Re: [Repeater-Builder] Understanding Intermod
Quite true Gary. There are a whole lot of situations that can mess up things for everyone, and the poisen pill does not have to be directly on site to be the problem. Milt - Original Message - From: Gary Glaenzer To: Repeater-Builder@yahoogroups.com Sent: Sunday, March 08, 2009 12:10 PM Subject: Re: [Repeater-Builder] Understanding Intermod Milt; Not to make light of your observation just the program spitting out all the combinations that COULD cause a problem But..this discussion brought to mind a statement made by a Motorola engineer at a school about multicouplers, splitters, combiners, etc back in 1984; they were discussing the multitude of repeaters on the John Hancock building (or maybe it was Sears Tower) in Chicago One 'wrong' frequency allowed onto a site can 'poison' the site for everyone I guess that means run the IM program BEFORE any new TX is added...not after problems crop up Gary - Original Message - From: Milt To: Repeater-Builder@yahoogroups.com Sent: Saturday, March 07, 2009 10:31 PM Subject: Re: [Repeater-Builder] Understanding Intermod Kevin, I recall a case where several UHF community repeaters were installed at a single site. This was not a pretty picture since there was no way to achive vertical seperation and transmitter combiners/receiver multicouplers were not as popular as they are today. An intermod study weas run on the site and at either the 5th or 7th order, every receiver on the site should have been experiencing intermod from a combination of all the transmitters. Of course this was not the case. It was just the program spitting out all the possible combinations that COULD cause a problem. All intermod programs just do the mathmatical combinations and report the outcomes, even outragous ones that are not likely to naturally occur. Some do a better job than others of taking into account things like receiver bandwidth and other factors. Actually finding the source where the mix is taking place much more difficult. The benefit of such programs is that they can easily do the math for you, but you have to decide which numbers are good possibilities and which ones are red herrings. Milt N3LTQ - Original Message - From: Kevin King kc6...@comcast.net To: Repeater-Builder@yahoogroups.com Sent: Saturday, March 07, 2009 8:59 PM Subject: RE: [Repeater-Builder] Understanding Intermod Bob Ok so much for the software. One other user today mentioned a possible issue with another repeater. 147.300. All I had been looking at with the software were the transmitters at his site. Ok so I plug that into the mix of xmiters in the software and bingo. A+B-C 146.70 + 146.70 - 147.30 = 146.100. The repeater 146.70 does have a circulator. Again this intermod is not my best area. I had the repeater owner keep the 146.700 xmitter up them had him hit the 147.30 with an HT and yep we got a burp of noise. But he heard at least 3 repeaters come up. So if this mix is the one that is causing all the issues, now we just need to find where the mix is occurring. Any ideas on where to look? As you can see in the full printout from the intermod tool. There are more that one set of possible offenders. -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of n...@no6b.com Sent: Saturday, March 07, 2009 3:47 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Understanding Intermod At 3/7/2009 08:59, you wrote: I did a analysis on a transmitter site using the TCS intermod tool to see what mixes could be causing some issues at this site. I ran the 1-3 order and 1 to 5 order. So I do not use these tools much and was wondering if any of my fellow engineers on here have used this tool and have comments on the output. For example this line from the output confuses me: A+B+C-D-E: 146.7 + 145.77000 + 145.09000 - 145.73000 - 145.73000 = 146.10 It is a hit right on the input but how do you minus 145.73 twice? Is this just an issue with the program I am using? I should say 146.700 + 145.770 + 145.090 - 2 * 145.730 = 146.100 It's actually an A+B+C-2D mix. Bob NO6B Yahoo! Groups Links Yahoo! Groups Links Checked by AVG - www.avg.com Version: 8.0.237 / Virus Database: 270.11.9/1989 - Release Date: 03/07
Re: [Repeater-Builder] Understanding Intermod
