Capture Effect and
Low Power Repeater Use
By: David E. Rudd, CET WC
AI4JI
What is Capture Effect?
In referring to radio, capture effect, or
FM capture effect, is a phenomenon
associated with FM reception wherein
only the stronger of two signals at,
or near, the same frequency will be
demodulated.
It’s What?
Capture effect is defined as the
suppression of the weaker signal at
the receiver limiter; where the
weaker signal is not amplified, but
attenuated.
How does it happen?
When more than one signal is nearly
equal in strength, or the signals are
fading independently, the receiver
may switch from one signal to the
other and exhibit picket fencing.
What in the blue blazes is
Picket Fencing?
Originally from amateur radio, the phrase
was used to describe the way an FM
transmitter will cut in and out as it nears
the capture threshold of the receiver, thus
chopping the speech of the transmitting
operator. It is not clear if the phrase was
intended to describe the loss of the
speech, or if it actually referred to the
chopping sound itself, which imitates the
noise produced by dragging a stiff object
across a picket fence.
Kidz Today..
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Picket fencing is slang for the chopping
effect sometimes heard by cell phone
users at the edge of a cell's coverage
area, or (more likely) by the landline user
to whom the cellphone is connected.
"Picket fencing" refers to the way portions
of speech are stripped from the
conversation, as if the listener was
walking by a picket fence, and hearing a
conversation on the other side that
changes audibily depending on the
position of the pickets relative to the
listener.
Back to Technical Reality..
The capture effect can occur at the
signal limiter, or in the demodulation
stage. Some types of radio receiver
circuits have a stronger capture
effect than others. The measurement
of how well a receiver can reject a
second signal on the same frequency
is called the capture ratio for the
specific receiver being measured.
How do we measure it?
Capture Effect is measured as the
lowest ratio of the power of two
signals that will result in the
suppression of the smaller signal.
What about Ancient Modulation?
Amplitude Modulation, or AM radio is
not subject to Capture Effect. This is
one reason that the aviation
industry, the military, and some
others, have chosen to use AM for
certain voice communications rather
than FM. Thus allowing multiple
signals to be heard on the same
frequency.
The Capture Effect Battle
While we are listening to our repeater and
conditions are favorable for ducting, a repeater
from Alabama comes booming in on our
frequency.
When our repeater un-keys you hear them in the
background. Our repeater has won the capture
effect battle.
When you are listening to the morning CRS net and
in the middle of someone grumbling about Brain
Donors, all of a sudden the Alabama repeater
comes in loud and clear; well, they won the
battle. Their signal was so strong, it overpowered
our local repeater’s signal at your receiver.
You can relate to this..
While we are happily using our repeater and
some individuals in another state decide
to point their antennas in our direction,
crank up their power, and operate in a
manner against the rules; you will hear
what sounds like our folks dropping in and
out, or being muffled. That is Capture
Effect and our repeater’s receiver trying to
switch between the stronger of the two
signals arriving at the antenna.
Low Power Repeater Use
The FCC Rules state that an operator
will use the MIMIMUM amount of
power to conduct reliable two way
communications.
What is RELIABLE?
Reliable
[ri-lahy-uh-buh l] –adjective
that which may be relied upon;
dependable in achievement,
accuracy, honesty, etc.
(reliable information.)
Full Power?
To engage in reliable communications,
a signal report of 2 and 2 or better is
all that’s needed.
Rarely should anyone need full power
when operating within 20 miles of
the repeater, unless you are mobile
and down in a deep valley with a
poor antenna!
QRM and QRN
QRM is the Q-Code for Interference. I
am Being interfered with. / Are you
being interfered with? (relating to
another transmission)
QRN is I am troubled by static. / Are
you troubled by static? (relating to
atmospheric noise)
QRM
When you hear someone using the
repeater and you intentionally key
over them or cause their
transmissions to become unreliable,
you are causing QRM.
When you fire up that amp, point your
beams and give your call over
everyone else in an attempt to break
a pile up, you are causing QRM.
QRN
QRN is caused by lightning, Sporadic
E, Meteors, the Aurora Borealis, one
of Arnold’s GA Power facilities,
cracked insulators on a power poles,
etc.
Natural and unintentional man made
noise / radiation.
Dealing with local QRM
Log it! Write it down, get as much info
as you can. Date, Time, Operator(s),
Location, any information you feel is
pertinent. Get it to the trustee.
Do Not discuss it over the air! Don’t
hint about it, don’t try secret squirrel
codes, ignore it, just go QSY or QRT!
If someone knows it’s working, they
will continue!
The Amateur’s Code
Paul M. Segal, W9EEA (1928)
The Radio Amateur is:
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CONSIDERATE..... never knowingly operating in such a way as to
lessen the pleasure of others.
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LOYAL..... offering loyalty, encouragement and support to other
amateurs, local clubs and the American Radio Relay League,
through which Amateur Radio in the United States is represented
nationally and internationally.
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PROGRESSIVE..... with knowledge abreast of science, a well
built and efficient station, and operation beyond reproach.
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FRIENDLY..... with slow and patient operation when requested,
friendly advice and counsel to the beginner, kindly assistance, cooperation and consideration for the interests of others. These are
the hallmarks of the amateur spirit.

BALANCED..... Radio is an avocation, never interfering with
duties owed to family, job, school or community.

PATRIOTIC..... with station and skill always ready for service to
country and community.
Good Amateur Practice
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Giving a little ground--even if you have a right not to--in order to help preserve
Amateur Radio and not cause it to get a bad name or hasten the day when it
becomes obsolete.
Respecting band plans, because they make it possible for every mode to have a
chance.
Not transmitting a 6-kHz bandwidth signal when there are lots of people on the band.
Not acting like an idiot just because you were interfered with.
Being aware that we all love Amateur Radio, and there's no need to damage or
disgrace it just to save face.
Keeping personal conflicts off the air. Settle your arguments on the telephone, the
Internet or in person. Just keep them off the air.
Cutting a net or a contester a break, even if you don't have to and even if you have
no interest whatsoever in nets or contesting.
Operating so that if a neighbor, niece or nephew or news reporter hears you, that
person will be impressed with Amateur Radio.
Realizing that every right carries responsibilities, and just because you may have a
right to do certain things doesn't mean it's right to do them in every circumstance.
You don't "own" or get preference to use any frequency even though you've been on
the same spot every morning for years shooting the breeze with Arnold.
Not operating so that whoever hears you becomes sorry they ever got into Amateur
Radio in the first place.
WCERC Club Rules
1.
2.
3.
4.
5.
6.
NO Radio or Club POLITICS!
Don’t do or say anything to another
member for which you should apologize.
Have Fun.
If you dish it out, you better be able to
take it; sooner or later, somewhere,
somehow, someone will get you back.
If in doubt, refer to rules 1, 2 and 3!
Enjoy the Hobby and Friendship.
Thank you for your time, patience,
understanding and participation.
We have one of the best and growing
clubs in the entire area, and it’s
because of our quality members!
The End