InterMod/Mix Image or ?

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MassFD
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InterMod/Mix Image or ?

Post by MassFD »

I have a site that has devoloped a problem in cold weather, since it went under 10 degrees the last few days I have a 470 repeater that is geting into the receiver of another 470 repeater (473 input) that is 3.075mhz above the original 470xmit.

There are mutilple 470 repeaters with 473 inputs on this site but it only gets into the one that is 3.075 removed from the others Xmitter. It most times sounds like it is dead on this input frequency when it is 10degrees outside and sounds like it starts to go off freq as it warms up outside. The problem repeater has been replaced with the problem remaining

Now for the fun stuff. Also at this site is an Airaya 4.9ghz microwave link.When I shut the link down the problem goes away.

Thinking It was the Airaya throwing some kind of spur we changed it's operation frequency from 4.9 to 5.3 and the problem remains. As it is a backup link we just shut it down and the repeaters have been operating with no problems since.

Now for the real question is this intermod, a mix image, a spur or something else and how would I find it? We have a spectum analiser (does not go up to 4.9 ghz) but have no idea what frequency range to look in as in would a 470 + a 3.075(generated by the microwave) = the 473 input of the other repeater.

All the equipment has been in place with no problems for years, it only started with the recent cold snap. The Airaya equipment is located on the tower exposed to the cold with only a Cat.5 and power injector in the radio room (heated)


So what should I look for??
Cause Motorola said so that's why
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KI4M
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Re: InterMod/Mix Image or ?

Post by KI4M »

First thing I have to ask is are you running any type of circulator/LPF on the output of you P.A.? If it is only doing it when your transmitter is keyed there might be some sort of mix going on in your P.A. That being said it doesn't make any sense that it only occurs during really cold weather if that is the case.

Also another thing that comes to mind could there be some really bad L.O. leakage in the uWave radio that is somehow being radiated out and mixed with?
MassFD
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Re: InterMod/Mix Image or ?

Post by MassFD »

I have to say yes to the circ and filters on the Xmitter, It's a Quantar. The receiver on the affected repeater is a CDM (Has been replaced) and it has a Pass/Reject duplexer in front of it.

I am thinking more along Microprocessor noise on the Microwave as I think the LO freq would change when I changed te freq from 4.9 to 5.3 ghz
Cause Motorola said so that's why
Jim202
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Re: InterMod/Mix Image or ?

Post by Jim202 »

Cold weather temp problems can be a real pain to locate. Now that you seemed to find part of your answer by powering down the microwave, you need to narrow it down even further.

I would start by trying to figure out if it is some sort of an IP connection problem with the microwave. if not, then take a heat gun and work through on chassis at a time to find the source. Once you find it, can you narrow it down to components in that chassis?

maybe a solution would be to just put a light bulb under the chassis causing the problem. Put in a thermostat to turn on the lamp when you reach the cold temp point where the issue starts.

Jim
MassFD
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Re: InterMod/Mix Image or ?

Post by MassFD »

Jim, Yea I would like to heat the microwave but it's 100 feet up the tower, was thinking about some kind of heater. They do make self regulating cabinet heaters but that would require running power up the tower as the micro runs on POE, not going to work for a heater.

Just looking to find a way to identify the freq the micro is producing that seems to me to be mixing with the 470 to produce 473.

If it was not so cold I would put a dummy load on the microwave output and see if it is something being radiated out the antenna but it's to cold to fly!!
I guess I could vary the power output on the micro and see if the 473 level changes with it.


I could mask the entire thing with a PL change but I do not want to leave something on the tower that is not operating properly as I could cause problems at at someone elses site. We do have a full blown P25 site 1/8 mile from us for the local PD, would not want to cause them any problems
Cause Motorola said so that's why
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Bill_G
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Re: InterMod/Mix Image or ?

Post by Bill_G »

If it sounds like it's dead on, then it probably is. And if it's only happening when the Airaya is on, then it's probably radiating from the microwave housing and getting into your master rx antenna up on the tower where there is probably little physical separation. You can prove that by pulling the rx ant line of the affected rcvr to see if it quits. You might also be able to see it on a spec an out of the multicoupler. Emphasis on might because most spec analyzers have a noise floor higher than -100db, and most rcvrs work down to -118db no problem. So, the spur might be below the spec an sensitivity, but hitting the rcvr perfect. You can determine the amount of degradation with an effective sensitivity test. As for the solution, I think you've already found it - replace the outdoor unit on the Airaya when the weather improves.

