2.4GHz: is it all it’s cracked up to be? Join the discussion

May 30, 2011 13 Comments by

We recently received this intruiging article from Dave Horvath and invite you to read it and then join in the discussion.  

A substantial number of unexplained crashes of radio control model airplanes on 2.4GHz frequency prompted me to write this article on the so-called “interference free” radio control systems on the 2.4GHz band.            
The electromagnetic wave spectrum is subject to the immutable laws of physics.
The propagation characteristics of the 2.4GHz wavelength and the environmental effects of this frequency are more complex than on the 72MHz band. To better understand this, we have to look at the electromagnetic wave spectrum where 72MHz band is in the broadcasting region and the 2.4GHz band is in the microwave region. It is easier to see the huge difference between 72MHz and 2.4GHz frequencies when we convert 2.4 gigahertz to megahertz. Now it is 2400MHz versus 72MHz. When frequency increases, wavelength decreases. Therefore, the 2.4GHz wavelength is shorter and closer to visible light on the electromagnetic wave spectrum. Since visible light is also an electromagnetic wave, 2.4GHz wavelength behaves more like visible light and travels in straight lines until it is reflected, deflected, diffracted or absorbed. Reflection and diffraction will create interference. When parallel rays of light are reflected by a concave mirror, it greatly increases the intensity of light at the focal point. A parabolic dish antenna works the same way for a 2.4GHz electromagnetic wave. Since we cannot focus a high gain directional parabolic dish antenna between our constantly moving model airplane and our transmitter, we have to use an omnidirectional vertical antenna system which has much lower signal intensity.  

Interference            
The FHSS (frequency-hopping-spread-spectrum) and the DSSS (direct-sequence-spread-spectrum) techniques can share the same band. However, they interfere with each other causing a degradation of performance. Range decreases as the number of clear channels decreases. Bandwidth drops each time FHSS encounters a blocked frequency on a crowded spectrum.          
The crowded spectrum on the 2.4GHz band reduces the bandwidth, increases the ever present background noise, increases the adjacent channel leakage ratio, reduces the range, and causes overlapping. Overlapping is a direct interference.            
Unlike the 72MHz wavelength which penetrates most objects, the 2.4GHz wavelength behaves more like visible light. Signal absorption from objects on a model airplane like the engines, electric motors, batteries, servos, pushrods, landing gears, switches, wires, etc., may cause path interference.            
Signal reflection from objects in the terrain, like fences, walls, buildings, trees, hills, power lines cause line of sight interference. High speed data transfer reduces the receiver’s sensitivity on 2.4GHz band. There is a trade-off between speed versus range.            
The signal strength decreases quadratically as distance increases at constant radiation levels. This is called path loss. When frequency increases, path loss also increases. This is one of the reasons why 72MHz radios have a better range than the 2.4GHz radios. We can see this clearly when we look at the Wireless Range Calculator:
Frequency                  Distance                      Loss
100MHz                     0.2 mile                      62 decibel  
2400MHz                   0.2 mile                      90 decibel  
These calculations are under non-existing ideal conditions, less Fresnel (pronounced Frehnel) effect. When we fly our model airplane on 2.4GHz, the area around us is known as the Fresnel zone. Since we have to use an omnidirectional antenna system, the electromagnetic waves will scatter and diffract from objects and from the terrain around us. When the diffracted wave reaches the receiver antenna, it is slightly lags behind the signal which traveled to the receiver antenna in a straight line that creates interference due to the phase canceling effect.            
The Fresnel effect also deals with the behavior of electromagnetic waves over a water surface. As mentioned before, the 2.4GHz radiation behaves more like visible light, so we have to think of reflections and shadows. Flying a 2.4GHz radio control model over a reflective surface like water, snow, ice or wet terrain negatively affects the radio link. Occasionally a 3D aerobatic model plunges into water while hovering. When the rudder is near the water surface, the prop wash creates a chaotic wave pattern which generates a myriad of false-signals.            
The Fresnel effect and the described interference on the 2.4GHz band work pretty well. We successfully tested this at different locations. Unfortunately, the “unbreakable Tx-Rx link” broke when our model was over 0.2 miles away at 45 degree angle. Despite the fact that a 90 decibel signal loss over a thousand feet (0.2 miles) is rather significant, we should have had control at this distance. There are too many factors that can determine the overall range on 2.4GHz. 2.4GHz receivers are not immune to ignition and electrical noise as advertised. Occasional arc from high tension insulators could break the bind.  

