting that the receiver or transmitter is off frequency enough to be a problem. Actually, the receiver pass band appears to be broad enough that I don't think receiver frequency can be the problem, but if the transmitter is off a bit, it could be away from the notches in the duplexer receive channel. Does this make sense? I'm not sure what to do next. Any suggestions? The spec on the duplexer (Motorola T-1504A) is 80 dB of receiver isolation at transmitter frequency. Also, the minimum separation is given as 2 MHz while we have 5 MHz. This would seem to give enough isolation, wouldn't it? Should I be considering six cans giving about 100 dB of separation? John -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of John Transue Sent: Saturday, July 19, 2008 4:10 PM To: [email protected] Subject: RE: [Repeater-Builder] Measuring Desense Skipp, We have new commercially made RG-400 cables on both the transmit and receive side, and we have RG-214 from the tee to the Heliax transmission line. The new cables could have a fault, of course. I am going to take a break from this - going to the beach. I'll get back to it after that, and I'll let you know what happens. Thanks for all the help. John -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of skipp025 Sent: Saturday, July 19, 2008 12:27 PM To: [email protected] Subject: Re: [Repeater-Builder] Measuring Desense > "John Transue" <[EMAIL PROTECTED]> wrote: > I made the desense measurements, and I can't believe the > results. I get about 12 to 14 dB of desense. Is that possible? May not all be desense but what the overall effect you experience is lumped in together in the same "desense" label. > 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 Never say never if the wrong cables are anywhere close to each other. Few if any cables regardless of type are "100% sin free" including hard-line. Ensure you have as much practical physical cable spacing as possible. It doesn't take much stray RF energy to really rain on your receiver parade. s. __________ NOD32 3192 (20080616) Information __________ This message was checked by NOD32 antivirus system. http://www.eset.com __________ NOD32 3192 (20080616) Information __________ This message was checked by NOD32 antivirus system. http://www.eset.com
