hissing noise in system

[johnny1225]

i have a repeater system that is installed in a large Hotel in downtown Toronto. Security is on (1) repeater tx 463.8500 and rx 468.8500 with a DPL of 627. We just put a second repeater in for engineering tx 452.8125 rx 457.8125 with a DPL 152, using the same antenna system with C3037 multicoupling cans from sinclair labs. I am wondering if i need something on the system to stop the hissing noise of the system now that i installed a second repeater with cans. Is a ferrite isolator something i would need on this system or would i need something else to get rid of the hissing background noise. If it is a ferrite isolator i need model # I3111A from sinclair what frequency would i tune it too and where in the line does it go. Any help would be appreciated.

thanks you

John

[MSS-Dave]

I'm taking a long shot here but...

I think the hissing might be from the radios. You don't say what radios or repeaters you are using but looking at frequencies your security may be a normal 5 Khz wide channel while the engineering may be on a 2.5 khz channel. I don't know if Canada is the same as the U.S. though. Had this issue years ago on a LTR trunk system using HT1250LS radios and RKR1225 repeaters. System was narrowband and the Xpand (compandering) was WAAAY wacked out.

If you get the hissing ONLY when the transmit on the other repeater is up, there could be a desense issue with your receiver(s) but being about 6 Mhz apart on 1 (ch 1 TX to ch 2 RX) and 16 Mhz (ch 2 TX to ch 1 RX)on the other, seems a bit goofy unless the second channel was added incorrectly to the existing system.

Could you give a little more info as to the type of radios being used and how the second repeater was added to the existing antenna system?

Dave

[Jim202]

I'm taking a long shot here but...

I think the hissing might be from the radios. You don't say what radios or repeaters you are using but looking at frequencies your security may be a normal 5 Khz wide channel while the engineering may be on a 2.5 khz channel. I don't know if Canada is the same as the U.S. though. Had this issue years ago on a LTR trunk system using HT1250LS radios and RKR1225 repeaters. System was narrowband and the Xpand (compandering) was WAAAY wacked out.

If you get the hissing ONLY when the transmit on the other repeater is up, there could be a desense issue with your receiver(s) but being about 6 Mhz apart on 1 (ch 1 TX to ch 2 RX) and 16 Mhz (ch 2 TX to ch 1 RX)on the other, seems a bit goofy unless the second channel was added incorrectly to the existing system.

Could you give a little more info as to the type of radios being used and how the second repeater was added to the existing antenna system?

Dave

Another question I would ask is do you hear a bunch of background noise when a user is trying to talk but has paused between words?
Like can you hear other people talking in the background, automotive noises, radio playing while in a vehicle and they have the portable
radio or mobile keyed?

We need some more feedback on the problem as MSS-Dave has mentioned.

[johnny1225]

I will have to check, i know the new repeater we put in is 12.5 khz which is a VR-7000 vertex repeater, the main repeater that has been there for years is a motorola GR1225, but might be 25 khz i will check on monday. They use a mixture of CP200 and BPR40 Radio's. The system if my memory is right, is 4 cans transmitter and 4 cans receiver. The Vertex is the second repeater that we added so the transmitter on the back of the equipment is hooked up to the equipment port on the tx cans and the receiver on the back of the repeater is hooked up to the equipment port on the receiver cans. The duplexer antenna port on the 1st repeater which is the GR1225 is hooked up to the feedthru port i believe on the tx cans and the antenna port on the rx cans are connected up to the splitter which feeds two antennas in the hotel i believe this is the way it is hooked up. The cables that connect the above connections are LMR400 cabling and the small cables that connect each can is factory RG214 cabling from sinclair labs. There is no noise such as people talking or anything like that just alot of hiss which makes it hard to hear. If a radio transmits and does not say anything the receiving radio will hear alot of hiss and almost like intermod as well. Do you guys think i have the cans hooked up right?

[Bill_G]

Which system has the hiss - the new one, the old one, or both?

As far as connecting to the correct cavities - what does your wattmeter say, and how good is the inbound/outbound range? Did someone sweep and tune the cavities for freq of operation?

[johnny1225]

yes the cavities were tuned by sinclair labs themselves right at the lab for 452.8125 and 457.8125 i never hooked a wattmeter up to it so i don't know that, where would i hook up the wattmeter to do a power test? i will have to find out if it is the new system or both systems i don't know that as of yet

[Jim202]

There is a long proven history of using LMR type cable in a repeater system that it does and
will cause all sorts of problems with time. My first suggestion is to get rid of any cables
between the radio, cavities and the good heliax cable going to the antenna. Replace
them all with double shielded coax or with some super flex type cables. Stay away from
anything that has a foil shield and copper braid together.

