[RFI] RFI Detector?
Frank N. Haas
kb4t at arrl.net
Mon Oct 3 18:15:20 EDT 2005
I am a Communications Specialist and Interference Investigator for a large
The company provides me with very esoteric and expensive tools specifically
designed to locate "sparking" interference sources typically caused by high
voltages carried on our transmission and distribution facilities.
When I visit a customer's home to investigate interference I always carry
one tool with me: an Icom T-90a. The T90a is a remarkable radio that I find
useful for many different applications but it is particularly useful in
tracking down interference sources.
The T90a receives from .5 MHz to 2 GHz and has amazing sensitivity across
the spectrum. Modes include AM, FM and Wide FM. The antenna that comes with
the unit is worse than a dummy load so I replaced it with an MFJ 15 inch
dual band (2M/440) thin and highly flexible whip. The radio is very small.
It fits in the palm of my hand nicely. The speaker produces loud clear audio.
Depending on the nature of the interference issue, I can start low in
frequency (in the AM broadcast band) or listen on whatever frequency
(including TV channels) about which the customer is reporting the problem.
As needed I can move up the spectrum to suit my needs.
True sparking interference can be heard well into the hundreds of MHz when
you are very close to the source. Typically you can hear the interference
at a low frequency from a distance and as your movement brings you closer,
you can increase frequency. Increasing frequency is much like using an
attenuator to reduce signal strength as you get closer and closer. When you
can hear the sparking interference at 300 to 500 MHz you are usually
standing right under or next to the source.
An increasing percentage of my "in-home" interference issues stem from
poorly designed switching power supplies found within inexpensive consumer
electronics like DVD/VCR combo units and the like. With my T90a, I can walk
around the customer's home and bring the flexible whip near the suspect
item and listen to the signal increase. I ask the customer to unplug (not
just turn off) the item and if the noise goes away, the customer is
surprised but pleased. If unplugging does not stop the noise, we keep
looking. If the source is in the customer's home, it's much easier to
search with the small handheld and very flexible antenna. This process is
successful 95% of the time for in-home issues. Rarely have I had to break
out the directional antenna to locate a source indoors.
In the recent past I have found the following causes of severe sparking
Door bell transformer
Switching power supply for an elevator
Switching power supply in a PC
Switching power supply in a DVD/VCR combo unit
Switching power supply in a cell phone charger stand
Sprinkler system timer/controller
High voltage electric fence for cow pasture
High voltage electric fence to keep dogs within lot
Photocell inserts in exterior lighting fixtures
In my line of work, my employer is guilty until I prove us innocent. In my
experience, only about 50% of the interference sources I locate are power
company related. I consistently find consumer electronic devices that cause
TVI and RFI.
True power line related sources are most easily found using equipment
designed for the purpose. The T90a listening in AM mode on ever increasing
frequencies will do nicely but a directional antenna (typically a 3 to 5
element UHF yagi) AND a switchable attenuator make the job MUCH easier. If
you don't have the luxury of using a $5000 scope-equipped DC to light
receiver or a $3000 receiver mounted directly to a yagi that listens to
300+ MHz in AM mode, try using a $249 T90a, a $35 homemade UHF yagi and a
$40 simple 4 to 6 switch attenuator. I'm sure many of you can find less
expensive solutions. Essentially any frequency agile, AM receiver with an
antenna jack should work.
Call your power company and ask for an interference investigation. My
employer does not charge customers for interference investigations...no
matter how long it takes...even if the problem ends up being something in
your home and not in any way related to the power lines. If the service is
free, take advantage of it. Your request should at least imply that the
power lines are causing interference to your equipment :-)
As long as I have deviated from the equipment theme, may I digress a bit
further? Interference investigators come in a wide variety of
effectiveness. Some are very skilled and highly motivated and care a great
deal about your issue. Some are just the opposite. You have to take what
you get. If you don't like it, there's little that can be done except
perhaps to ask for another investigator. You can usually tell very easily
who is skilled and cares. Ideally, your investigator would be an active
Amateur Radio Operator with an extensive background in RFI location.
Important things to remember:
1. Always be polite. The service you get will be directly and inversely
proportional to the pleasantness of your attitude.
2. Most power company investigators actually have other primary
responsibilities. Interference investigation is only a small part of what
they usually do.
3. Try not to tell the investigator how to do his job. If the investigator
seems incompetent to you, remain friendly and courteous but when he leaves
call customer service and request someone else.
4. Your patience will be tested. Be ready to wait. If the investigation
reveals a power company problem, it can take weeks, even months for a
repair crew to be dispatched to correct your problem. Interference
investigations and their resolution provide no income so they tend to be
assigned fairly low priority. Remember #1. Once you start behaving angrily
or with impatience and a sour attitude, you will only aggravate the problem
and extend the delay. Being kind, polite and courteous in the face of
enormous frustration requires immensely powerful self-control but it does
5. Know your options. If your state has a Public Service Commission or
other power company regulating entity, learn how they can help you. Call
them, visit their web site, read the law...do whatever it takes to learn
and understand your options. Knowledge and a good attitude will get the
problem solved as quickly as possible.
6. The FCC is not your friend. Don't waste your time calling the FCC about
interference issues. Unless you are another governmental agency, public
safety agency, congressman or senator, the FCC will do nothing to help you
solve your interference issue. They will always refer you to someone else.
Their actions re: BPL should clearly show you they care nothing about RF
7. Keep good records. Log your interference occurrences. Patterns can be
very helpful in locating the source. If you call in the pro's, get names
and phone numbers. Log their visits, phone calls and actions. If you need
to call in reinforcements, good records grease the skids.
I hope this note is of value to at least some of you. This note is
something of a catharsis for me as I've wanted to say these things time and
again after reading various notes on this list. RF pollution is widespread
and frustrating to hams everywhere. When those I QSO find out that I am an
interference investigator, they often express the wish that I could come
visit them and help resolve issues with which they must cope. I wish I
could do that too but it's simply not practical or cost-effective. I
believe most hams can solve their own interference issues if they are
willing to employ readily available resources and use the simple tools and
services described herein.
Back to lurking mode...
Frank N. Haas KB4T
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