Latency            
Latency is the time between stimulation and the beginning of response caused by propagation delays. There is a huge time difference in latency claims by different radio manufacturers. Some latency claims are in milliseconds, others are in microseconds! This is confusing since one millisecond is one thousandth of a second and one microsecond is one millionth of a second.            

Velocity of electromagnetic waves is 186,283 miles per second. The velocity of the electric signal through conductors is nearly at the speed of light. With an adequate power output, our radio signal will travel one microsecond which is one millionth of a second to reach our model airplane one thousand feet away. This applies to all brands of radios on 72MHz or on 2.4GHz. As we know, nothing travels faster than the electromagnetic waves. Therefore, I don’t see how latency could be improved “50 %” over the leading competitors regardless of different processing.

 A seven millisecond latency or a fourteen millisecond latency claim is irrelevant since the human being, the RC pilot, has a painfully slow 200 millisecond latency and can not differentiate between seven or fourteen milliseconds.  

Conclusion            
At huge events, like Nationals, the 2.4GHz pin-free radio system makes life easier for competitors and organizers. However, there is a huge difference between flying on 2.4GHz band in the beautiful country side near Muncie where chances are good that there won’t be any noticeable interference and flying on 2.4GHz band in the middle of one of the largest concentration of population and industries in Los Angeles or other urban areas.            
2.4GHz radios under harsh conditions work most of the time, however most of the time is unacceptable. Illegal signal boosting, ham radio, and rolling hills further aggravate the situation. 2.4GHz wavelength has higher incurred losses than 72MHz wavelength. In any case, we should hold on to our assigned frequencies on 27MHz, 50MHz, and 72MHz bands.            
Despite glowing reviews, the so-called “bulletproof 2.4GHz technology” has had range and reliability problems since day one. A bench test inside a building in a controlled environment where the receiver is a few inches away from the transmitter is meaningless.            

 2.4GHz wavelength is not the best choice to control model airplanes. Furthermore, we have ended up with complex radio systems on an overcrowded band on the electromagnetic wave spectrum.            

The bottom line is that glitch-free software, error-free computers, and an interference-free radio link is only an illusion.

References:  

www.google.com electromagnetic spectrum Images for electromagnetic spectrum Videos for electromagnetic spectrum  

www.google.com Frequency-hopping spread spectrum DSSS and FHSS-Spread Spectrum tutorials   www.google.com 2.4 GHz interference Interference in the 2.4GHz ISM Band: Challenges and Solutions by N Golmie 20 Myths of Wi-Fi Interference (RF Solutions)  

www.google.com path loss in the 2.4GHz Speed vs. Distance ISA 900 MHz versus 2.4GHz – Learning Center  

www.google.com fresnel zone Images for fresnel zone ZyTrax-Fresnel Zones and their Effect   www.google.com diffraction Images for diffraction Diffraction-Wikipedia    

www.google.com polarization of light Images for polarization of light Videos for polarization of light MIT Physics Demo-Microwave Polarization  

www.google.com 2.4GHz spread spectrum problems 2.4GHz Spread Spectrum problems   www.radiolabs.com Wireless Range Calculator Free Space Loss

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13 Responses to “2.4GHz: is it all it’s cracked up to be? Join the discussion”

  1. Simon Bradford says:

    This is very interesting and explains some of the issues that I’ve read about on the forums. I haven’t encountered any of these problems personally.

    Even if it isn’t perfect (what really is?), isn’t 2.4ghz significantly better than 72mhz?

  2. Bill H. says:

    OF COURSE 2.4GHZ isn’t totally glitch-free. And as long as R/C flyers are HUMANS, we’ll continue to have dumb-thumbs CRASHES that are blamed on radio interference, bad servos, badly built ARFs … you name it!

