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2-way interference #160133 06/05/05 06:52 PM
Joined: Jun 2005
Posts: 124
poorboy Offline OP
In an office trailer beside a truck weighing scale the use of the two way radio (CB I guess, but I'll double check next trip)that the dispatcher uses at one end of the 34 foot trailer knocks out the phone the guy on the other end is using every time it's used. The antenna is on the roof over the dispatcher's end, and these are cord sets I believe, although I better double check that, too. (I was hurrying off when they dropped this gem on me)'

All wired in unshielded Cat 5 under the trailer. It doesn't just bleed over the phone while mike is keyed, but actually cuts off the connection. Any thoughts?

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Re: 2-way interference #160134 06/05/05 07:19 PM
Joined: Dec 2001
Posts: 444
Sandro Offline
Hrm...sounds like an interference problem. First thing to check would be if any wires are nicked or damaged at any point. Also a long shot, but, any sort of battery chargers in the area?

Re: 2-way interference #160135 06/05/05 07:45 PM
Joined: Dec 2003
Posts: 886
hbiss Offline
What's the history here. Did this problem just start or has it been happening all along? What kind of radios are these (can't believe they are CB's) and what are they used for? EXACTLY what kind of phones are being used?

Without this information it would be just a guessing game.


Re: 2-way interference #160136 06/05/05 08:04 PM
Joined: Jul 2004
Posts: 783
LarryC Offline
Several things can cause this problem.

Radio caused problems:
Has this always been a problem since the beginning? Has anything changed? Especially up on the roof. If this is a magnetic mount antenna, is there a uniform piece of steel underneath the antenna? Something roughly about the size of 3 feet by 3 feet. That wold be the ground plane for the antenna.

Check VSWR between the radio and the antenna. If there is a gross mismatch between the antenna and the radio, the RF energy is going to go everywhere else than out the antenna. Check for loose connections, damaged cable, jury rigged connections, wrong antenna i.e. cell phone external antenna for CB etc.

Phone side problem:
"Land line" I assume you mean a newer phone system that can have multiple phone lines, intercom, hold function, speaker phone, all in a little plastic housing with 4, 6, or 8 conductors feeding it. If so, these local systems are very susceptable to strong Electro-Magnetic Interference (EMI). Do all of the lines light up when this happens? Even the ones not connected. Your best option is try to eliminate the source of interference. Next route would be to stop the interference from entering the phone system. This includes running the phone cable in a grounded steel conduit to as close to the phone as physically possible.
Adding a ferrite choke to the phone cable as close to the phone as possible. Putting the phone in a steel enclosure to shield it.


Re: 2-way interference #160137 06/06/05 12:33 AM
Joined: Jul 2002
Posts: 8,374
Trumpy Offline
I'd go along with LarryC's comments.
Check the coaxial cabling to the antenna, first off.
Signal leakage and SWR problems are rife with dodgy connections.
For some odd reason, CB installations seem to have a lot of suspect wiring.
Other side of the coin, if this is a 27MHz set, I'd advise a Low-Pass filter be fitted between the transciever and the antenna.
This can cure a lot of EMI problems, it sounds like there could be a Harmonics issue with the transciever.

Re: 2-way interference #160138 06/06/05 11:57 AM
Joined: May 2005
Posts: 177
John Crighton Offline
I'm going to make a wild guess here.

The 2-way radio is not CB, it's a 900 MHz trunked system, and the cordless phones are also 900 MHz FM. The phones aren't designed well enough to reject higher-power adjacent channels, and they choke.

Solution: None.

Am I close?

Re: 2-way interference #160139 06/07/05 04:46 AM
Joined: Jun 2005
Posts: 124
poorboy Offline OP
i will be back at this facility in a few days and will get the specifics. Sorry to post this with so few details but glad for the feedback so I will know what to look for. BTW, this is a brand new install and has done this from the moment it began operating.

Re: 2-way interference #160140 06/07/05 08:13 AM
Joined: Dec 2003
Posts: 886
hbiss Offline
I believe he said that the phones were corded. Also, now we know that this has been a problem from day one.

I wouldn't knock myself out over this, I doubt there is anything that will help other than moving the antenna or changing the phones. The RF is getting into the phones themselves and causing severe problems, some phones are better with RF than others. Not much you are going to do other than putting more distance between them.


Re: 2-way interference #160141 06/15/05 01:01 PM
Joined: May 2005
Posts: 177
John Crighton Offline
*** bump ***

Was there any resolution to this? Any further info?

Re: 2-way interference #160142 06/16/05 04:36 AM
Joined: Jun 2005
Posts: 124
poorboy Offline OP
Thought I would have gone back to this place by now, have to finish a small project there. We are so busy now, however, that I haven't been able to get there. It is not an earth shattering problem to them, but I would like to figure it out for my own education and curiosity and also so I can convince them that I am "some ol' smart". They don't have to know you guys helped me figure it out!

They will be screaming at me to finish their little project soon, and I'll be sure to get you some more facts.

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