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Author Topic: Welcome to RFI!  (Read 3034 times)
N2MG
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Posts: 122



« on: January 06, 2003, 02:19:16 PM »

Welcome to the RFI forum.  Please use it as you see fit to discuss anything at all about interference... all legal, social, technical topics are welcome.  Gripe about your lousy PC or monitor, praise a new cordless phone, ask questions or give tips on how to rid your shack/home/neighbor's home of interference.

73 Mike N2MG
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WB2WIK
Member

Posts: 20547




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« Reply #1 on: January 06, 2003, 05:26:33 PM »

I'll start with a quick tip:

I own, and these are in use in my home, five cordless telephones, in addition to some fully-wired ones.  For no particular reason, no two of the ones I have are the same.  They were purchased at different times, and in most cases, from different stores.

Of the five, the only one that is 100% absolutely "bulletproof" with regard to RFI is an older 900 MHz unit labeled "AT&T" (and made by "who knows?").  It's off-white in color, and contains a digital voice recording-type answering machine in its base (no tape).  Since I've had it for five years, I'm certain it is not a current model.  No matter what band I use, or how much power, this particular unit receives no interference whatever, without my having had to add any filtering devices to it.

The others are branded SONY (2, but different models), and two are nondescript brands.  It doesn't really matter, but all the other four cordless phones were BAD receptors of RFI until I add tons of filtering to them.  In one case, I added an in-line filter having both inductors and capacitors, and in addition to that, it needed THREE clamp-on ferrite chokes in series, with the base unit telephone cord wrapped through each one three times (total of 9 turns!) to finally rid myself of interference, which for some reason peaked on 30 meters (maybe a resonant line length somewhere).

Moral of the story: When selecting a cordless phone, bring an RF-generating device with you, and get a demonstration of the telephone plugged into an active land line.  Dial a local number, hold your handheld transmitter up against the base unit, and transmit to see what effect that has.

I've found that four out of my five phones would "fail" this test.  The AT&T-branded unit, without modification, "passes" this same test.  In my home experiment, I used a little Yaesu VX-150 5W 2m handie for the RF generator.

Based on the obvious correlation between RFI from a 2m source and RFI from HF sources, it appears a reasonably valid test.

The "failure" is obvious: In phones that are RFI-prone, the 2m handie signal causes a buzzing in the telephone headset.  It cannot demodulate the FM signal, just the carrier alone causes the problem.

WB2WIK/6
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W5RB
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Posts: 565




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« Reply #2 on: January 06, 2003, 05:48:14 PM »

Great idea for a new topic , Mike . I'll throw in one observation , and one sure-fire fix . My guess would be that the newer spread-spectrum cordless phones are a lot less subject to RFI . I'd be interested to hear from folks using them . To fix RFI on the phone line , a single .01 ufd disc capacitor across the line , as near the phone as possible , will get rid of even extreme RFI .( I used to live in the shadow of a 10kw AM radio station , and it fixed me right up.) This may not help the cordless phones , but it'll get the RF off the wire .If you're moving into a new QTH , ask the Telco for an RF supression kit . They'll install it at no charge .

Russ, W5RB
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KB3IFH
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Posts: 3


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« Reply #3 on: January 09, 2003, 12:00:40 PM »

Does anyone know about a Kenwood MC-50 mic needing to have the ground rewired to help eliminate RF problems?

Thanks,
Randy
KB3IFH
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W0FM
Member

Posts: 2052




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« Reply #4 on: January 09, 2003, 02:06:00 PM »

Thanks for the new one Mike!  This should prove to be a popular spot on eHam.

I have had ongoing line noise in my neighborhood for years.  I've gotten to know the power company "ghost buster" by first name he's been out so many times.  The noise gets really bad and I call Mark the engineer.  Mark sends out Bruce the "ghost buster" and, within a few days, the noise goes away.  Weeks later, we repeat the same scenario.

The other day however, I heard heavy noise, like line noise, all over the spectrum (80M past 2M).  I got in the car and tuned between two AM stations (560 kHz) on the car radio.  I drove around the neighborhood looking for the culprit.  No luck.  Upon returning home, I noticed that the noise on the car radio got really bad as I pulled into my garage.  It went away when I backed out.

I took a portable AM receiver and began searching the house, garage, shack.  I have found some "hot spots" where the noise is so bad, it's worse than the power line problems I've experienced.  But the locations are spotty and very wierd.  

