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CTCSS vs. CDCSS/DCS for high noise/RFI environments?

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BlueDevil

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Any input on whether CTCSS or CDCSS/DCS is better for high noise/RFI environments? I was thinking that CTCSS may work slightly better in this situation since it is not a digital format.

I am assisting a client in cleaning up a mess where another vendor installed a VHF Conventional Analog Repeater in a networking closet of a school that is producing a significant amount of noise/RFI in the immediate area of the repeater. I have cleaned it up a little bit with better double shielded coax running between the duplexer and the receiver. However it still isn't performing to the standard I believe it should be. I believe the receiver is partially deaf due to the noise/RFI in the immediate area. The other part to this equation is that the previous vendor used LMR400 as the feed line from the duplexer to the antenna.

The organization didn't anticipate or budget for a really expensive or costly system so I am doing what I can to try and get the repeater to operate at satisfactory performance without starting all over from scratch.

Thoughts/Suggestions/Ideas?
 

BlueDevil

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Yeah, we may end up moving the repeater however I figured I would try as many things as I could before immediately ripping everything out.

Right now their system is using DCS code on their frequencies.

I was thinking about switching them to a PL/CTCSS tone to see if that was easier for the receiver to decode.
 

BlueDevil

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The repeater is a Motorola SLR5700 running VHF Conventional Analog Voice only.
 

RKG

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Not having encountered (or tested) the precise situation you describe, I'm nonetheless inclined to think that there is no difference between PL receiver qualification and DPL receiver qualification insofar as it affects the performance of your receiver in a noisy environment.

Historically, there has always been a phenomenon known as "PL talkoff" or "PL roll off" where a speaker's voice could cause a PL-gated receiver to lose PL detection momentarily. The solution was an anti-rolloff timer, which effectively negated receiver shutdown upon loss of PL detection, at least for the duration of the anti-rolloff timer. In all my years, I never encountered PL rolloff, but the opinion of the wise old dudes I worked with was that DPL was less susceptible of rolloff than PL.

I suspect your situation has more to do with receiver desense, which causes the receiver to lose RF qualification, versus PL or DPL qualification. My vote is with the fellow who recomendd moving the receiver.
 

RFI-EMI-GUY

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I certainly agree about moving away from the network closet AND wiring. I have some experience with DCS and on channel interference. The DCS creates annoying warbling when beating against an interfering carrier. As far as reliability, though DCS was designed to be a very reliable signalling format,

DPL is designed so that out of every three cycles of the word, only one is required for decode.

My suspicion is that persistent interference can render it unreliable compared to CTCSS.


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chief21

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In my experience, DCS is very robust and reliable. It appears that what you're hearing from most posters is that any decoding problems are most likely related to the immediate environment of the repeater and the resulting desence/noise. As I understand it, the SLR 5700 is a business class repeater, so the receiver might not be as protected as a typical public safety model. Probably best to move it to a more RF-quiet location.

Suggestion: If this is straightforward install, move the repeater temporarily and test for improvement before you commit to a complete relocation.

John
 

RFI-EMI-GUY

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The repeater is a Motorola SLR5700 running VHF Conventional Analog Voice only.
Where is the antenna located in this installation? Equally important is the isolation of the antenna from the network closet. RF radiating from the network equipment and cables. Also TX energy mixing with the network and re radiating back to the receiver.

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jim202

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You probably can't reduce the data network noise to a level that will allow the radio repeater system to function as it should. Look for a new home for the repeater.

With that said, let me point out another major noise problem that will cause you untold hours looking to resolve. Do a search on the Internet about using LMR400 for repeater service. It may work well for a couple of years. But once moisture starts getting under the jacket of that coax cable, your in for the ride of your life trying to resolve noise on the receiver.

What is taking place is that corrosion starts to set in between the aluminum foil wrap used as the first layer of shielding. Then on top of it is a copper braid. The dissimilar materials oxidize and start to create a small diode action and start to rectify the current flow. This causes wide band noise that makes the radio repeater receiver become deaf. The solution is to junk the feed line and any jumpers made from the LMR400. Go to a "heliax" type coax cable. It will cost more, but you won't have the noise problem in a couple of years after installing the cable.

Now you will see postings from people that say you can weather proof the connectors and not get this problem. Yup the weather proofing helps, but the cable will still have moisture leaking into it given enough time.

The other problem areas that can cause radio noise to the receiver is rusted metal items neat the antenna. Loose metal moving around neat the antenna. Loose gutters and down spouts. The list goes on and it takes a keen nose and searching to find all the causes.

Good luck on your efforts. But I would start by moving the repeater and getting rid of the LMR400 coax cable. Don't forget that a bad antenna can also cause problems. You didn't even mention what kind of antenna the repeater has on the other end of the coax cable.
 

jonwienke

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DCS is less likely to randomly trigger the repeater than CTCSS, because it is more complex.