Kevin, I recall a case where several UHF community repeaters were installed at a single site. This was not a pretty picture since there was no way to achive vertical seperation and transmitter combiners/receiver multicouplers were not as popular as they are today. An intermod study weas run on the site and at either the 5th or 7th order, every receiver on the site should have been experiencing intermod from a combination of all the transmitters. Of course this was not the case. It was just the program spitting out all the possible combinations that COULD cause a problem. All intermod programs just do the mathmatical combinations and report the outcomes, even outragous ones that are not likely to naturally occur. Some do a better job than others of taking into account things like receiver bandwidth and other factors. Actually finding the source where the mix is taking place much more difficult. The benefit of such programs is that they can easily do the math for you, but you have to decide which numbers are good possibilities and which ones are red herrings. Milt N3LTQ - Original Message - From: Kevin King kc6...@comcast.net To: Repeater-Builder@yahoogroups.com Sent: Saturday, March 07, 2009 8:59 PM Subject: RE: [Repeater-Builder] Understanding Intermod Bob Ok so much for the software. One other user today mentioned a possible issue with another repeater. 147.300. All I had been looking at with the software were the transmitters at his site. Ok so I plug that into the mix of xmiters in the software and bingo. A+B-C 146.70 + 146.70 - 147.30 = 146.100. The repeater 146.70 does have a circulator. Again this intermod is not my best area. I had the repeater owner keep the 146.700 xmitter up them had him hit the 147.30 with an HT and yep we got a burp of noise. But he heard at least 3 repeaters come up. So if this mix is the one that is causing all the issues, now we just need to find where the mix is occurring. Any ideas on where to look? As you can see in the full printout from the intermod tool. There are more that one set of possible offenders. -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of n...@no6b.com Sent: Saturday, March 07, 2009 3:47 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Understanding Intermod At 3/7/2009 08:59, you wrote: I did a analysis on a transmitter site using the TCS intermod tool to see what mixes could be causing some issues at this site. I ran the 1-3 order and 1 to 5 order. So I do not use these tools much and was wondering if any of my fellow engineers on here have used this tool and have comments on the output. For example this line from the output confuses me: A+B+C-D-E: 146.7 + 145.77000 + 145.09000 - 145.73000 - 145.73000 = 146.10 It is a hit right on the input but how do you minus 145.73 twice? Is this just an issue with the program I am using? I should say 146.700 + 145.770 + 145.090 - 2 * 145.730 = 146.100 It's actually an A+B+C-2D mix. Bob NO6B Yahoo! Groups Links Yahoo! Groups Links
Re: [Repeater-Builder] Understanding Intermod
Milt; Not to make light of your observation just the program spitting out all the combinations that COULD cause a problem But..this discussion brought to mind a statement made by a Motorola engineer at a school about multicouplers, splitters, combiners, etc back in 1984; they were discussing the multitude of repeaters on the John Hancock building (or maybe it was Sears Tower) in Chicago One 'wrong' frequency allowed onto a site can 'poison' the site for everyone I guess that means run the IM program BEFORE any new TX is added...not after problems crop up Gary - Original Message - From: Milt To: Repeater-Builder@yahoogroups.com Sent: Saturday, March 07, 2009 10:31 PM Subject: Re: [Repeater-Builder] Understanding Intermod Kevin, I recall a case where several UHF community repeaters were installed at a single site. This was not a pretty picture since there was no way to achive vertical seperation and transmitter combiners/receiver multicouplers were not as popular as they are today. An intermod study weas run on the site and at either the 5th or 7th order, every receiver on the site should have been experiencing intermod from a combination of all the transmitters. Of course this was not the case. It was just the program spitting out all the possible combinations that COULD cause a problem. All intermod programs just do the mathmatical combinations and report the outcomes, even outragous ones that are not likely to naturally occur. Some do a better job than others of taking into account things like receiver bandwidth and other factors. Actually finding the source where the mix is taking place much more difficult. The benefit of such programs is that they can easily do the math for you, but you have to decide which numbers are good possibilities and which ones are red herrings. Milt N3LTQ - Original Message - From: Kevin King kc6...@comcast.net To: Repeater-Builder@yahoogroups.com Sent: Saturday, March 07, 2009 8:59 PM Subject: RE: [Repeater-Builder] Understanding Intermod Bob Ok so much for the software. One other user today mentioned a possible issue with another repeater. 