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crosstalk in digital

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#1
we currently having a problem with crosstalk and most of the users complain about it.

actually i dont have an idea if the portables has the problem if it really switch automatically in different channel or if it the system. We invite some technical that could help us and they said that the receive and transmit is reverse and the other two duplexer has low power than the other two. Before we invite other people but they didnt tell us this thing maybe because they supplied us the duplexer. And now they recommend us to reverse the frequency and replace the duplexer. Im just wondering if this is really the cause of the problem or if the recommendation is really the solution to the problem?

the setup is there's 4 repeater in the same place with duplexer with one antenna each.

sample frequency

tx rx
434.025 429.025
427.35 422.35
436.025 431.025
439.55 431.025

439.5 432.5


Thanks
 

N1GTL

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#2
I'm kind of confused as to what your asking. What do you mean "crosstalk"?

If you've had a radio shop in, I would hope they tested the output power of each repeater and measured the signal required to key it up (sensitivity). Once those things are verified, measure those same things with the duplexer connected. If the repeaters and duplexers are the same make and model, the output power should be the same. If not, the duplexers may not be tuned correctly. Measure the SWR between the duplexer and the transmitter using dummy load in place of an antenna. If the duplexer is perfect, there should be nothing reflected. Move to the feed line and antennas.

Also, I am not sure where you are but those would be some oddball frequencies here. The first 3 are 5 mhz splits which are normal. The next two you list are 8mhz and 7 mhz splits?
 
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#3
i mean they hear other channel but the selector is in their own group or channel..

they measured it but not with dummy load, they used the existing cables and antennas when measuring the swr and power. The output power of the two repeater is 25 watts while its swr is 1.5 and the other two is 10 watts and the swr is 2.1.

So if in case that the repeater side has the problem does that mean that it can cause that kind of problem?
 
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#4
i mean they hear other channel but the selector is in their own group or channel..

they measured it but not with dummy load, they used the existing cables and antennas when measuring the swr and power. The output power of the two repeater is 25 watts while its swr is 1.5 and the other two is 10 watts and the swr is 2.1.

So if in case that the repeater side has the problem does that mean that it can cause that kind of problem?
 
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#8
the separation distance of each antenna is not that big maybe 3 to 5 feet apart, the tower is not that high so spacing is a problem
 
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#9
Are your color codes and talkgroups on different numbers? Like in the analog days, we used different PL tones to cover up poor rf engineering...... sounds lime you have a mess on your hands. What part of the world are you in? TT
 
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#10
yes they are, each repeater has different color codes as well as group id. In portables, channel color code is selected according to the repeaters color code as well as the time/repeaters slot. Im not an engineer so i cant tell really tell everything i just based on what i saw , and i said we hire consultant to help us and i just like to verify if that's really the problem.
 

buddrousa

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#12
the separation distance of each antenna is not that big maybe 3 to 5 feet apart, the tower is not that high so spacing is a problem
Then the bad news is you are going to have problems until you get the proper antenna spacing or move the other repeaters to other sites. The other thing you can do is buy a RX TX combiner set it up proper and put all the repeaters on the same antenna. I will also point out look at the cell towers near you they cover a 360 path at one height then about 40 to 60 foot down the tower is another group of antennas this is so they do not receive interference from the set of antennas above.
 
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902

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#13
Jen02,

I have experience in identifying and mitigating mixing products - which is what this sounds like to me. Before I (or any of our other folks who've worked on this kind of thing) can drill into the issue, I need more information:

Where are you? US or Canada?
Is this an amateur radio, public safety, or industrial system?
What data protocol are you using? Have you tried using different NAC or color code values for each system?
What types of repeaters are you using? - What is their power level and receiver sensitivity?
Are you using IMD suppression panels (circulator) between your transmitter and duplexer?
Are you using a harmonic filter directly after the circulator?
Does your site include rusty, corroded, or oxidized hardware?
What kind of duplexers are you using (make and model, please - or are you combining your transmitters and using a receiver multicoupler)?
Your separation - is that vertical or horizontal (or a combination of)?
What kind of coaxial cable are you using from the duplexers to the antennas?
What kind of coaxial cable are you using from the repeaters to the duplexers?
What kind of antennas are you using (and proximity to tower, if applicable)?

