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. Second thought---Be careful connecting anything other than something intended for this application like a service monitor to the antenna output of your duplexer------there is or are lots of watts of power there with the transmitter operating and the possibility exists of taking out the front end of a conventional spectrum analyzer or the back end of a generator etc. An iso-T backed way out should be OK but be careful. Scott ----- Original Message ----- From: John Transue To: [email protected] Cc: [EMAIL PROTECTED] Sent: Sunday, August 03, 2008 11:51 AM Subject: RE: [Repeater-Builder] Measuring Desense Scott and Skipp and others, Thanks, Scott, for the detailed instructions. I went about the testing a little differently and might have been misled. I put the isotee between the antenna (or the dummy load) and the duplexer. Is it better to put it directly in front of the receiver? Also, I didn't use a modulated signal. I just tried to judge the amount of noise I was hearing. This was difficult because the ambient noise was loud and changeable. My signal generator will provide the modulated signal you suggest, so I can do as you specified. Skipp, the repeater is a Yaesu FTR-5410. I have tried another receiver (FT-8800) without the transmitter transmitting. I don't think I tried it when transmitting. The repeater receiver plus pre-amp seems to be more sensitive than the FT-8800 when there is no transmit energy to interfere. I'm sorry I don't have a direct comparison without the repeater pre-amp. I made the measurements at two different times and didn't get all the combinations that could help decipher this problem. I don't know the brand and model of the pre-amp (label is covered). I will attach the record of the testing I did just a couple days ago. It might tell you something that I have missed. Thanks for the very good help. You guys are great. John -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Scott Overstreet Sent: Sunday, August 03, 2008 12:38 PM To: [email protected] Cc: Scott Overstreet Subject: Re: [Repeater-Builder] Measuring Desense 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 __________ NOD32 3192 (20080616) Information __________ This message was checked by NOD32 antivirus system. http://www.eset.com
