RE: [Repeater-Builder] Desense Problem on 222 MHz Repeater
How are you checking for desense? Are you using an isolated T between the duplexer and antenna line and doing the same when measuring desense on the dummy load? Are you measuring site noise? Do this the same way you would measure desense with the isolated T in the line. But first see what the receiver sensitivity is with the dummy load connected in place of the antenna. Then replace the dummy load with the antenna, do not key the transmitter, and measure the difference in receiver with the antenna connected verses the dummy load. With TV stations present you may be surprised at the amount of site noise present. Then key the transmitter and again measure receiver sensitivity thru the isolated T. If you haven't done this you may find that a lot of the problem is site noise rather than desense problems. 73 Gary K4FMX _ From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Stu Benner Sent: Friday, January 25, 2008 12:52 PM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Desense Problem on 222 MHz Repeater Our group has substantial technical knowledge and experience, but we've been just about beaten down by a problem with our repeater. A brief overview of our situation follows. We have a 222 MHz repeater comprised of a converted Micor mobile, Telewave TPRD-2254 BpBr duplexer, AM-6155 PA modified for class C operation at 250W, and a DB-264JJ antenna at 80 ft. fed by 1/2' Heliax on a commercial FM broadcast tower . With the duplexer terminated into a load, we have about 1 dB degradation in sensitivity when transmitting. However, with the antenna connected to the duplexer, we experience in excess 15 dB of desensitization. We have eliminated other narrowband transmitters and analog TV transmitters as contributing factors. We are left with a channel 12 digital TV transmitter at an adjacent site as a key contributor to the problem. Our hypothesis is that we have broadband IMD products from the mix of our transmitter and the DTV transmitter that are appearing in and near our receiver passband. Is it a rusty bolt problem or is there some other non-linear component somewhere on the site or in our system that is the mixing point - we don't know. I'd be interested in beginning a dialog with anyone who might be able to give us some further insight into this problem. Regards, Stu Benner W3STU Boonsboro, MD
RE: [Repeater-Builder] Desense Problem on 222 MHz Repeater
Just make sure to pull the unused loop out of the cavity. Steve / K6SCA Gary Schafer [EMAIL PROTECTED] wrote: If you have an extra band pass cavity then you also have a notch cavity. Just connect a T to one port of the band pass cavity and ignore the other port on the cavity. This will work as a notch cavity for your testing. 73 Gary K4FMX - From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Stu Benner Sent: Friday, January 25, 2008 4:54 PM To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Desense Problem on 222 MHz Repeater Thanks to everyone for the replies so far. Please see comments embedded below. I think that they address most of the comments, questions, and recommendations posed by all to this point. If you suspect IMD between the channel 12 DTV transmitter and your Tx carrier, work on attenuating the DTV signal. Using cavity notch filters to reject the entire 6 MHz of DTV isn't too practical, so instead, try adding pass cavities on your Tx before the duplexer input. That will help determine if the IM is originating in your PA. An isolator *may* help, but with channel 12 being the better part of 20 MHz away (about 10%), it may not afford full protection -- isolators don't have infinite bandwidth. Likewise, finding a 250 watt 220 MHz isolator may not be easy. I think I have some 220 isolators that came off a combiner (Sinclair), but doubt they're good for 250 watts judging by their size. [Stu] Agree, notches don't work well for that plus we don't have handy any cavities that will tune there. Have used up to two BP cavities in the TX path along with a 2-stage isolator. No difference in desense is observed. If you suspect a rusty bolt mix, use an alternate antenna for testing. If nothing else, try a quarter-wave whip (suitable for operation at your 250 watts TPO), even if it's just temporarily mounted on the tower (be sure it's at a sufficient height to prevent desense due to close proximity to the repeater itself). [Stu] We're presently on split antennas. One is at about 80 ft., the other is at about 15 ft. This improves the desense on the order of 6 dB. Another good possibility is IM in your receiver front end (or preamp, if you're using one). Again, pass cavities are your friend here. Attenuate the channel 12 signal as much as possible and see if it makes a difference. Have you looked at what sigs are reaching your receiver input on a spectrum analyzer? With 15 dB of desense, you should be able to see the culprit(s); it's not like they're going to be buried in the noise if it's causing 15 dB of desense. [Stu] The desense is significant with or without