But that is a separate issue than receiver desense due to high RF interference.
 

BlueDevil

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Thanks for all the feedback! The vendor who originally installed the system didn't do a very good job. They are a well known communications company who should have known better than to put the repeater in the networking closet. They should have also known better than to use LMR400 for a duplex operation. They used standard RG58 to run between the duplexer and the receiver and transmitter ports. The antenna is not well placed for peak operation but rather for most convenience.

This organization already has more into this system than they were originally thinking so I am just doing my due diligence and checking to see if we can get the system to perform satisfactory for them without completely moving everything. However is that is what must be done then that will be my recommendation.

By the way does anyone have much experience with the Motorola SLR5700 Repeater?
 

RFI-EMI-GUY

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Thanks for all the feedback! The vendor who originally installed the system didn't do a very good job. They are a well known communications company who should have known better than to put the repeater in the networking closet. They should have also known better than to use LMR400 for a duplex operation. They used standard RG58 to run between the duplexer and the receiver and transmitter ports. The antenna is not well placed for peak operation but rather for most convenience.

This organization already has more into this system than they were originally thinking so I am just doing my due diligence and checking to see if we can get the system to perform satisfactory for them without completely moving everything. However is that is what must be done then that will be my recommendation.

By the way does anyone have much experience with the Motorola SLR5700 Repeater?
Some more stuff to worry about:

1) What is the transmit to receive frequency separation for this VHF system?
2) Is the duplexer designed for this frequency separation and repeater model?
3) Have you measured duplex sensitivity with the TX keyed? How much desense?
4) What is desense measured in 3) above into a 50 Ohm dummy load?
5) What is desense measured in 3) above into the antenna system?

The choice of VHF frequency band is odd for a number of reasons. Is there a reason they are on VHF?
 

N4DES

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Ever think about snapping ferrite cores on the data cables to try to reduce the noise from the interfering devices? They can be found pretty cheap and you can clamp a lot of cables in the snap in style.

 

BlueDevil

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VHF systems are very popular where I live. They had an existing simplex VHF system with all VHF equipment already in place. They wanted to expand the coverage of their radios so they elected to install a repeater. There is approx 5.2MHz split which the duplexer is designed to handle. The SWRs look good on everything and forward and reflected power is good. I don't have all the necessary equipment to test the desense but I can say that it appears everything is matched up well using my Bird Wattmeter and measuring forward and reflected power.

I thought about trying some ferrite cores but it would take several hundred. I did swap out the RG58 cable for some RG214. That seemed to help out a lot.
 
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So I've been chasing a similar issue at one of my customer's sites (also VHF). Not too long ago, their split base was upgraded to a full duplex repeater (to allow them to ditch the wireline). Ever since then, it has been issue after issue. Long story short, since then a new LTE carrier has gone on the tower, a broadcast transmitter has gone up (60 feet above) and it seems the tower owner no longer was aware of LMR customers still on the tower.

In my case it's a MTR2000 with RFS BP-BR duplexer. Running at 60W it's got .2 dB of desense on a load. On the antenna, 36 dB…SWR is 1.2:1, antenna/line sweeps are good. Beginning to look like a PIM issue with the antenna but…eh.

Just out of curiosity, is this installer/reseller a nationwide company? If so, I may actually work for them (via an acquisition) and I've noticed some things about the quality of some installs. PM me if you wish to confirm my suspicions.


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RFI-EMI-GUY

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VHF systems are very popular where I live. They had an existing simplex VHF system with all VHF equipment already in place. They wanted to expand the coverage of their radios so they elected to install a repeater. There is approx 5.2MHz split which the duplexer is designed to handle. The SWRs look good on everything and forward and reflected power is good. I don't have all the necessary equipment to test the desense but I can say that it appears everything is matched up well using my Bird Wattmeter and measuring forward and reflected power.

I thought about trying some ferrite cores but it would take several hundred. I did swap out the RG58 cable for some RG214. That seemed to help out a lot.
I hope nobody tried to tweak the duplexer.

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BlueDevil

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The duplexer seems to be operating fine. And I wouldn't touch it without having the right equipment to do so.
 

jim202

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One note that I thought many of you may not be aware of, in using a DPL on your receiver, there is just a small oops that Mother M made in using it. Have found out that in using a PL of 136.5, it will mute any receiver that was listening to a DPL encoded transmission. Your PL signal must be strong enough to cause a slight capture of the signal enough for the DPL receiver to hear the 136.5 PL code.

Found this out many years ago when there was a taxi service using the same business frequency that I was. They never shut up and had diarrhea of the mouth. If I made a transmission in the brief time between theirs, it would mute their receivers and produced all sorts of comments from them about not being able to hear the dispatcher. So it does work and is a weak spot in the use of a DPL code.

The 136.5 PL code is close enough to the turn off digital code that it causes the DPL receiver to shut off till the 136.5 PL tone goes away.
 
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