For example, the noise seems to follow the steel I-beam along the length of the house.  AC wiring in that area is quiet, as is low voltage stuff.  I also get strong noise along certain lengths of round duct work in the basement, but the furnace and humidifier motors are dead quiet.  So are the florescent lights and halogens.  This is not static like static caused by dry winter air, or ignition noise, but, rather, the familiar popping noise like power line noise.

I have probed lights, motors, switches, fans, outlets, TV's (on and off) dimmers, etc with no luck.  The closest I have come to identifying anything likely to be the culprit, is the digital clock on the front of the oven in the kitchen.  But that noise goes away when I move the receiver more than 6" from the clock.

The only conclusion I can come up with is that the steel I-beam and hunks of duct work might be acting as passive radiators of the neighborhood power line noise.  Maybe the lengths are harmonically related or something.  But, what would cause these metal items to re-radiate the noise LOUDER inside than it is outside?

This weekend I plan to to the same test while flipping circuit breakers, but so far I am baffled.  It doesn't seem electrically related to any device or appliance in the house.

Anyone ever experience anything like this?

73, Terry, WØFM
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N8UZE
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Posts: 1524




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« Reply #5 on: January 18, 2003, 12:39:49 PM »

Garage door opener?

Any thing with a thermostat in it could be a problem.  Transformers such as found in many doorbell systems can also be a problem.
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N3ZOC
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Posts: 2


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« Reply #6 on: January 18, 2003, 10:32:48 PM »

We had the ham (RFI guru)from the DC/MD area give a talk on powerline RFI at a club meeting.  I think he recently taught a course for Power Company RFI reps thru ARRL.  Don't have his call/name handy.  One of the things they found once was the aluminum siding on a house was being energized by a nearby power line and the induced voltage was causing arcs between pieces of the siding (at the gable on the end of the house where the two pieces were mitered I think was one source).  Sounds like you might have a house full of metal ducts etc being subjected to high voltage from a nearby powerline.  Just a thought/possibility.  Hard to pin down.
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KC7BDP
Member

Posts: 14




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« Reply #7 on: February 05, 2003, 08:54:19 AM »

I work for the phone company in my home town. Since the boss knows I'm a Ham, guess who gets sent on all the "strange noise" problems?

I read with interest the topic of cordless phones and RFI; surprisingly (maybe), most of the calls I go on actually do NOT involve the cordless units!  The "el cheapo" corded units are almost ALWAYS involved.  My favorite was the lady who called us out to figure out why (only) her bedroom phone would suddenly spout a very muffled voice which she deciphered to be saying, "Take your clothes off (repeated at least 3 times). I just love it. (repeated, again, at least three times)."  Yes, this is what this very upright, correct lady had determined the voice to be saying; and her phone I had seen on sale previously at Wal-Mart for about $6.00. I'm just managing to keep a straight face while I'm asking questions; finding out that this occured on weekends that sounded REALLY familiar from my having read the (then) latest issue of CQ.  When I mimiced the sound by holding my hand tightly over my mouth and saying, "CQ contest...." her eyes lit up. Since this lady lived "in the shadow" about a half block away from Yuma's most famous contestor - at whose house I had taken my VE exam - this was the easiest call I'd ever taken!  A quick call to Jackie (Dave's wife) put the two of them together; with assistance following. Haven't heard from the lady since.

Yes the phones themselves can be problematic; but by far, the single most common telephone RFI problem is simply - bad connections.  Your neighborhood  "phone guy" could - SHOULD - start with connections at the terminal box that feeds your house (most common) and then at the "protector side" of the Interface (j-box) where the line enters your house.  He - or you if, like I've ALWAYS been since well before becoming a phone guy,you're too cheap to have a house wire maintanence plan - should then move on to each individual jack.  Look for typical corrosion and loose screws.  Remember, almost ANY cruddy electrical connection can turn into a nice little rectifier; converting RF into pulstating DC which permeates the whole phone circuit in your home.  It's almost stupidly simple, but you'd be surprised how many people - knowledgeable or otherwise - sail right past that fact.

Now if I could just get the neighbors "cable ready" VCR out of 2M SSB; or figure out the trick to keeping 6M SSB (even at 10 watts) from turning my wife's 17" computer monitor inside out!  Something besides replacing both units; as I already stated, I'm CHEAP.

Jimmy
KC7BDP
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