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56 May 2012 ARRL – the national association for Amateur Radio www.arrl.org

Technical

by Mark Spencer, WA8SME

Hands-On Radio

H. Ward Silver, N

AX, n0ax@arrl.org

Experiment 112

RFI Hunt

This month’s story begins with a clank — a

rather loud sound emanating from some-

where inside our gas cooking stove. I’d just

put up a 105 foot dipole about 30 feet above

the kitchen. Operating on 20 and 15 meters

was no problem but when I dropped to 30 or

40 meters — clank. The oven door latch

solenoid was activating whenever a dot or

dash was sent, holding for a few seconds

after the RF stopped and — clank releasing,

only to — clank activate again on the next

transmission. Ellen is tolerant but not so

tolerant that I could ignore it, and so the

story begins.

General Approach to RFI

Let’s back up a little bit. Solving RFI prob-

lems requires a somewhat organized

approach — without it you’ll chase your tail

forever or until you give up and go off the air

entirely. We can start by dividing RFI into

two categories: RFI caused by your ham

radio equipment and RFI to your ham radio

equipment. This was clearly in the former

category.

In the cases for which your transmissions are

causing RFI, there are a series of cases to

evaluate. First, is the victim device experi-

encing the RFI designed to receive a radio

signal as part of its normal operation — such

as a TV or cordless phone? If so, the first

thing you must do — even according to the

FCC rules — is to determine or ensure that

your station is not generating a spurious

signal on the frequencies intended to be

received by the victim. For example, in the

case of RFI to a weather radio, your station

should not be generating any signals on the

frequencies of the weather alert transmis-

sions. If it is, go no further and either fix the

transmitter or add the necessary transmit

filters.

Assuming that the device is a receiver of

some sort and your transmissions are clean

(free of spurious emissions), determine

whether or not it is simply the strength of

your signal causing the problem. For exam-

ple, a commercial broadcast FM receiver’s

front end circuitry could be overwhelmed by

a strong 2 meter FM signal from your

nearby mobile rig. This is called fundamen-

tal overload and is a symptom of a receiver

being overwhelmed by a strong signal via its

normal receive path. The usual fix here is a

filter in the receive signal path to the receiver

(such as in the antenna feed line). Obviously,

the stove was not designed to receive RF

signals so it was on to the final case.

This final case is the most common form of

RFI from ham transmissions now that most

TV reception is not via over-the-air analog

signals. In this case, the signal from your

station is being picked up as common-mode

current by external cables or wires attached

to the victim device. (See Hands-On Radio

experiment #91, “The Common-Mode

Choke,” for a discussion of common and

differential mode signals.)

Those currents

are then conducted into the device where

they disturb its normal operation in some

way. If you can block those signals from

getting into the device — usually by using

some kind of common-mode choke or

shielding — you can solve the problem.

That’s what I expected would solve my

problem with the stove.

There is one additional frequent RFI case

— direct pickup — in which the wiring

inside the device picks up the signal without

any external cables or wires. This is often the

case for battery powered devices and can be

very difficult to solve. Guess which mine

turned out to be?

Let’s Get Cooking

The stove is made of heavy sheet metal and,

as with every other appliance these days, has

a microprocessor that controls its functions.

The only external wiring is the ac line cord.

As Figure 1 shows, the stove sits directly

under the antenna. Testing showed that the

RFI only occurred at power levels greater

than 25 W and only on the bands at and

below 10 MHz. With the antenna so close,

the stove was clearly in enough V/m of field

strength to cause interference.

My first — and only — candidate for pick-

ing up common-mode signals was the ac

line cord to a socket directly in back of the

stove. Opening up my shack notebook and

starting a troubleshooting log for recording

each step, I grabbed some ferrite split

cores (the common variety available at

RadioShack) and snapped them on the ac

power line where it entered the back of the

stove through a hole in the sheet metal.

[While the split core type of ferrite bead is

All previous Hands-On Radio experiments

are available to ARRL members at

www.arrl.org/hands-on-radio.

Figure 1 — The gas stove experiencing the RFI was directly underneath the HF dipole less than

30 feet above it. Even though the stove was made of sheet metal and the wiring completely

enclosed, there was still enough RF picked up by the latch solenoid wiring to cause a problem.

1 2 3

RFI Hunt 1
Additional Digital Content 3

Summary of Contents

Page 1 - RFI Hunt

56 May 2012 ARRL – the national association for Amateur Radio www.arrl.org Technicalby Mark Spencer, WA8SMEHands-On RadioH. Ward Silver, N

Page 2

QST – Devoted Entirely to Amateur Radio www.arrl.org May 2012 57pigtails with a plug and receptacle (a metal junction box could also be use

Page 3 - Additional Digital Content

58 May 2012 ARRL – the national association for Amateur Radio www.arrl.org Figure 3 — The controller board with bypass capacitors and a fe

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