John, As I and several others have noted earlier, the Motorola T-1500-series duplexers are barely adequate even with a 5 MHz split. Nevertheless, Motorola shipped thousands of Micor repeater stations with this duplexer, and they worked fine. But, keep in mind that a Micor 75-100 watt PA was a very good design and far more stable and noise-free than most aftermarket power amplifiers. The crystal-controlled Micor exciters are inherently less noisy than the synthesized exciters used in newer and less-expensive repeaters. The extremely sharp, crystal-controlled Micor receivers were better able to shrug off nearby carriers or noise than many- if not most- modern synthesized receivers.
Even if your T-1500-series duplexer is optimally tuned, you can possibly improve its performance a bit by using adhesive-backed aluminum tape to cover the slots where the loop and probe adjustments penetrate the side walls of the cavities. The metal tape will eliminate a possible leakage path through the slots. Your jumper cables should be made of RG-400/U or RG-214/U cable with the proper connectors on each end- no adapters. Crimped-on, silver-plated connectors are more reliable than those that are soldered or clamped. Please confirm that the bandpass cavity you are going to put on the transmit side is a true bandpass cavity filter and not a pass-notch cavity. A pass-notch cavity has a very broad bandpass response; a bandpass (only) cavity is much sharper. One possibility that has not been mentioned yet is that the cables inside the Yaesu FTR-5410 itself might be leaky. I have a Yaesu/Vertex VXR-5000 GMRS repeater that originally suffered from a small amount of desense. I found that single-shield cable was used inside the cabinet to connect the internal receiver and PA output modules to the bulkhead connectors on the cabinet wall. Even though these in-cabinet factory jumpers were barely six inches long, they leaked enough to impair the performance of the repeater. Once I upgraded the internal jumpers to RG-400/U cable, the desense was gone. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of John Transue Sent: Saturday, August 23, 2008 9:09 AM To: [email protected] Cc: [EMAIL PROTECTED] Subject: RE: [Repeater-Builder] De-Sense and Circulator Question Robert, Thank you. Your suggestions are very reasonable. I hope to find a spectrum analyzer to verify that the output of the repeater and PA are clean. I just had the duplexer tuned a few weeks ago but I realize this does not guarantee that is its tuned properly now or that the duplexer has the isolation that it is supposed to have. When it was tuned, there was no careful check of the isolation. I have just replaced all the interconnecting cables with RG-400, one of the types that has wide acceptance from Repeater Builder correspondents. I dont want to replace them with Superflex until other possibilities are exhausted. Adding a circulator (isolator perhaps) to the output of the PA: I dont understand how this would help the de-sense. It would protect the PA from a badly tuned duplexer. Hold on! Now that I re-read your email, I see that the circulator would prevent a signal received at a close-by frequency from mixing with the output of the PA. But wouldnt the mixing already have taken place in the duplexer? Perhaps, being a passive component, the duplexer cant be the site of mixing? Yes, I am becoming convinced that the duplexer we have is not up to the job. I guess it would be good to get it on a vector network analyzer or other suitable instrument to verify that its isolation is or is not adequate. Our frequency pair is 448.375 TX and 443.375 RX. I would have to check for near-by repeater frequencies. I doubt that this is the problem though because the 440 repeaters around here have a rather low duty cycle, and our problem seems to be continuous. Also, we still have de-sense when testing into a dummy load. Your suggestions and explanations are very helpful, Robert. Please stick with us. Sooner or later we are going to get this repeater working properly. John AF4PD Vienna Wireless Society -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of KD4PBC Sent: Wednesday, August 20, 2008 1:16 PM To: [email protected] Subject: RE: [Repeater-Builder] De-Sense and Circulator Question John, Assuming that the output of the repeater and amp are clean. This would be my suggestions do this in order to avoid spending any more than needed. 1) Verify clean output from TX and amp. 2) Verify tuning on the duplexer. Both of these are most likely fine. 3) Replace all interconnect cables with ¼ of ½ superflex 4) Add circulator to the output of Amp (dual at least maybe even a triple) 5) Replace duplexer with something from this century. I read the post about the spur from adding a cavity to the tx output . the only way that should happed is if there is a problem in the amp in the 1st place. Any RF that makes it back through the transmit side of the duplexer can mix with your frequency and anything else that can get in there. An isolator is like an RF diode with a drain. Anything that comes toward your amp from the outside will be diverted into a dummy load (drain) to be burned off as heat. You did not say what frequency that you are operating on but how close in frequency are the nearest repeaters ? And example if is you are on 464.025 and there is another repeater on 464.050 then your duplexer no matter how good offers no protection itself that would be the isolators job. I hope this was helpful and while yes I do have some stuff available my post is to be helpful not sell stuff. Regards, Robert Mitchell From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of John Transue Sent: Wednesday, August 20, 2008 11:37 AM To: [email protected] Subject: RE: [Repeater-Builder] De-Sense and Circulator Question First a big thank you to Eric, David, Rodney, and Robert. It seems clear now that a circulator is not what I need. I plan to re-tune a pass cavity and put it on the TX side as suggested by Eric. David has warned of the possibility that this can cause a spur that might interfere with another system so I will try to have a spectrum analyzer look at the output. Robert asked for some additional information. Here it is: RG-400 (new) in the cabinet, RG-214 (new) to the Heliax feed line. The RF environment is intense. I do not know what all is transmitting, but there are numerous antennas within 100 feet. The repeater is a Yaesu Musen FTR-5410. It drives a Mirage PA to about 90 watts. John Transue -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of KD4PBC Sent: Wednesday, August 20, 2008 7:50 AM To: [email protected] Subject: RE: [Repeater-Builder] De-Sense and Circulator Question John, A couple questions. What are you using for interconnect cables, antenna, feedline ? Tell us something about the RF environment Hostile or not ? What type of radio are you using ? That Motorola duplexer is a very poor duplexer. With all the holes in them after a few years they may generate more noise than the suppress. All interconnecting cables need to be superflex or some other solid shielded cable. Any RG type cable the braided shield becomes illuminated and will cause noise and if there is a non linear junction can produce intermod. Send me the details and I will try and make some suggestions. Robert / KD4PBC __________ NOD32 3192 (20080616) Information __________ This message was checked by NOD32 antivirus system. http://www.eset.com <http://www.eset.com>