147.300. All I had been looking at with the software were the transmitters at his site. Ok so I plug that into the mix of xmiters in the software and bingo. A+B-C 146.70 + 146.70 - 147.30 = 146.100. The repeater 146.70 does have a circulator. Again this intermod is not my best area. I had the repeater owner keep the 146.700 xmitter up them had him hit the 147.30 with an HT and yep we got a burp of noise. But he heard at least 3 repeaters come up. So if this mix is the one that is causing all the issues, now we just need to find where the mix is occurring. Any ideas on where to look? As you can see in the full printout from the intermod tool. There are more that one set of possible offenders. -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of n...@no6b.com Sent: Saturday, March 07, 2009 3:47 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Understanding Intermod At 3/7/2009 08:59, you wrote: I did a analysis on a transmitter site using the TCS intermod tool to see what mixes could be causing some issues at this site. I ran the 1-3 order and 1 to 5 order. So I do not use these tools much and was wondering if any of my fellow engineers on here have used this tool and have comments on the output. For example this line from the output confuses me: A+B+C-D-E: 146.7 + 145.77000 + 145.09000 - 145.73000 - 145.73000 = 146.10 It is a hit right on the input but how do you minus 145.73 twice? Is this just an issue with the program I am using? I should say 146.700 + 145.770 + 145.090 - 2 * 145.730 = 146.100 It's actually an A+B+C-2D mix. Bob NO6B Yahoo! Groups Links Yahoo! Groups Links -- Checked by AVG - www.avg.com Version: 8.0.237 / Virus Database: 270.11.9/1989 - Release Date: 03/07/09 18:43:00
Re: [Repeater-Builder] Understanding Intermod
At 3/7/2009 20:31, you wrote: Kevin, I recall a case where several UHF community repeaters were installed at a single site. This was not a pretty picture since there was no way to achive vertical seperation and transmitter combiners/receiver multicouplers were not as popular as they are today. An intermod study weas run on the site and at either the 5th or 7th order, every receiver on the site should have been experiencing intermod from a combination of all the transmitters. Of course this was not the case. It was just the program spitting out all the possible combinations that COULD cause a problem. ...which is why I've never bothered with IMD programs. Put it up; if it works it works. If there's IMD usually the demoded audio in the mix will tell you who's involved in the mix. Even then, knowing what frequencies are involved doesn't help you determine which rusty tower joint is actually producing the mix. The last mix problem I was faced with ended up being a 2A-B, with my output being the 2A the B a commercial repeater 5 MHz above my output. The mix ended up occurring in the front end of an Icom VHF/UHF multimode radio plugged into an antenna about 30' from the repeater antenna; unplugging the UHF port of the radio eliminated it. Knowing the frequencies involved in the mix played no part in the determination of the source. Bob NO6B
Re: [Repeater-Builder] Understanding Intermod
At 3/7/2009 08:59, you wrote: I did a analysis on a transmitter site using the TCS intermod tool to see what mixes could be causing some issues at this site. I ran the 1-3 order and 1 to 5 order. So I do not use these tools much and was wondering if any of my fellow engineers on here have used this tool and have comments on the output. For example this line from the output confuses me: A+B+C-D-E: 146.7 + 145.77000 + 145.09000 - 145.73000 - 145.73000 = 146.10 It is a hit right on the input but how do you minus 145.73 twice? Is this just an issue with the program I am using? I should say 146.700 + 145.770 + 145.090 - 2 * 145.730 = 146.100 It's actually an A+B+C-2D mix. Bob NO6B
RE: [Repeater-Builder] Understanding Intermod