I know these are a lot of questions, but all of this information is relevant to the issue and possible solution.
 
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#14

mikewazowski

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#15
I will also point out look at the cell towers near you they cover a 360 path at one height then about 40 to 60 foot down the tower is another group of antennas this is so they do not receive interference from the set of antennas above.
Sorry but this is not the case.

The only time we locate antennas further down the tower is to focus on an area closer in to the tower.

Since the OP is running different colour codes on each repeater, antenna spacing is more than likely not the issue.
 
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#16
Jen02,

I have experience in identifying and mitigating mixing products - which is what this sounds like to me. Before I (or any of our other folks who've worked on this kind of thing) can drill into the issue, I need more information:

Where are you? US or Canada?
Is this an amateur radio, public safety, or industrial system?
What data protocol are you using? Have you tried using different NAC or color code values for each system?
What types of repeaters are you using? - What is their power level and receiver sensitivity?
Are you using IMD suppression panels (circulator) between your transmitter and duplexer?
Are you using a harmonic filter directly after the circulator?
Does your site include rusty, corroded, or oxidized hardware?
What kind of duplexers are you using (make and model, please - or are you combining your transmitters and using a receiver multicoupler)?
Your separation - is that vertical or horizontal (or a combination of)?
What kind of coaxial cable are you using from the duplexers to the antennas?
What kind of coaxial cable are you using from the repeaters to the duplexers?
What kind of antennas are you using (and proximity to tower, if applicable)?

I know these are a lot of questions, but all of this information is relevant to the issue and possible solution.

Location: Southeast Asia
Oil Refinery
We used color code and its different in each repeater
Repeater:Mototrbo R8200
I only a saw a duplexer in each repeater, i didnt take note of the model but it has four cyclinder tube i dont know the name
Antenna Separation: Horizontal
Cable used from duplexer to antenna : Heliax cable
Cable used repeater to duplexer: mixed the other repeater has rg-8 cable and the other has thin coaxial cable
 

902

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#17
Location: Southeast Asia
Oil Refinery
We used color code and its different in each repeater
Repeater:Mototrbo R8200
I only a saw a duplexer in each repeater, i didnt take note of the model but it has four cyclinder tube i dont know the name
Antenna Separation: Horizontal
Cable used from duplexer to antenna : Heliax cable
Cable used repeater to duplexer: mixed the other repeater has rg-8 cable and the other has thin coaxial cable
Thank you. My knowledge of DMR is limited. I'm only a casual user of it on amateur radio, and that is infrequently.

With respect to the RF stuff, I would replace the RG-8 cable with RG-214 and use silver plated connectors, rather than nickel plating.

The smaller cables, if they are 1/4" FSJ or RG-142/RG-400 are fine. If they are LMR-240 or RG-8X or even LMR-400 (NOT the same as RG-400!), I would replace those, as the gaps between the foil and the woven braid of the shield may produce arcing that would impact duplex service.

I would run each of the frequencies in a mixing product analysis program, such as ComSitePro (which is no longer supported and has been cut loose by Black and Veatch) to see if the arithmetic products of the transmitters (accounting for more deviation in harmonics and for receiver passbands) to see whether they end up on a receiver frequency. You may need to install isolation devices between your transmitter ports and duplexers or literally move your antennas to achieve greater physical separation.

Finally, I would look to vertically separate the antennas. Vertical separation provides much higher isolation than horizontal separation, and the same amount of isolation can be achieved in tens of feet in vertical, whereas horizontal would require hundreds of feet. Here is an article on that separation: Isolation between the repeater receiver and RF sources

Good luck!
 
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