    Use commonsense, check your aircraft before each flight and fly in a safe, responsible manner. I love my programmable 2.4 radio!! If there’s something better out there, we may have to wait ten years to get it.

  3. Richard D. says:

    Bill – I agree. I think 2.4Ghz is a natural progression beyond the 72Mhz realm. One thing he forgets to mention is that multiple users on 2.4Ghz are able to fly at the same time. Only one flier can be on the 72Mhz band width. Everyone has their own opinion, but I believe Mr Horvath should do more research on his own. The 27Mhz is mostly for surface vehicles, 50Mhz is hardly used anymore. Not perfect, nothing is!!

  4. High Nitro says:

    Nice article, and interesting reading. I would offer some barnyard logic to go along with the technical stuff.

    I’ve never had radio interference on 72 or 2.4 as far as I am aware. This is year #39 in the hobby for me. I fly several times a week, 12 months, helicopters and airplanes mostly. I fly conservatively, I build/assemble carefully, and most of my models are 6-7 servos or less. I still use NiMH batteries and never had one quit in the air. I fly nitro and electrics, no gas. That sound you hear is me knocking on wood. I’ve seen a number of crashes blamed on radio interference.

    The “I ain’t got it” statement invariably is followed — after the crash — by… “the radio works now”. Hmm.

    On a really busy day, there are 20-30 guys at the field. Most are now on 2.4. Most of them never crash except when they make a pilot error. I’ve definitely done some of those.

    Like fishing, where 10% of the fishermen catch 90% of the fish, I’d say that’s about the right percentage of the crashes I’ve seen. 10% of R/Cers have 90% of the crashes.

    Not counting 700 fliers at Joe Nall, or 1,000 fliers at IRCHA. No wonder there are issues there. Our radios are masterpieces of engineering in my opinion, and if you have any idea of what they used to be like, you certainly agree with me.

  5. Michelle says:

    Very Informative! Thank you Dave Horvath.

  6. Dave says:

    You know it is interesting… I was sent here to read the commentary being misled that this was written by the magazine. It wasn’t, The arguments used were used for those who are trying to get the 900mhz radios going. I even agree somewhat. But to insert comparisons with 72 (and I had to laugh when he also mentioned the a.m. and 50 mhz that was never for flying in the first place. Is 2.4 perfect? of course not. Is 900 the way to go? Not necessarily. Are both 900 AND 2.4 far better than 72??? ABSOLUTELY. Try and distance your 72 to 3-5 miles like my Airtronics SD10-G or my 8 channel Spectrum. LOL In theory things are much different than in the real world.
    Imagine… If it is wonderful (2.40 in crowded areas where 72 would die a sudden and painful death) then how much nicer it is outside Muncie…. Free of any issues you will face in the real world… (unless you can fly your plane miles away and do a Jedi trick of keeping it flying out of site).. Perhaps there is a reason so many of us old timers as well as newcomers love the 2.4 but hated their old 72′S?

  7. Nitro-Tom says:

    Well, I have been saying for YEARS we should go back to HAM Radio….which requires a license (Which could generate some revenue for someone) and is not an open “Wild West” free for all like the 2.4 band is.

    As long as guys like JR, Futaba, and Hi-tec made Tx units I would switch to HAM band in a second. I would buy ALL NEW Rx units, even though it would cost me well over a thousand bucks, just because 2.4 is the most polluted cess-pool in the band and I can’t believe I am stuck there.

    That being said, I have had 100% flawless results with my 2.4 and many using the same gear have not. Can’t explain that other than to say there is alot to having a good setup and testing. I have been flying 2.4 weekly for about 6 years with perfect result…..I will keep knocing on wood and praying for the return of the HAM band :-)

  8. davepcb says:

    Good article. The transmitter manufacturers are working on signal hopping to minimize the signal loss issues by changing bands every 20 milliseconds or less. You can lose one or two and not see the loss of signal affect airplane. Things are better as technology is improved and applied.
    I do have an issue with :Therefore, I don’t see how latency could be improved “50 %” over the leading competitors regardless of different processing.” The issue of latency is not with signal speed, but signal processing speed of transmitter and receiver pair. These are measured in the tens of milliseconds where 50% can be realized. Think about this, in 1983 computers ran 6 Mhz, today they run 2Ghz +. Significant, noticeable, difference in processing speed. Fly a responsive airplane with a JR then the same plane with Airtronics and see if you can feel the difference in speed of signal from your hand to servo. It may surprise you.