a preamp. Worse with but I can't find my notes to quote numbers. Used up to 2 BP cavities on RX with no perceptable difference in desense. Have also installed a DCI 4-pole filter on RX and TX with no effect. Have looked at the receiver input with a spectrum analyzer. The most significant signal is the one FM broadcast transmitter at the site. Running power down on it or turning it off has no effect on the desense. Our TX signal at our RX input is consistent with our measured duplexer isolation (about -88 dBc or -34 dBm). Within several hundred kHz of the RX frequency there are no detectable narrowband signals. Even that 1 dB of desense would give me some agita. I'd verify that the duplexers are properly tuned and the transmitter is clean before even starting down any other paths related to the channel 12 issue. IIRC, the Telewave cavities have adjustable coupling. If necessary, sacrifice a little extra loss for additional rejection if necessary. [Stu] I tuned the duplexer myself with a network analyzer and the transmitter looks clean. I have coupling set where I get about 1 dB through loss and the notches are at about 88 dB on TX and about 90 dB on RX. I also assume you're using all known-good interconnect cables (no foil+braid or other cables not suitable for duplex operation). [Stu] All cables are either Heliax or double braided. Are you using a Polyphaser or other type of surge arrestor? If so, try bypassing it. I've seen gas discharge tube type surge arrestors become noisemakers after absorbing a strike. [Stu] Yes but there is no difference in desense when it is removed. Has the VSWR changed at all on your antenna? If so, it could indicate water in a connector or the harness which will cause all kinds of grief, including wideband noise. [Stu] The problem has existed since the repeater was installed. It exists whether we duplex on a DB264 at 80 feet or a G7-220 at 15 feet, both fed with Heliax Finally, does the desense change appreciably if you vary transmitter power output (it probably will). Do you any have desense when running on just exciter power? [Stu] The desense is roughly proportional to transmit power. Barely perceptable at 20W (exciter only
RE: [Repeater-Builder] Desense Problem on 222 MHz Repeater
If you have an extra band pass cavity then you also have a notch cavity. Just connect a T to one port of the band pass cavity and ignore the other port on the cavity. This will work as a notch cavity for your testing. 73 Gary K4FMX _ From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Stu Benner Sent: Friday, January 25, 2008 4:54 PM To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Desense Problem on 222 MHz Repeater Thanks to everyone for the replies so far. Please see comments embedded below. I think that they address most of the comments, questions, and recommendations posed by all to this point. If you suspect IMD between the channel 12 DTV transmitter and your Tx carrier, work on attenuating the DTV signal. Using cavity notch filters to reject the entire 6 MHz of DTV isn't too practical, so instead, try adding pass cavities on your Tx before the duplexer input. That will help determine if the IM is originating in your PA. An isolator *may* help, but with channel 12 being the better part of 20 MHz away (about 10%), it may not afford full protection -- isolators don't have infinite bandwidth. Likewise, finding a 250 watt 220 MHz isolator may not be easy. I think I have some 220 isolators that came off a combiner (Sinclair), but doubt they're good for 250 watts judging by their size. [Stu] Agree, notches don't work well for that plus we don't have handy any cavities that will tune there. Have used up to two BP cavities in the TX path along with a 2-stage isolator. No difference in desense is observed. If you suspect a rusty bolt mix, use an alternate antenna for testing. If nothing else, try a quarter-wave whip (suitable for operation at your 250 watts TPO), even if it's just temporarily mounted on the tower (be sure it's at a sufficient height to prevent desense due to close proximity to the repeater itself). [Stu] We're presently on split antennas. One is at about 80 ft., the other is at about 15 ft. This improves the desense on the order of 6 dB. Another good possibility is IM in your receiver front end (or preamp, if you're using one). Again, pass cavities are your friend here. Attenuate the channel 12 signal as much as possible and see if it makes a difference. Have you looked at what sigs are reaching your receiver input on a spectrum analyzer? With 15 dB of desense, you should be able to see the culprit(s); it's not like they're going to be buried in the noise if it's causing 15 dB of desense. [Stu] The desense is significant with or without a preamp. Worse with but I can't find my notes to quote numbers. Used up to 2 BP cavities on RX with no perceptable difference in desense. Have also installed a DCI 4-pole filter on RX and TX with no effect. Have looked at the receiver input with a spectrum analyzer. The most significant signal is the one