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Author Topic: Home security System Problem while TXing....Help?  (Read 4421 times)
KD5KZN
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Posts: 6




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« on: April 03, 2003, 10:37:47 AM »

Hey All,

I have been working on getting my MARS certification and was trying to get into a net the other day for the first time....

I use a Icom IC-718 and a a 40 meter Hamstick Dipole at 17' AGL. I can't get it any higher due to antenna restrictions as it will be noticable from the street...

When I key up to check my SWRS, my house alarm beeps like a door or window has been opened...

The antenna is 6' away from the single story house I live in.

The alarm uses magnetic sensors on the windows , so I figured the wires going to the control box were picking up the RF, and causing the control box to thing a window had been opened...

I ferrited the sensor wires going into the control box at the control box end, but no joy. I also ferrite the control panel at the other end of the house, but also no joy...

Does anyone have any siggestions on how to stop this RFI? The WYL is losing her sense of humor about it... Wink

Thanks
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KD5KZN
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« Reply #1 on: April 03, 2003, 10:41:21 AM »

One more think...

The house alarm was installed when the house was built in 1992.

I am the second owner of this house....
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W5RB
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Posts: 564




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« Reply #2 on: April 03, 2003, 10:59:39 AM »

While I've never used it on an alarm system , I'd bet the simple trick I've solved plenty of telephone RFI with would work . Simply put a .01 ufd disc capacitor across the suspect line (between the two wires for that sensor , at the panel ).It'll be invisible to anything but RF , which it'll bypass to ground .Use them on any , or all ,of your sensor lines .Also , check in the RFI forum here , there may be something that applies already posted there .

Good luck ,
Russ, W5RB
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KA5S
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Posts: 229




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« Reply #3 on: April 03, 2003, 11:31:17 AM »

Yeah, this is a common problem.  Best way to fix it is to modify the alarm controller and (possibly) sensors that are responding to RF. Most of us are precluded from doing this by the alarm company's terms, or concerns about a UL-uncertified device voiding fire insurance.  The only other alternatives are to byapss RF before it gets into the device, or raise the impedance of the path it gets in from.

Most of this kind of pickup is common-mode and NOT between the two wires from each protected area.  Bypassing each wire to ground can work -- but it has to be a ground that's better than the path to whaetever the RF has turned on.  

Usually, common mode return is through the AC power wiring (or FROM it!) and a good AC power line filter can help open the path this current takes. But it can also be between different alarm inputs, sort of a dipole, and so you should put common-mode chokes in each input  as well.

The most common common-mode choke is a ferrite bead. Hoever, msot generic beads are ineffective in the HF bands; you need higher permeability and loss.  The most common EMI beads run 850 to 1000 permeability, and help only a little as low as 7 MHz. However, low-frequency ferrites like the cores of flyback transformers and magnetics from switching power supplies can do better.

There are companies selling ferrites into the Amateur market who identify bead permeability, and the frequency they're meant to help.

Good luck!


Cortland
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KD5KZN
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Posts: 6




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« Reply #4 on: April 03, 2003, 12:14:51 PM »

Hmmm..OK...

One thing I did notice is the the sensor wires at the control box circuit board entry have resistors [100K ?] in series with one of the wires...the installer looks to have used common resistors with teh full linegth legs on them...

probably straight out of the Radio Shack card packaging....

I am just trying to figure out the best way to rid myself of it...I also noticed that AM does it all the time and SSB does it about 60 watts of power...

I am willing to try the cap idea, as it should be a fairly easy install...

Just try the .01 ufd across the sensor inputs?

HF Ferrites? Might have to look for some....

Thnaks guys
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KD5KZN
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Posts: 6




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« Reply #5 on: April 03, 2003, 10:12:46 PM »

Update...

Grabed some caps on the way home out of the .01ufd bin at the local electronics store bin...got home and found out they were .01pf, 1KV...tried it anyway...

IT WORKED!
YOU GUYS ROCK! THANKS!
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KA5S
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« Reply #6 on: April 03, 2003, 10:17:31 PM »

If he put in that high a value of resistors, that is a BIG advantage for you in getting rid of your problem.

Likely, neither side of the sensors is grounded at the sensor end. In that case, you can replace each single resistor with TWO of half the value; if they're 100k (that's a lot) use 47k or 51k in each lead. There's no ferrites anywhere that will give you that much isolation -- and the alarm will not notice the difference, except for common mode noise. But be SURE of the resistor value.


Cortland
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N9AVY
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Posts: 67




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« Reply #7 on: April 09, 2003, 06:08:34 PM »

Been in the alarm businees about 33 years now and have come up against this problem on several occasions. Last year I worked with ham in NJ to fix his RFI problems on his security system.

