RF interference — is it interference to you? Is it interference by you?
Possibly both! What does this
interference consist of? And how can
you tell what type is present? A topic
that starts off with so many questions
is bound to cover a lot of ground, so
let’s get started.
The phenomenon known as RF
interference — RFI to its many friends
and acquaintances — is a sub-type of
electromagnetic interference, or EMI,
which is itself a topic that is part of
the general subject of
electromagnetic compatibility, or
EMC. Specifically, RFI involves
interference caused by signals
propagated wirelessly as radio waves,
but even this sub-type of a sub-type
of a subject still has a surprisingly
subtle and broad reach.
Is RFI Really a
Problem?
Undoubtedly. It’s a growing
problem beginning to have an impact
on the broadcasting and wireless
data/telephony industries.
Electromagnetic “smog” is pervasive
in urban and suburban areas across
the entire frequency range, from
long-wave signals through
microwaves.
The most widespread effect is
higher random noise levels all across
the spectrum, just as atmospheric
smog affects an entire region. This
raises the noise floor (the noise
present at any frequency), meaning
weak signals are increasingly
obscured and stronger signals
degraded by bits and pieces.
Why would, say, a mobile
telephony company care about
analog noise since the phones
communicate digitally? Because, at
some level, just about everything is
analog. Inside that phone is a radio
receiver looking for analog radio
signals from the local base station.
If I mix in a little more noise —
let’s say, I raise the noise level by a
measly 3 dB at the receiver input —
the distance at which the phone’s
digital protocol breaks down will
decrease by about 30%. That means
the coverage area for the base station
has decreased by half!
The service provider then has to
either build more towers or you have
to transmit at a higher power, or
both. That’s money out of your
pocket, one way or the other. The
same is true for stations broadcasting
programming, or public safety
agency communications.
On a personal basis, most
people don’t know (or care) about
RFI until their garage door opener
won’t work or an audio system starts
speaking in tongues. As readers of
Nuts & Volts, however, you should
become aware of the
problem, understand its
causes, and know how to
solve it. As day-to-day users
of the spectrum, ham radio
folks are intimately familiar
with the symptoms and
cures for RFI.
RFI Basics
The definition of RF
itself (radio frequency)
spans electromagnetic
signals over a range of just
above audio (about 20
kHz) through infrared light
at several hundred THz. To keep this
column from occupying the entire
magazine, we’ll stay in the range of
500 kHz through the upper end of
UHF (ultra-high frequency) around 3
GHz — still a 2,000-fold range.
RFI is discussed as having
sources and victims with a path
between them. The relationship
should be pretty obvious. The goal —
paraphrasing Poor Richard’s Almanac
— is to “neither a source or victim
be.” To deal with RFI effectively, you
must be able to identify all three:
1. What system or device is
creating the interfering signals
and how — the source?
2. What system or device is
being interfered with and how
— the victim?
3. What is the path by which the
RF is radiating from the source
and getting to the victim?
Even across the many octaves,
the basic techniques to achieve the
goal are the same: Keep RF out and
keep RF in. The techniques for
implementing those two seemingly
simple tactics are themselves
deceptively simple sounding: Avoid
and block.
hwardsil@gmail.com
■ BY WARD SILVER N0AX THE HAM’S WIRELESS WORKBENCH
RF Interference
It's everywhere! It's everywhere! Fortunately, you
can take a bite out of RFI.
What is the Pin 1 Problem?
In the pro audio world, the “pin 1 problem” was
identified by Neil Muncie in 1994. It is caused by the
shield pin of the ubiquitous XLR connector (pin 1) being
attached directly to circuit common. This connection
provides an RF highway for signals being picked up on the
cable’s shield directly into the equipment where they can
cause interference. Simultaneously, any noise or spurious
signals present on the circuit common are given a terrific
antenna to radiate from.
The Mackie Company — a professional audio
equipment manufacturer — has addressed this in the
application note, “Grounds, Shields, Hums, and Buzzes”
at www.mackie.com/pdf/CMRefGuide/Tips_Ch6.pdf. More
information is available in several papers and tutorials by
Jim Brown K9YC (a retired professional audio engineer)
on his website at http://audiosystemsgroup.com/
publish.htm.
PRACTICAL TECHNOLOGY FROM THE HAM WORLD
62 September 2015