FM broadcast transmitter at the site. Running power down on it or turning it off has no effect on the desense. Our TX signal at our RX input is consistent with our measured duplexer isolation (about -88 dBc or -34 dBm). Within several hundred kHz of the RX frequency there are no detectable narrowband signals. Even that 1 dB of desense would give me some agita. I'd verify that the duplexers are properly tuned and the transmitter is clean before even starting down any other paths related to the channel 12 issue. IIRC, the Telewave cavities have adjustable coupling. If necessary, sacrifice a little extra loss for additional rejection if necessary. [Stu] I tuned the duplexer myself with a network analyzer and the transmitter looks clean. I have coupling set where I get about 1 dB through loss and the notches are at about 88 dB on TX and about 90 dB on RX. I also assume you're using all known-good interconnect cables (no foil+braid or other cables not suitable for duplex operation). [Stu] All cables are either Heliax or double braided. Are you using a Polyphaser or other type of surge arrestor? If so, try bypassing it. I've seen gas discharge tube type surge arrestors become noisemakers after absorbing a strike. [Stu] Yes but there is no difference in desense when it is removed. Has the VSWR changed at all on your antenna? If so, it could indicate water in a connector or the harness which will cause all kinds of grief, including wideband noise. [Stu] The problem has existed since the repeater was installed. It exists whether we duplex on a DB264 at 80 feet or a G7-220 at 15 feet, both fed with Heliax Finally, does the desense change appreciably if you vary transmitter power output (it probably will). Do you any have desense when running on just exciter power? [Stu] The desense is roughly proportional to transmit power. Barely perceptable at 20W (exciter only) with split antennas. A little worse at 20W (exciter only) using one antenna. Have tried both tube-type and solid state amplifiers at various power levels. --- Jeff WN3A
RE: [Repeater-Builder] Desense Problem on 222 MHz Repeater
Stu, It looks to me as if you have covered almost all of the bases, and have eliminated every one of the typical causes of desense. However, there is one possibility that has not been mentioned: Leakage inside the Micor radio. Even with a careful duplex conversion, there are several sneak paths for extraneous signals to enter the Micor receiver. One ingress point is the tiny slots around the RCA plug at the input to the helical resonator block. A wrap of metal tape around that plug will seal the plug. Additional bypass capacitors and ferrite beads on DC supply leads may help. Try using a separate DC power supply for the receiver. Moreover, I suspect that the receiver needs more shielding. 73, Eric Lemmon WB6FLY -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Stu Benner Sent: Friday, January 25, 2008 1:54 PM To: Repeater-Builder@yahoogroups.com Subject: RE: [Repeater-Builder] Desense Problem on 222 MHz Repeater Thanks to everyone for the replies so far. Please see comments embedded below. I think that they address most of the comments, questions, and recommendations posed by all to this point. If you suspect IMD between the channel 12 DTV transmitter and your Tx carrier, work on attenuating the DTV signal. Using cavity notch filters to reject the entire 6 MHz of DTV isn't too practical, so instead, try adding pass cavities on your Tx before the duplexer input. That will help determine if the IM is originating in your PA. An isolator *may* help, but with channel 12 being the better part of 20 MHz away (about 10%), it may not afford full protection -- isolators don't have infinite bandwidth. Likewise, finding a 250 watt 220 MHz isolator may not be easy. I think I have some 220 isolators that came off a combiner (Sinclair), but doubt they're good for 250 watts judging by their size. [Stu] Agree, notches don't work well for that plus we don't have handy any cavities that will tune there. Have used up to two BP cavities in the TX path along with a 2-stage isolator. No difference in desense is observed. If you suspect a rusty bolt mix, use an alternate antenna for testing. If nothing else, try a quarter-wave whip (suitable for operation at your 250 watts TPO), even if it's just temporarily mounted on the tower (be sure it's at a sufficient height to prevent desense due to close proximity to the repeater itself). [Stu] We're presently on split antennas. One is at about 80 ft., the other is at about 15 ft. This improves the desense on the order of 6 dB. Another good possibility is IM in your receiver front end (or preamp, if you're using one). Again, pass cavities are your friend here. Attenuate the channel 12 signal as much as possible and see if it makes a difference. Have you looked at what sigs are reaching your receiver input on a spectrum analyzer? With 15 dB of desense, you should be able to see the culprit(s); it's not like they're going to be buried in the noise if it's causing 15 dB of