Had him trace the ground wire (yes, it's important!) from the control panel down to basement where he found ground wire hanging in mid-air next to cold water pipe. Putting ground back on cured much of the RFI. (Found one done by "professional" a few months ago where the guy put ground strap on PVC pipe!)

The .01 disc caps across the alarm circuits usually work well as do ferrite beads. You might also want to filter the incoming power to panel. Check smoke detectors on the system, if any, as the ionization type (radioactive label on back) are high impedence devices which will pick up RFI like a magnet; switch them to photoelectric types.

You might also check any splices out in field wiring. Over the years many installers have simply twisted the wires together and wrapped some tape around them. A bad practice as the adhesive on tape and oils on skin can corrode wires. If you must tape wires, then solder them.

There may be other things to look for depending on the type of system and wire used. These are some basics which may help.

Good luck !

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N9AVY
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« Reply #8 on: April 09, 2003, 06:14:25 PM »

100K end of line resistors Huh  Not in any panel I've ever seen.  Might be 1K or 2K, but 100K would leave you system in alarm all the time.
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KA5S
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Posts: 229




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« Reply #9 on: April 10, 2003, 12:27:51 AM »

Yeah, 100K IS high. Took his word on it. But even 1K, split in two, 500 ohms per wire, would isolate more than any ferrite bead at 40 meters.

Cortland
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N9AVY
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Posts: 67




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« Reply #10 on: April 10, 2003, 12:21:45 PM »

Don't think splitting end of line resistance is a solution as the circuits are designed to see that resistance (20% tolerance) and anything outside of that would cause problems (troubles, alarms). The end of line resistors are supposed to be at the end of line (out in field wiring), but most installers put them at panel. These resistors prevent tampering of alarm circuits by keeping a small current running through loop at all times. Variations are seen as alarms/troubles depending on programming.

Typically, alarm panels have 6 or more zones with a 1-2K resistor on each zone. Fire alarm panels usually run from 1.8K upwards to 33K. Wrong resistances in fire loops keep fire depts. busy at times (until the owner gets fined for too many false alarms).

Perhaps I may have misunderstood you comments on splitting resistances, but it would at first hand appear to be something that would cause futher prblems.

de n9avy
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KA5S
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Posts: 229




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« Reply #11 on: April 11, 2003, 12:12:52 PM »

Perhaps I've been the one to  misunderstand, for it seemed to me the resistor mentioned was in series with the line (NC sensor). If it is in parallel (sensor normally open), things get more complex.
 
In THAT case, one might try three resistors, each about one third the total value, one in series with each wire at the controller and one across the sensor, serving the same function as a single parallel resistor at the sensor. This would still give 280 or 330 ohms series impedance for RF, again, more than a ferrite at 40 meters, at the alarm controller. I don't know if the 660 ohms or son "on" resistance would still let a sensor trigger the alarm, though. Might show up as tampering, not an alarm.  You might have to use lower values for the series resistors. Could be a cheap way to build immunity into controllers, though.
 
Cortland
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W9WHE
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Posts: 254




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« Reply #12 on: May 08, 2003, 11:11:03 AM »

Don't forget the telephone and power inputs to the CPU. They also need caps and ferrite. Also, how close is the earth ground to the CPU?  Can you improve it?  
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VE3EGA
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Posts: 12




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« Reply #13 on: June 10, 2003, 10:47:57 AM »

Hi,

I had RFI on my TV's from my small vertical ANT and recently changed my ATU from a small Dentron unit to an MFJ - violla! TVI gone - probably better harmonic rejection - give it a try (borrow a friends)....

73
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K8JW
Member

Posts: 1




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« Reply #14 on: June 12, 2003, 10:53:17 AM »

I had a similar problem here at my house.  I put ferrites and bypass caps at the main alarm box with no effect.  After exploring different ways to isolate the problem I tried shielding the Alarm Keypads ... My house has three of them.  The alarm system here is made by NAPCO.  I removed the keypads from the wall.  The keypad consists of a pc board screwed to the plastic front panel where the buttons and displays show through.  There is a thick paper insluator on the back of the pc board to protect it. I took out the screws holding the paper insluator to the pc board.   I took TIN FOIL and carefully covered the back of the insulated paper being careful not to short out any connections that went through holes in the Insluating paper backing.  I made sure that the tin foil covered the area where the screws held the paper on.  I then pushed the mounting screws through the tin foil to make contact and connected a wire from under one of the screw heads to the ground wire terminal... It is one of the four wires going to your keypad from the main alarm box.  You will have to determine which one it is.   This fixed the problem for me at this location.  Good luck  73  K8JW
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