The Amateur Amateur: Choking on RF
By Gary Hoffman, KB0H
The intrepid newcomer encounters common mode problems in a mobile
September 14, 2001
My automobile is
my ham radio laboratory. It was not a conscious decision. That's just
where I spend the most time trying to fix things. My wife Nancy and I
have similar cars, each equipped with the same model of dual-band
Amateur Radio transceiver. Between the two of them, practically
everything that can go wrong has
There are several
attitudes you can take when dealing with problems. One is to curse,
complain, and make everyone around you miserable ("Dag-blasted
con-sarn wretched son-of-a-gun is broken again!
Another approach is to try to learn something ("Okay, why
is smoke pouring out of it?"). Frankly, I do both. I burn up the
adrenaline cursing, then when I calm down I try the logical approach.
I get a great deal of satisfaction if I can actually figure out a
problem and then fix it. (And later I write a column about it. It's a
win-win situation for me.)
The problem that
plagued me the most with our mobile radios was interference. (Real
hams call it RFI but I'll stick with calling it "interference.")
I couldn't blame external sources since this was self
or more properly, "auto-interference." Let me explain.
My mobile radio
was actually two radios built into one chassis. One was for 2 meters,
and the other was for 70 cm. They shared a single antenna, a single
power supply, a single control mechanism and display, and a single
microphone. They did, however, have individual speakers. It was
possible to hear conversations on both bands simultaneously, or to
hear a conversation on one band while talking on the other.
The problem was
that sometimes when I transmitted on one band, I would hear an
on the other
band. It sounded like someone dragging a phonograph needle across a
record. It didn't happen every time I transmitted, so I ignored it
for as long as I could. Eventually it started to drive me crazy, and
it would often happen when I was in the middle of an important
My first thought
was that the dag-blasted con-sarn wretched son-of-a-gun radio was
busted, but it happened both on my radio and on Nancy's radio. My
second thought was that it was weather related, since I noticed it
most often when I was trying to report poor road conditions, but
after a few BRRAAAAAAAAAAPPPPPP!s
on sunny days,
I changed my mind. The third thing that occurred to me was that
neither radio had exhibited the problem until I'd switched from
glass-mount antennas to magnetic-mount antennas. Was something wrong
with the new antennas? Hmmmmm. I thought hard, trying to remember.
I'd switched antennas because the glass-mount ones were terribly
Time for a Test
The chokes on the power cable (left) and speaker cables (right).
The glass-mount antennas were still in place, just not connected to
anything. I tried to recreate the problem, but as you all know,
appliances never fail when the repairman is there. Eventually,
though, I was able to produce a mild buzz while transmitting. I
switched to the glass-mount antenna, transmitted, and heard no buzz
on the other band, but there was a subtle hiss. My analysis was that
the problem was
present with both of antennas, but much more
evident when I used the magnetic-mount antenna. Ha! Problem solved!
All I had to do was move the magnetic-mount antenna from the trunk to
That didn't help,
of course. The problem wasn't the proximity
of the antenna to
the transceiver, it was merely the efficiency
antenna--the better the signal, the greater the interference. All
right. I hadn't fixed the problem, but I'd sure learned a lot. I knew
that the radios had gizmos called "diplexers" that were
supposed to prevent a signal transmitted from one band of the radio
from getting into the other band. So, I assumed that the interference
wasn't getting in through the antenna cable. How about grounding?
Manuals always say that grounding is important. I thought the
transceiver was well grounded, but made absolutely sure by connecting
it with a thick, ugly metal braid. My voltmeter confirmed a good
connection, but that didn't solve the problem.
I stared at the
spaghetti-like mass of wires in the trunk. Then I had an epiphany
(translation: the light bulb over my head lit). Could one or more of
these wires be acting like an antenna, picking up my transmissions
and feeding them back into the radio? A brief and abortive experiment
with tin foil convinced me that I could not effectively shield the
Sitting on my
haunches staring into the trunk wasn't helping, so I went inside and
sat in my La-Z-Boy instead. The recliner was next to a bookshelf, and
I my eyes happened to rest on the ARRL's Operating Manual
Idly I picked it up, thumbed to the index in the back and looked up
"interference." Moments later I jerked upright and re-read
the section I'd just perused. The very problem I was having was
described in detail, and there was a solution! It was called a common
The isolated speaker cable choke.
This is a "Common Mode Chokes for Dummies" (for amateur
sounded familiar. Right. I remembered. It was one of the questions on
a license exam I had taken some years earlier--common mode chokes, 43
mix ferrite toroids, stuff like that. I closed my eyes and tried to
visualize it. Inductors tend to allow lower-frequency signals to pass
but block higher-frequency signals. So, if I put inductors on the
speaker wires, the audio, being low frequency, should go through
unharmed. Any stray radio signals, being much higher in frequency,
should be blocked.
I will tell
you right now that I am not an electrical engineer. If you talk about
microhenrys or picofarads I look like a deer caught in headlights.
But in this case I didn't have to calculate anything and barely had
to build anything. It was all in the books (The ARRL Operating
and The ARRL RFI Book
). I bought some 43 mix
ferrite toroids (easier than you might think). They were
semi-metallic doughnuts, about an inch and a half across. I ran a
speaker wire through a doughnut about 15 times (like I was trying to
wrap up the doughnut with the wire), and voila! I had an inductor!
More specifically, I had a "common mode choke" designed to
keep out evil interference.
And did it work?
You betcha! Once I'd "choked" both speaker wires the
diminished to a mere hiss.
Later I was able to eliminate even the hiss by putting a choke on the
transceiver's power cable. I still receive some auto-interference
when I drive through one particular part of town, but hey! That just
means I have more to learn.
note: ARRL member Gary Hoffman, KB0H, lives in Florissant,
Missouri. He's been a ham since 1995. Hoffman says his column's
name-- "The Amateur Amateur"--suggests the explorations of
a rank amateur, not those of an experienced or knowledgeable ham. His
wife, Nancy, is N0NJ. Hoffman has a ham-related Web page. Readers may contact the
author via e-mail, [email protected].
© 2001 American Radio Relay League