It's not the first time I've seen this. Ethernet anything throws off a lot of trash across the bands. I've had perfectly quiet VHF sites suddenly have +10db of site noise because someone put up a wifi link. I've had a wireless camera land a dead carrier on a railroad channel that could be heard for a quarter mile. I've found public wifi 5.8 backhauls that swamp the gps band. I've found an ethernet switch that clobbered a station tapout rcvr, and ip phones that made the bda oscillate. This stuff is here to stay, and all we can do is locate it, and kill it.
MassFD
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Re: InterMod/Mix Image or ?

Post by MassFD »

Guess I know what I have to do, just for kicks I fired the Airaya up today and since the temp was in the 20's the problem was gone.

Bill, no master antenna system, each repeater has it's own antenna system. With the antenna off the receiver it does not hear the 470, also tryed it on an inverted antenna that is 10' below the original antenna thats located on top with the Airaya antennas but closer to the Airaya box and the quieting was about the same

Still not sure if the mix freq is being radiated by the Airaya antenna or the system box 10' below.


Any idea what fequency I would be looking for that would add with the 470 to produce a 473 signal
Cause Motorola said so that's why
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Bill_G
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Re: InterMod/Mix Image or ?

Post by Bill_G »

My crystal ball is broken. (big grin) But!

Is the offending signal constant or intermittent?

If it's constant, then the Airaya is generating a birdie, and probably the sole offender. If it's intermittent, then it may coincide with another transmitter at the site. Hopefully you access to the entire site, all the towers, all the transmitters, etc. Otherwise, you have just half the equation. Which is still good news. Still the same solution. Too bad the Airaya has gone bad. When the weather improves, you could always swap ends, and cross your fingers the opposing Airaya ain't broken too.
MassFD
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Re: InterMod/Mix Image or ?

Post by MassFD »

Bill, when the outdoor temp is low the problem is constant it only needs the 470 from the other system to generate the 473 and bring up the other repeater so I assume something generated by the Airaya is mixing with the 470.

I know it will not effect the outcome (replacing the Airaya) but I would like to see the signal anyway to see if it drifts with the outdoor temp or starts and stops at a temp.
Cause Motorola said so that's why
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Bill_G
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Re: InterMod/Mix Image or ?

Post by Bill_G »

Since intermod can be formed by signals mixing algebraically, it could be a A+B-C kind of problem with the Airaya contributing the B-C so that you get this constant dead carrier thrashing your rcvr only when it's cold enough to deform something in the Airaya. If you are really interested in finding it, you'll have to put your spec an on the rx antenna centered on the affected rx freq with a span that could be hundreds of megs wide watching for changes as you turn the Airaya on and off.

And of course you have to do it when the problem is occurring.

And your spec an has to be sensitive enough to see the offender without getting clobbered by other nearby transmitters.

The real problem might be PIM (passive intermod) caused by physical deformation in an antenna or line as the temperatures drop. It may be caused by the Airaya housing, the tower structure, an antenna or the connector to the antenna, or even the compound fence. Read the provided link for PIM test procedures. It may be advisable to conduct a full site return loss, DTF, and PIM tests of all lines and antennas when time permits. The cold may have uncovered some gremlins, or created some.
MassFD
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Re: InterMod/Mix Image or ?

Post by MassFD »

Bill, it gets deep real fast. I am just glad it's something at my site, I went into this thinking it could be anyware. I could just see knocking at a site neighbors door and telling them their swiming pool is causing radio interferance.

It does seem to me that if it was an Araya antenna problem it would have changed when I changed from 4.9 to 5.3 ghz as a test. I am beting on a microprocessor in the Airaya system box as the cause.

Anyway, most of the questions have been answered, I was just looking to educate my self for the future on how frequencys mix to cause problems like this
Cause Motorola said so that's why
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Bill_G
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Re: InterMod/Mix Image or ?

Post by Bill_G »

Well, you're probably right that it's something other than the rf section in the Airaya that is mixing with something else, and reradiating to the affected rx antenna. And it's something that changes only when it gets cold enough. But, locating that could be difficult. You did a great job of identifying the Airaya as a culprit. But, that's about as far as you can go without getting into the weeds. The physics of it can be daunting, and only the guys in the pocket protector league really understand it. The cell providers go after it with a vengeance because it effects their bottom line. But, they have the money to chase it. Kind of important, that part. Most of us just hope to get lucky enough to find it, kill it, and get out of Dodge without losing too much skin.
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FMROB
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Re: InterMod/Mix Image or ?

Post by FMROB »

MassFD wrote:I have a site that has devoloped a problem in cold weather, since it went under 10 degrees the last few days I have a 470 repeater that is geting into the receiver of another 470 repeater (473 input) that is 3.075mhz above the original 470xmit.