desense. [Stu] The desense is significant with or without a preamp. Worse with but I can't find my notes to quote numbers. Used up to 2 BP cavities on RX with no perceptible difference in desense. Have also installed a DCI 4-pole filter on RX and TX with no effect. Have looked at the receiver input with a spectrum analyzer. The most significant signal is the one FM broadcast transmitter at the site. Running power down on it or turning it off has no effect on the desense. Our TX signal at our RX input is consistent with our measured duplexer isolation (about -88 dBc or -34 dBm). Within several hundred kHz of the RX frequency there are no detectable narrowband signals. Even that 1 dB of desense would give me some agita. I'd verify that the duplexers are properly tuned and the transmitter is clean before even starting down any other paths related to the channel 12 issue. IIRC, the Telewave cavities have adjustable coupling. If necessary, sacrifice a little extra loss for additional rejection if necessary. [Stu] I tuned the duplexer myself with a network analyzer and the transmitter looks clean. I have coupling set where I get about 1 dB through loss and the notches are at about 88 dB on TX and about 90 dB on RX. I also assume you're using all known-good interconnect cables (no foil+braid or other cables not suitable for duplex operation). [Stu] All cables are either Heliax or double braided. Are you using a Polyphaser or other type of surge arrestor? If so, try bypassing it. I've seen gas discharge tube type surge arrestors become noisemakers after absorbing a strike. [Stu] Yes
RE: [Repeater-Builder] Desense Problem on 222 MHz Repeater
--- Eric Lemmon [EMAIL PROTECTED] wrote: Stu, I think 250 watts is far too much power for that duplexer to properly isolate. Try running just your exciter, or try no more than 30 watts or so. My 220 repeater runs just 18 watts, and it is almost perfectly balanced. I am using the same Telewave duplexer, and I have zero desense. According to my CommShop program, you need more than 90 dB of isolation for 250 watts TX and 0.25 uV RX- and that's assuming a tube amplifier. The Telewave TPRD-2254 duplexer is spec'd at 85 dB, so it is borderline, even when perfectly tuned. You might try a sharply-tuned bandpass cavity to clean up the transmitter output, to see if sideband noise is causing the desense. Also, try a bandpass cavity on the RX input. As has been explained many times on this list, a BpBr duplexer has practically no bandpass effect, and what little effect there is, is very broad. 73, Eric Lemmon WB6FLY That was my thoughts also. Seems too much power for the ammount of isolation. Especially if a preamp is used on the receiver. I would like to know the modifications to the amplifier to get it to run that much power in repeater operation. I have one that will run around 500 watts but use it for ssb. As this was origionaly a 50 watt AM amp, I might see it running 100 to maybe 150 watts out in FM repeater service. Inside the amp are several pieces of coax. Is this double shielded or has it been changed to double shielding ? Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ
RE: [Repeater-Builder] Desense Problem on 222 MHz Repeater
Stu, I think 250 watts is far too much power for that duplexer to properly isolate. Try running just your exciter, or try no more than 30 watts or so. My 220 repeater runs just 18 watts, and it is almost perfectly balanced. I am using the same Telewave duplexer, and I have zero desense. According to my CommShop program, you need more than 90 dB of isolation for 250 watts TX and 0.25 uV RX- and that's assuming a tube amplifier. The Telewave TPRD-2254 duplexer is spec'd at 85 dB, so it is borderline, even when perfectly tuned. You might try a sharply-tuned bandpass cavity to clean up the transmitter output, to see if sideband noise is causing the desense. Also, try a bandpass cavity on the RX input. As has been explained many times on this list, a BpBr duplexer has practically no bandpass effect, and what little effect there is, is very broad. 73, Eric Lemmon WB6FLY -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of Stu Benner Sent: Friday, January 25, 2008 9:52 AM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Desense Problem on 222 MHz Repeater Our group has substantial technical knowledge and experience, but we've been just about beaten down by a problem with our repeater. A brief overview of our situation follows. We have a 222 MHz repeater comprised of a converted Micor mobile, Telewave TPRD-2254 BpBr duplexer, AM-6155 PA modified for class C operation at 250W, and a DB-264JJ antenna at 80 ft. fed by 1/2' Heliax on a commercial FM broadcast tower . With the duplexer terminated into a load, we have about 1 dB degradation in sensitivity when transmitting. However, with the antenna connected to the duplexer, we experience in excess 15 dB of desensitization. We have eliminated other narrowband transmitters and analog TV transmitters as contributing factors. We are left with a channel 12 digital TV transmitter at an adjacent site as a key contributor to the problem. Our hypothesis is that we have broadband IMD products from the mix of our transmitter and the DTV transmitter that are appearing in and near our receiver passband. Is it a rusty bolt problem or is there some other non-linear component somewhere on the site or in our system that is the mixing point - we don't know. I'd be interested in beginning a dialog with anyone who might be able to give us some further insight into this problem. Regards, Stu Benner W3STU Boonsboro, MD