Bob Ok so much for the software. One other user today mentioned a possible issue with another repeater. 147.300. All I had been looking at with the software were the transmitters at his site. Ok so I plug that into the mix of xmiters in the software and bingo. A+B-C 146.70 + 146.70 - 147.30 = 146.100. The repeater 146.70 does have a circulator. Again this intermod is not my best area. I had the repeater owner keep the 146.700 xmitter up them had him hit the 147.30 with an HT and yep we got a burp of noise. But he heard at least 3 repeaters come up. So if this mix is the one that is causing all the issues, now we just need to find where the mix is occurring. Any ideas on where to look? As you can see in the full printout from the intermod tool. There are more that one set of possible offenders. -Original Message- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of n...@no6b.com Sent: Saturday, March 07, 2009 3:47 PM To: Repeater-Builder@yahoogroups.com Subject: Re: [Repeater-Builder] Understanding Intermod At 3/7/2009 08:59, you wrote: I did a analysis on a transmitter site using the TCS intermod tool to see what mixes could be causing some issues at this site. I ran the 1-3 order and 1 to 5 order. So I do not use these tools much and was wondering if any of my fellow engineers on here have used this tool and have comments on the output. For example this line from the output confuses me: A+B+C-D-E: 146.7 + 145.77000 + 145.09000 - 145.73000 - 145.73000 = 146.10 It is a hit right on the input but how do you minus 145.73 twice? Is this just an issue with the program I am using? I should say 146.700 + 145.770 + 145.090 - 2 * 145.730 = 146.100 It's actually an A+B+C-2D mix. Bob NO6B Yahoo! Groups Links
Re: [Repeater-Builder] Understanding Intermod
On Mar 7, 2009, at 6:59 PM, Kevin King wrote: Bob Ok so much for the software. One other user today mentioned a possible issue with another repeater. 147.300. All I had been looking at with the software were the transmitters at his site. Ok so I plug that into the mix of xmiters in the software and bingo. A+B-C 146.70 + 146.70 - 147.30 = 146.100. The repeater 146.70 does have a circulator. Doesn't matter if it has a circulator or not. That 146.70 signal doesn't have to mix in THEIR PA, or even in a PA at all, to be a mess. All you need is something outside on the tower -- rusty bolts, bad guy wire connection, tin on the roof touching at seams, colinear antenna coming apart internally, cracks in aluminum dipole antennas, corrosion of just about anything -- to act as a diode at RF. When you put more than one repeater at a site with the same OFFSET, you're set up for that kind of mixing. Multiple Amateur machines at a site means, it's ready to go. (Check the math sometime... three 600 KHz offset VHF machines, three 5 MHz offset UHF machines... doesn't matter what frequency. Once you have two on the same offset, you're set up for mixes external to both systems to hit whoever's input is unlucky enough to get hit.) To find out which system is really mixing, you may have to either a) temporarily lower the power output of systems one at a time to see when the mix diminishes, or b) the more active way... be there when it happens and slap the power switch/TX disable switch and see if it instantly stops when you apply the kill switch to any particular transmitter. (This of course, doesn't make whoever was talking on it real impressed with your technical skill, since they have no idea why you just bounced their repeater. And you need buy-in from the folks who own the things... to do stuff like that.) Lowering power on systems involved in the mix can give great clues to where the mix is physically occuring. For example, if you drop to exciter power and the mix is still happening... and you can reproduce it. Either the antennas are high-gain and RIGHT on top of each other on the tower... or the mix is being amplified somewhere. Start looking at PA's and receive pre-amps... If the mix disappears from the receiver being hit, with a very slight reduction in power of one of the mixing partners, the problem is not likely happening in an active component. But those are just hints, there's too many variables to know 100% at that point. It's just some of the homework you have to do. The fact that it seems to happen only in cold weather would tend to indicate to me, that it's a metal to metal joint that pulls apart when the metal gets cold and shrinks. Fixing that type of problem can rapidly turn into a hunt for a needle in a haystack. Been there done that. It sucks. Looking at all the transmitters for noise with a spectrum analyzer is also a key homework component. Make sure something simple just isn't broken... if a dirty TX is at the site, all bets are off until it's fixed. And of course, always think about what changed right when this started? Did someone up their power? Put up a new antenna? New feedline? Is there a new tenant on the site? Did anything about the tower or antenna structure get changed? These and similar questions can all be clues to the ever-important question -- When did this start? The software will spit out so many possibilities it'll make your head spin -- you have to add those to the other things you know about the site, and it's just tough... no matter how you slice it. Nate WY0X