There are mutilple 470 repeaters with 473 inputs on this site but it only gets into the one that is 3.075 removed from the others Xmitter. It most times sounds like it is dead on this input frequency when it is 10degrees outside and sounds like it starts to go off freq as it warms up outside. The problem repeater has been replaced with the problem remaining

Now for the fun stuff. Also at this site is an Airaya 4.9ghz microwave link.When I shut the link down the problem goes away.

Thinking It was the Airaya throwing some kind of spur we changed it's operation frequency from 4.9 to 5.3 and the problem remains. As it is a backup link we just shut it down and the repeaters have been operating with no problems since.

Now for the real question is this intermod, a mix image, a spur or something else and how would I find it? We have a spectum analiser (does not go up to 4.9 ghz) but have no idea what frequency range to look in as in would a 470 + a 3.075(generated by the microwave) = the 473 input of the other repeater.

All the equipment has been in place with no problems for years, it only started with the recent cold snap. The Airaya equipment is located on the tower exposed to the cold with only a Cat.5 and power injector in the radio room (heated)


So what should I look for??

I am going to +10 on what the other guys have stated. I have tired more than a few flavors of ethernet radios from many differnet quality manufacturers. Most of them are less than desireable when it comes to just plastering the site with noise that is everywhere. This is especially true with the 900 mhz equipment. I/we have noticed that tower mounted equipment causes more interference then shelf mounted radios, and during cold weather the tower top radios are prone to just crap out or go ballastic with noise.

My thought is that all sites should have filtering and circulators as good practice. The installation of a micorwave hop should not cause you to have to provide extra filtering, especially on the front end of your equipment. I certainly dont want any extra loss on my RX side.

With that in mind, and within our area considering most poles are about 120' or less due to topography and local needs, never go with tower mounted equipment. It is the death of all good systems, lol. I have all together stopped using tower mounted radios, it's just prohibitive for many reasons.

Get a good quality dual polarized antenna to fit your needs, two runs of LMR600 and a short run into the conditioned building to a connectorized radio and your good to go. Work on the radio like a gentlemen, reduce temperature induced noise and it is just easier to diagnose problems.

My warning label::: I say all of the above based solely on loosing hair and a few years off my life messing around with many brands and flavors of links and radios. Good brands including rad, ubiquiti, lynx, solectek, exalt, bridgwave, etc. They all have there good and bad points. Obviously a link budget needs to be assesed and path calc before introducing 100'+ of cable into a link, but I can tell you we now try to standardize on this method. We have some long links 10+ miles in 4.9 using 100+ feet of LMR on both side to 2' dishes and they are rock soild with high thruput. For your application you should have no issues at all.

Rob
MassFD
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Re: InterMod/Mix Image or ?

Post by MassFD »

Rob, I agree. It would be nice to have all the radios in the equipment room. At the time we went Airaya (about 8 years ago) they did have that option but thought we would do better with tower top equipment.

The Airaya as at it's EOL anyway as it does not have the bandwidth to support the new Video systems installed last year, it's been limping along badly overloaded.

We started to change over to Dark Fiber between buildings on Jan 2nd so the loss of the Airaya realy does not hurt us. We will leave it on the tower as a backup to the fiber but only power it up if needed.

The Fiber realy flys compaired to the Airaya
Cause Motorola said so that's why
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FMROB
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Re: InterMod/Mix Image or ?

Post by FMROB »

Kev, I remember when you guys put them up. You got a good run, and agreed on the EOL on that equipment. Fiber is awesome, most areas dont' have dark fiber available yet so we use alot of micro out east. We have even started running SM fiber between the dispatch office and back end within a bulding in addition to the cat5.

Rob
dag
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Re: InterMod/Mix Image or ?

Post by dag »

I am curious about the type cable used to feed the Airayan. Is it shielded and if so properly grounded? You may have this issue no matter what you do to the equipment if the CAT5 cable is not shielded with a proper ground.

Too many times cheap unshielded CAT5 is used on towers by folks trying to save a little money and not knowing any better.
David Gosselin
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MassFD
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Re: InterMod/Mix Image or ?

Post by MassFD »

It's a good point, the cable is outdoor rated gel filled but is not shelded. It does however enter a steel monopole within 2' of the outdoor unit so I would think there would be some shelding there.

It's not your standard ethernet noise (Chunk Chunk Chunk), It's a dead quiet carrier mixing with my 470 xmit to produce a 473 frequency. The noise is there even with no network traffic (cable disconnected from switch)

As said before, it only happens when it's real cold outside.
Cause Motorola said so that's why
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