Re: [Repeater-Builder] Desense Problem on 222 MHz Repeater
Couple of thoughts from one who is not to technically oriented. 1. Have you tried putting any notch filters between the duplexer and rcvr. to notch the TX freq. ? 2. Have you checked all of your cables and heliax connecters for proper installation ? I only ask this because I was having erratic performance and desense on a 440 rptr. and found some bad duplexer cable connector installations when I started wiggling and moving them around. Another time while checking Fwd/Ref. power and SWR on o 160' peice of 1/2 heliax that I thought was good (and getting satisfactory readings on my Bird mtr.), I pulled the connectors of each end to inspect them. I found one connector had been improperly installed, the heliax shield had been twisted inside the connector to the point where it was almost touching the center conductor. Even though there was no direct contact between shield and the center conductor the RF on xmit. was desensing the rcvr. when the xmtr. keyed up. Reinstalling the bad cables and connectors solve the desense problem for me. -- Doug N3DAB/WPRX486/WPJL709 Stu Benner [EMAIL PROTECTED] wrote: = Our group has substantial technical knowledge and experience, but we've been just about beaten down by a problem with our repeater. A brief overview of our situation follows. We have a 222 MHz repeater comprised of a converted Micor mobile, Telewave TPRD-2254 BpBr duplexer, AM-6155 PA modified for class C operation at 250W, and a DB-264JJ antenna at 80 ft. fed by 1/2' Heliax on a commercial FM broadcast tower . With the duplexer terminated into a load, we have about 1 dB degradation in sensitivity when transmitting. However, with the antenna connected to the duplexer, we experience in excess 15 dB of desensitization. We have eliminated other narrowband transmitters and analog TV transmitters as contributing factors. We are left with a channel 12 digital TV transmitter at an adjacent site as a key contributor to the problem. Our hypothesis is that we have broadband IMD products from the mix of our transmitter and the DTV transmitter that are appearing in and near our receiver passband. Is it a rusty bolt problem or is there some other non-linear component somewhere on the site or in our system that is the mixing point - we don't know. I'd be interested in beginning a dialog with anyone who might be able to give us some further insight into this problem. Regards, Stu Benner W3STU Boonsboro, MD
Re: [Repeater-Builder] Desense Problem on 222 MHz Repeater
Hey Jeff: I think there are some out there that might question agita. Haven't heard that term in a while (and don't want to have it). Ciao, Tony Jeff DePolo wrote: We have a 222 MHz repeater comprised of a converted Micor mobile, Telewave TPRD-2254 BpBr duplexer, AM-6155 PA modified for class C operation at 250W, and a DB-264JJ antenna at 80 ft. fed by 1/2' Heliax on a commercial FM broadcast tower . With the duplexer terminated into a load, we have about 1 dB degradation in sensitivity when transmitting. However, with the antenna connected to the duplexer, we experience in excess 15 dB of desensitization. We have eliminated other narrowband transmitters and analog TV transmitters as contributing factors. We are left with a channel 12 digital TV transmitter at an adjacent site as a key contributor to the problem. Our hypothesis is that we have broadband IMD products from the mix of our transmitter and the DTV transmitter that are appearing in and near our receiver passband. Is it a rusty bolt problem or is there some other non-linear component somewhere on the site or in our system that is the mixing point - we don't know. If you suspect IMD between the channel 12 DTV transmitter and your Tx carrier, work on attenuating the DTV signal. Using cavity notch filters to reject the entire 6 MHz of DTV isn't too practical, so instead, try adding pass cavities on your Tx before the duplexer input. That will help determine if the IM is originating in your PA. An isolator *may* help, but with channel 12 being the better part of 20 MHz away (about 10%), it may not afford full protection -- isolators don't have infinite bandwidth. Likewise, finding a 250 watt 220 MHz isolator may not be easy. I think I have some 220 isolators that came off a combiner (Sinclair), but doubt they're good for 250 watts judging by their size. If you suspect a rusty bolt mix, use an alternate antenna for testing. If nothing else, try a quarter-wave whip (suitable for operation at your 250 watts