  9. Fokker Flyer says:

    Great Article ! The unexplained crashes we have seen on 2.4 systems at our “rural” field have all been on DSM-2 and each time we see it, it is the same scenario. We have not seen crashes with the Futaba or the Airtronics 2.4 systems. I know they operate differently and I suspect they are more robust in their ability then the DSM-2 design. I switched reluctantly to 2.4 a few years ago because of pin policing issues at some of the larger events. I dont trust the honor system when the impound shuts down at 5 pm………lol

  10. keith says:

    i AM A Extra Class Ham and I find your article very accurate. I have used my spektrum analyzer at the field and you can see the block or group of channels that are active ( Hitec example 20 channels in the sweep ) .. As more radios are active I see the noise floor rise.. I also noticed that if you rebind the radio when several are active it will select the clearest group of channels at the time. ( hitec ) It never changes until re-bound.. Also Spektrum / JR have improved some of the problems with DSM by going to DSMX.. I believe that dual antennas at 45 degrees from each other will help with path/phasing loss.. Nothing is perfect and watch out for the over powered (illegal ) wireless modems . As the noise floor rises watch the degredation of range.. Great article – I enjoyed it Thank you

  11. Gene Davis says:

    Very interesting piece. What is forgotten by many flyers is the fact that the pulse from the 2.4 receivers to the servos is only about 3.2 volts and not 4.8 volts or 6 volts as is the battery. This can sometimes cause poor and erratic servo operation if the servos are older types. Correction, use a signal booster if you have this problem. Hobby King has these boosters.
    Also, high servo peak currents can cause the so called “brown out”. High current peak requirements from the servos can case the battery voltage to dip below about 3.5 volts and below where the microprocessor in the receiver will operate. This may cause the receiver to stop operating for a moment and the microprocessor may not reset in time to save a crash. Place a large filter capacitor on the battery lead. five thousand micro Farads or so. They are sold at many places and some sites advertise them in MAN.
    Some receiver come with filters. I use them all the time. You can also operate your receiver on 6 Volts to help with this problem but make sure your servos can handle 5 cells fully charged. This may be over 7 volts. Many small servos will not handle this high voltage.
    You can test the voltage dip problem by operating all your servos at once, just move the transmitter sticks in a circle. I have done this with my Hytec, and sometimes I can get a low voltage beep from the transmitter cause by a low voltage dip in the receiver battery.
    If you take precautions you will not have any trouble but nothing can fix dumb mistakes like I have had..
    Regards,
    Gene

  12. Don Weiss says:

    My first flight on 2.4 ended in a catastrophic crash due to radio failure. (Spektrum 6).
    I have since gone back to 50 Mhz where I never had a problem and am not leaving.

  13. Mach9 says:

    Nobody seems to be addressing the fact that strong WiFi links and everything under the sun is using 2.4GHz. I think in the U.S. the max power for 72MHz is 1 watt and for 2.4GHz it is .1 watt. Also, microwave ovens use 2.4GHz because it is efficient (resonant) at heating water. Why is that important? Your eyes (being mostly water and sensitive to radiant energy) can be slowly permanently damaged without you feeling it by this frequency. It is a matter of range vs power. The more power, the farther away you need to be to not damage your eyes. Hotels, restaurants, etc have large hidden antennas, some 5 feet tall, to make people happy with WiFi coverage. If you are looking in the direction of that wall or ceiling, your eyes are being damaged without you knowing it. Nobody cares because, hey, I have a signal! With enough power it will cook you like a microwave oven…before then you will be blind. The 72MHz tends to pass through your body. What does that do? The ONLY reason for the switch is because companies have a plan to acquire as much bandwidth as possible for smart phones, etc. They plan to take all of the TV frequencies as well. They are nearly maxed out now. There will always be a physical natural limit to bandwidth. 72MHz gobbles much more (50 channels available) than 2.4GHz (80 channels available). In the end, it is always about the money!

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