Other than that, you will just have to start trying to eliminate the sources or probable
sources of your problem. You should start by placing a dummy load right at the TX and
see if the noise is still there. Then move it to the antenna port of the duplexer. Again
see if the noise is still there. Pull the RX cable off the duplexer and test again.

The best tool for working on this problem is a coupler to feed a signal generator into the
antenna port of the duplexer. You need to have a signal of a know level that can be
used to create an input signal to the RX. You can make one of these couplers by using
a coax T connector and pulling the center pin from the side port. This way you don't
have any DC or direct connection to the antenna line. This protects the signal generator
from the transmitter power.

You should get the routine. Without some more feedback, it is just a waist of time for
us to speculate.

Jim

[Bill_G]

I haven't had bad cables cause a hiss. They generally make a crackle or a crunching noise. Based on what he has written so far, it doesn't sound like he has heard the noise himself yet. So, he may be arming himself for a service call next week based on a report from a customer. Hissing may be their term to describe the problem, and may very different from how those of us who appreciate noises would describe it. The earball has become finely tuned to the difference between a portable with the crunchies and vco rumble.

Years ago I went out to help an inexperienced tech with a mall security repeater he couldn't fix. Everything he tried would not get rid of the noise. Almost every transmission was plagued by crackling and loud bursts of noise. He had checked everything including doing a full pm on every portable, the base, and the repeater. As soon as I heard the noise over their base, I asked that officer to return with his radio. I took his speaker mic apart, greased the seams and the PTT bar slide, and sent him back out. Problem solved. Perfectly clear transmissions. They had listening to the creaking of the speaker mics, calling it repeater noise, and our tech wasn't quick enough to recognize the cause.

I took a very nice manufacturing account away from our competition because they had been chasing interference that disrupted both repeaters on premise. From the very first listening, I could tell they had maintenance issues, not interference. Bad mic cords, bad mic connectors, crunchy portables, low batteries, bad antennas, etc, etc. I gathered up the worst offenders, fixed them, returned them, and went after more. It took about two weeks to cycle everything through, and I never found any intermittent interference causing disruption. All noises were gone.

I guess Johnny will tell us more next week after he has had some face time with the repeaters.

[RFguy]

I suspect that your issue is related to improper filtering of the original repeater now that you have added another transmitter. With C3037 multicouplers, you should have installed 4 of them. 2 for the new repeater (Rx and TX) and 2 for the existing repeater (Rx and TX).

I suspect your existing duplexer is a basic reject type that only provides protection for its own transmitter. It will not provide any filtering for the new transmitter.

[FatBoy]

preselector, preselector, preselector..............and if you can get a recording of the noise, that would be great too. FB

[RFguy]

Preselector may not help if the noise is on-frequency noise generated by the transmiter. If they are in fact using a reject type duplexer, there is no TX sideband noise filtering.

[johnny1225]

so would you say a sinclair ferrite isolator would do the trick, we have many laying around the shop and can be tune to the tx frequency of the tx cans

Regards,


John

[Bill_G]

No. An isolator is a directional device that helps prevent intermod. But, an isolator won't help with desense in the receiver. I suppose theoretically it might, but I have never tried it. All good transmitter combiners incorporate isolators on each line. It is good design to have an isolator on each transmit line when you have more than one repeater at a site even if they do not share an antenna, but it is not mandatory. On the other hand, the simple reject duplexers shipped with lower tier product only keep the transmitter output from desensing the receiver of the repeater. It stops self clobbering. The xmit side rejects the rx freq, and the rcvr side rejects the tx freq. They have a steep curve about 200khz wide and pass all other freqs quite well. So, a strong in band transmitter could desense a rcvr especially if the freqs are well outside the notch of the duplexers. When you have more than one repeater in close proximity (ie: the same roof, or in the same plane within a few tenths of a mile), you need pass duplexers. In severe cases where you need to cut adjacent channel, you need pass/reject duplexers.

[johnny1225]

So if i am trying to stop the receiver from desense what should i put on the system to correct this problem would it be a preselector

[Bill_G]

No. A preselector is even broader. It's a barn door. A UHF preselector, for example, will pass the UHF band, but cuts the VHF and 800 preventing the rcvr front end from being saturated with strong out of band energy. However, it will pass strong in band energy just fine with a minimum of loss, and thus be useless against UHF desense. It sounds like one of your senior personnel should accompany you to look at the hissing, and perhaps review the way everything is hooked up.