I HAVE BEEN fielding several questions
this summer from members pertaining to
loss of control or loss of link with 2.4 GHz
radios. They usually start out in a similar
manner with a statement such as, “we
thought this stuff was bullet proof, but
lately we’ve been experiencing problems
and having some unexplained crashes.”
Often they can be attributed to battery
problems, or setup, or just plain dumb
thumbs, but still there are problems with no
answer.
These problems have been discussed
many times within the Electronic
Technology Committee (ETC) and theories
have been tossed around.
Warren Plohr has been on the ETC for
many years and is a well-known expert in
the field of radio frequencies and, in
general, a radio guru. I sent a series of
questions for his review and comment and
the answer I received was so well-written
and informative I felt it wise to print his
comments (with permission) for your
review. It may shed some light on some of
the issues we’re dealing with regarding the
use of 2.4 GHz radios.
Thank you, Warren!
“Reading the mail, I sense that that there
is a degree of ignorance about the 2.4 GHz
spectrum we use. The primary users of the
band are high-power users, holding a FCC
License. RC is a secondary user of the
band, operating under FCC Title 47, Part 15
(47 CFR 15) low-power rules, with no
rights against interference from the primary
user.
“The rules say that RC must cease
operation if it interferes with the primary
user. Go to: http://en.wikipedia.org/wiki/
Electromagnetic_interference_at_2.4_GHz
for general information about the band we
use.
“RC is permitted use of the low-power
band, provided the RC components meet
specific radiation requirements. RC
systems that do, display an FCC ID on the
component: all transmitters and some
receivers. All non-RC systems using the
band are also required to meet the same
standard. If all systems in use are designed
to the FCC standards, there will be no
interference between systems—with some
limits.
“One limit is a crowded spectrum.
Think of the 2.4 GHz radio signals at a
flying field as a cloud of noise-like signals.
The task of a receiver is to find the wanted
signal, and reject the unwanted. High local
activity makes it more difficult for a
receiver to find the wanted signal from its
companion transmitter.
“Cal Orr performed two experimental
tests of 2.4 GHz systems that provide a
real evaluation of receiver performance in
a 40-transmitter environment. Cal’s tests
are documented, and are considered a
valuable contribution to the evaluation of
spread spectrum RC on 2.4 GHz. See:
www.jrradios.com/Content/PDF/Challenge
_AD_2.pdf
“Use of a high RC frequency, like 2.4
GHz, introduces both additional
advantages and problems. An advantage
exists because receivers using 2.4 GHz
will reject low-frequency noise: the kind of
noise generated by airborne components
such as ignition engines, servos, and
electric motors. Aircraft noise suppressors,
often needed for 72 MHz aircraft, are not
required when using 2.4 GHz.
“The 2.4 GHz transmitter-receiver link
is impacted by the light-beamlike
characteristics of the RF signal, in two
ways. The very directional antenna of the
transmitter must see the receiving antenna
as the aircraft dances around in the sky.
The industry has found that a multiplereceiver-
antenna installation is a solution,
provided the installation in the aircraft is
properly done.
“The other 2.4 GHz problem,
reflectivity, creates a two-signal-dual-path
link that confuses the receiving decoder.
Reflections from nearby metal structures
are the usual cause. The problem is
significant, when flying inside a metal
structure. Perhaps the Lockheed-Martin
structure discussed is the source of
reflective interference at that specific
location.
“What to do about it? Fortunately, the
tools we have used for decades still work.
Perhaps the oldest—a full-power groundrange
check—is an effective way to
evaluate 2.4 GHz directivity problems:
both antenna-link and reflectivity.
Interference from non-RC sources can be
recognized by using the received servo
response as a detector.
“A forgotten tool is the missing-pulsecounter.
It’s use in detecting outside
interference and link issues is described in
detail in two AMA Technical Reports,
published in the 1990s, written by
Hershberger, Steiner, Plohr, and
Underwood.
“A spectrum analyzer (SA) is a groundbased
receiver that visually displays the
received signals of a band of frequencies. It
has been used to identify interference on
the 72 MHz band and can be used in the
same way for the 2.4 GHz band.
“The good news is that 2.4 GHz SAs
are low in cost, if you don’t count the
investment in a laptop computer needed to
visually display the received spectrum. A
typical device plugs into a USB port of the
computer and runs on provided software.
Two on the market are the Wi-Spy 2.4i
(www.metageek.net/products/wi-spy) and
AirView (http://ubnt.com/airview).
“The bad news is that any SA requires
an RF-knowledgeable operator for best
results. I urge anyone trying to use a 2.4
GHz SA to request help from a local
amateur radio club.
“One option for a 2.4 GHz RC user is to
change operating frequency to the 72 MHz
band. RC is the primary license holder on
these 50 frequencies. No other users are
legally permitted use, and experience
confirms that illegal use of our frequencies
was never a problem.
“It is well known that any RF
interference on 72 MHz came from other
RC users. Quoting Pogo, ‘We have met the
enemy and he is us.’ It is unfortunate that
discipline in the use of our narrowband
frequencies has been so difficult to
achieve. You might give that move some
consideration as another way to solve your
2.4 GHz interference issue.” MA
Edition: Model Aviation - 2011/11
Page Numbers: 161
