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TESTING WITH 40 DX7S TURNED ON!

We subjected the JR R921 and AR 9100 receivers to bench tests, ground-range and flight tests with more than 40 Spektrum DX7 transmitters turned on. We measured "latency," which is the time required to execute transmitter-to-servo commands. I built a circuit that plugged into the receiver in place of a servo so that the mechanical speed of a servo was not a factor. I used an analog servo amplifier whose output was connected to an LED in an opto-coupler instead of a servo motor. Then, the opto-coupler sensor signal was fed into a chip to "clean up" the pulse, which was immediately viewed on an oscilloscope. The receiver’s ground was not tied to the oscilloscope’s ground and/or the transmitter’s ground. I took the back off each transmitter being tested and connected the oscilloscope’s trigger to the back of the landing-gear switch. The landing-gear channel was used because it is a snap action; how fast I moved a joystick was not a factor.

With all 40 DX7s turned on, there was no evidence of an increase in "response time" or latency for either the R921 or the AR 9100 receiver. Our 2.4GHz band runs from 2.400 to 2.4835 in 79 1MHz frequencies. Since the Spektrum transmitters are said to use two frequencies at once, one would think that 40 transmitters would use up the entire band. I turned on 44 Spektrum transmitters, and my test radio still didn’t have any problems linking up and operating on the bench. There seems to be a time-share going on that permits more than 40 systems to be "on the air."

ANTENNA EXPERIMENTS

Ground-range tests were done to see whether antenna placement, orientation, or metal obstacles had an effect on range tests, and whether 40 transmitters turned on would affect range. I also tested to determine whether the maximum range was affected when the transmitter’s antenna was pointing at the model or if the transmitter antenna is bent at the knee so that it is at a right angle to the model. In all cases, the maximum range test showed a slight reduction in range (insignificant), as the transmitter’s antenna was pointing at the model—just as has been commonly seen with 72MHz systems, but not below the recommended 30 paces.

Metal barriers were placed between the transmitter and airplane; I used a large metal coffee can, a car and a large garage with a big metal door. The maximum range was reduced somewhat but never to less than the recommended 30 paces. With 40 transmitters turned on, however, the R921 started to show a range reduction to just below 30 paces. The JR 9303 2.4 system with the AR 9100 (also sold as the JR 922) Power Safe receiver system with the fourth remote receiver remained above 30 paces with all 40 DX7 transmitters turned on. This is a striking success story.

FLIGHT-TESTING

We flew test airplanes in a predictable manner, holding altitude and flying a repeatable big oval around the field. So that I’d be able to perceive any possible receiver holds, I flew my ovals with a constant rhythmic wing rocking, left, right, left, etc. We were able to fly both receiver systems with all 40 DX7 transmitters turned on and didn’t find any evidence of a hit.

Spektrum offers a tool called a “Flight Log” that plugs into the data port of their 9-channel receivers. The Flight Log displays receiver battery voltage, the number of antenna fades for each of the (up to) four receivers, the number of lost frames and the number of holds. Neither one of these receiver systems ever went into hold. In the case of the R921 2.4 receiver, there were a few frame losses (among 18,000 frames in a 6-minute flight) as the latter half of 40 DX7 transmitters were turned on, but the receiver never went into hold. The AR 9100 Power Safe Receiver system did not lose a single frame and had zero holds. I must say, I am very impressed with the AR 9100/JR 922 Power Safe RX System—truly, the most robust RF link I have seen under harsh testing. I want one! HobbyShopNow

Posted: 4/22/2008
Written By: Cal Orr- Fly RC Contributing Editor
Provided By: Fly RC.com



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