TPO), even if it's just temporarily mounted on the tower (be sure it's at a sufficient height to prevent desense due to close proximity to the repeater itself). Another good possibility is IM in your receiver front end (or preamp, if you're using one). Again, pass cavities are your friend here. Attenuate the channel 12 signal as much as possible and see if it makes a difference. Have you looked at what sigs are reaching your receiver input on a spectrum analyzer? With 15 dB of desense, you should be able to see the culprit(s); it's not like they're going to be buried in the noise if it's causing 15 dB of desense. Even that 1 dB of desense would give me some agita. I'd verify that the duplexers are properly tuned and the transmitter is clean before even starting down any other paths related to the channel 12 issue. IIRC, the Telewave cavities have adjustable coupling. If necessary, sacrifice a little extra loss for additional rejection if necessary. I also assume you're using all known-good interconnect cables (no foil+braid or other cables not suitable for duplex operation). Are you using a Polyphaser or other type of surge arrestor? If so, try bypassing it. I've seen gas discharge tube type surge arrestors become noisemakers after absorbing a strike. Has the VSWR changed at all on your antenna? If so, it could indicate water in a connector or the harness which will cause all kinds of grief, including wideband noise. Finally, does the desense change appreciably if you vary transmitter power output (it probably will). Do you any have desense when running on just exciter power? --- Jeff WN3A Yahoo! Groups Links
RE: [Repeater-Builder] Desense Problem on 222 MHz Repeater
We have a 222 MHz repeater comprised of a converted Micor mobile, Telewave TPRD-2254 BpBr duplexer, AM-6155 PA modified for class C operation at 250W, and a DB-264JJ antenna at 80 ft. fed by 1/2' Heliax on a commercial FM broadcast tower . With the duplexer terminated into a load, we have about 1 dB degradation in sensitivity when transmitting. However, with the antenna connected to the duplexer, we experience in excess 15 dB of desensitization. We have eliminated other narrowband transmitters and analog TV transmitters as contributing factors. We are left with a channel 12 digital TV transmitter at an adjacent site as a key contributor to the problem. Our hypothesis is that we have broadband IMD products from the mix of our transmitter and the DTV transmitter that are appearing in and near our receiver passband. Is it a rusty bolt problem or is there some other non-linear component somewhere on the site or in our system that is the mixing point - we don't know. If you suspect IMD between the channel 12 DTV transmitter and your Tx carrier, work on attenuating the DTV signal. Using cavity notch filters to reject the entire 6 MHz of DTV isn't too practical, so instead, try adding pass cavities on your Tx before the duplexer input. That will help determine if the IM is originating in your PA. An isolator *may* help, but with channel 12 being the better part of 20 MHz away (about 10%), it may not afford full protection -- isolators don't have infinite bandwidth. Likewise, finding a 250 watt 220 MHz isolator may not be easy. I think I have some 220 isolators that came off a combiner (Sinclair), but doubt they're good for 250 watts judging by their size. If you suspect a rusty bolt mix, use an alternate antenna for testing. If nothing else, try a quarter-wave whip (suitable for operation at your 250 watts TPO), even if it's just temporarily mounted on the tower (be sure it's at a sufficient height to prevent desense due to close proximity to the repeater itself). Another good possibility is IM in your receiver front end (or preamp, if you're using one). Again, pass cavities are your friend here. Attenuate the channel 12 signal as much as possible and see if it makes a difference. Have you looked at what sigs are reaching your receiver input on a spectrum analyzer? With 15 dB of desense, you should be able to see the culprit(s); it's not like they're going to be buried in the noise if it's causing 15 dB of desense. Even that 1 dB of desense would give me some agita. I'd verify that the duplexers are properly tuned and the transmitter is clean before even starting down any other paths related to the channel 12 issue. IIRC, the Telewave cavities have adjustable coupling. If necessary, sacrifice a little extra loss for additional rejection if necessary. I also assume you're using all known-good interconnect cables (no foil+braid or other cables not suitable for duplex operation). Are you using a Polyphaser or other type of surge arrestor? If so, try bypassing it. I've seen gas discharge tube type surge arrestors become noisemakers after absorbing a strike. Has the VSWR changed at all on your antenna? If so, it could indicate water in a connector or the harness which will cause all kinds of grief, including wideband noise. Finally, does the desense change appreciably if you vary transmitter power output (it probably will). Do you any have desense when running on just exciter power? --- Jeff WN3A
