Focus on Competition
Spatial issues pertaining to the use of 2.4 GHz radios
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, 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 a few of the issues we’re dealing with regarding the use of 2.4 GHz radios.
Thank you, Warren!
"Reading the mail, I sense 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 an 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—within 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-beam-like 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 multiple-receiver-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 ground-range 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-pulse counter. Its 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 ground-based 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 there, and experience confirms that illegal use of our frequencies was never a problem.
"It is well known that any RF interference on 72 MHz can come from other RC users. Quoting Pogo, 'We have met the enemy and he is us.' It is unfortunate that the narrow-band nature of the 72 MHz signals has been so easy to abuse by a few users. You might give that some consideration as another way to solve your 2.4 GHz interference issue."
MA
Transcribed from original scans by AI. Minor OCR errors may remain.