RE: [Repeater-Builder] Desense Problem on 222 MHz Repeater
Thanks to everyone for the replies so far. Please see comments embedded below. I think that they address most of the comments, questions, and recommendations posed by all to this point. If you suspect IMD between the channel 12 DTV transmitter and your Tx carrier, work on attenuating the DTV signal. Using cavity notch filters to reject the entire 6 MHz of DTV isn't too practical, so instead, try adding pass cavities on your Tx before the duplexer input. That will help determine if the IM is originating in your PA. An isolator *may* help, but with channel 12 being the better part of 20 MHz away (about 10%), it may not afford full protection -- isolators don't have infinite bandwidth. Likewise, finding a 250 watt 220 MHz isolator may not be easy. I think I have some 220 isolators that came off a combiner (Sinclair), but doubt they're good for 250 watts judging by their size. [Stu] Agree, notches don't work well for that plus we don't have handy any cavities that will tune there. Have used up to two BP cavities in the TX path along with a 2-stage isolator. No difference in desense is observed. If you suspect a rusty bolt mix, use an alternate antenna for testing. If nothing else, try a quarter-wave whip (suitable for operation at your 250 watts TPO), even if it's just temporarily mounted on the tower (be sure it's at a sufficient height to prevent desense due to close proximity to the repeater itself). [Stu] We're presently on split antennas. One is at about 80 ft., the other is at about 15 ft. This improves the desense on the order of 6 dB. Another good possibility is IM in your receiver front end (or preamp, if you're using one). Again, pass cavities are your friend here. Attenuate the channel 12 signal as much as possible and see if it makes a difference. Have you looked at what sigs are reaching your receiver input on a spectrum analyzer? With 15 dB of desense, you should be able to see the culprit(s); it's not like they're going to be buried in the noise if it's causing 15 dB of desense. [Stu] The desense is significant with or without a preamp. Worse with but I can't find my notes to quote numbers. Used up to 2 BP cavities on RX with no perceptable difference in desense. Have also installed a DCI 4-pole filter on RX and TX with no effect. Have looked at the receiver input with a spectrum analyzer. The most significant signal is the one FM broadcast transmitter at the site. Running power down on it or turning it off has no effect on the desense. Our TX signal at our RX input is consistent with our measured duplexer isolation (about -88 dBc or -34 dBm). Within several hundred kHz of the RX frequency there are no detectable narrowband signals. Even that 1 dB of desense would give me some agita. I'd verify that the duplexers are properly tuned and the transmitter is clean before even starting down any other paths related to the channel 12 issue. IIRC, the Telewave cavities have adjustable coupling. If necessary, sacrifice a little extra loss for additional rejection if necessary. [Stu] I tuned the duplexer myself with a network analyzer and the transmitter looks clean. I have coupling set where I get about 1 dB through loss and the notches are at about 88 dB on TX and about 90 dB on RX. I also assume you're using all known-good interconnect cables (no foil+braid or other cables not suitable for duplex operation). [Stu] All cables are either Heliax or double braided. Are you using a Polyphaser or other type of surge arrestor? If so, try bypassing it. I've seen gas discharge tube type surge arrestors become noisemakers after absorbing a strike. [Stu] Yes but there is no difference in desense when it is removed. Has the VSWR changed at all on your antenna? If so, it could indicate water in a connector or the harness which will cause all kinds of grief, including wideband noise. [Stu] The problem has existed since the repeater was installed. It exists whether we duplex on a DB264 at 80 feet or a G7-220 at 15 feet, both fed with Heliax Finally, does the desense change appreciably if you vary transmitter power output (it probably will). Do you any have desense when running on just exciter power? [Stu] The desense is roughly proportional to transmit power. Barely perceptable at 20W (exciter only) with split antennas. A little worse at 20W (exciter only) using one antenna. Have tried both tube-type and solid state amplifiers at various power levels. --- Jeff WN3A
Re: [Repeater-Builder] Desense Problem on 222 MHz Repeater
Any chance you can connect a spectrum analyzer to the duplexers receieve port? Make tests to look at the IF range and the receiver range with and without the transmitter active. This will tell you alot. For example on a 2m repeater we had a dirty SMPS in the vicinity of the antenna that opperated at approx. 600kHz. As soon as the transmitter came on at any level 2 sidebands were created + and - 600kHz from the transmitters carrier.
