35Mhz Interference Tests

Dec 30, 2011   //   by Nigel   //   Tech Info  //  No Comments
  • 35Mhz. Interference Tests

The new Exeter & District Radio Control Club flying site at Wrayford Field, near Kingsteignton, is only 2.1 miles from the  flying site at Little Haldon and only 2.3 miles from the Teign Valley flying site at Newton Abbot Racecourse. It was considered necessary to conduct actual flying tests to see if co-channel interference on 35 Mhz would be a problem, or not. Our concerns were due to both sites being each side of a large valley and easily visible each way with the naked eye.

 
 

 

Wrayford Field by Moto-Cross Track

On Saturday afternoon, 12th February 2011, I prepared my old Junior 60 fitted with a Futaba FP-R115F PPM receiver and servos, to operate from two different Futaba 35 Mhz transmitters. A “J” Series six channel transmitter, and a Gold “F” series seven channel transmitter. Both of early 1980’s vintage. Both transmitters had equal power outputs and RF bandwidth as verified on a spectrum analyser. The Gold “F” series transmitter had full alignment tests five years ago at the Ripmax service centre, along with a written technical report that verified full compliance with all EC regulations for 35 Mhz radio control equipment. Both transmitters were operating on Channel 70, which is in the middle of the 35 Mhz band.

Richard Ray picked up the Futaba Gold transmitter and the club “Mainlink” 35 Mhz frequency scanner from my house and then drove to the new club site with his wife. I drove to Little Haldon with my wife, assembled the Junior 60 and we walked to the runway intersection. Roger Perryman ( E&DRCC Vice Chairman) met us there to assist and observe the tests. Telephone contact was established between our repective wives and Richard confirmed that no 35 Mhz signals were being received on the scanner, now located on his car roof. My transmitted signals from Little Haldon were not being received by the scanner. So far so good. 

I started the OS FP20 fitted with a wooden 11” X 4” propeller, and with a short takeoff run, it climbed briskly into a 10 mph south-westery breeze to an estimated altitude of over 500 ft, at about 500 yards distance towards Richard’s location. The height and range was at the limit of my “comfort zone”. Richard was instructed to switch on and hold the transmitter high and rotate the aerial to the horizontal plane and back again. Nothing happened…..Not a single glitch…..Full control was maintained. By this time, I had fed in full down trim and shut the engine down to tick over. The Junior 60 as still climbing in the slope lift like a homesick angel.!!

I then switched my transmitter off, the engine immediately went to full throttle and the plane commenced a left hand spiral dive…….I switched on again quickly, and full command was resumed immediately. This was exactly the control inputs that Richard was giving…..with 50% rates ON.

I switched my transmitter off again. As the plane commenced a full power left hand spiral dive again, I requested, via our telephone operators, that full right rudder be given. The Junior 60 immediately banked to the right. I then called for half throttle, but this was misunderstood and the plane reverted to a left hand spiral dive. I then switched on again and full control was immediately resumed. This was as Roger voiced his concerns, just a moment before the wings might have been ripped off !

 I could have stayed in the slope lift but did a long traverse with the aerial pointed straight at the plane giving minimum signal propogation and glided down with a dead stick engine for a landing just 25 yards away.

 Results:

:         With the plane at over 500ft height in distant visual range and at an estimated horizontal distance in excess of  500 yards towards Richards location, the main transmitter was in full control at all times with no observed glitches.

 2:         When the main transmitter was switched off, control was immediately established by the “interfering” transmitter.

 3:         Each time the main transmitter was switched on again, full control was immediately re-established.

 Conclusions: 

1:         The known FM radio receiver phenomenon known as “capture effect” whereby the receiver “locks-on” to the strongest signal was apparent during this test.

 2:         The observation that control was established by the distant transmitter when the main transmitter was temporarily switched off, showed that the PPM receiver was sensitive enough to lock-on to the other transmitter at an estimated one and a half miles range.

3:         Full control of the model could be maintained at all times when the main transmitter was switched on.

 4:         The COMPULSORY use of ODD or EVEN channels on 35 Mhz at our local flying sites need NOT be imposed.

 Recommendations:

1:         These test results and recommendations shall be distributed amongst the local flying clubs.

2:         It is a recommendation that EVEN channels are used at Little Haldon for all model aircraft.

 ( This due to prior use of EVEN channels on a national basis by model gliders)

3:         It is a recommendation that ODD channels are used at the E&D RCC flying site at Wrayford Field.

4:         It is a recommendation that EVEN channels are used at the Teign Valley Flyers site at Newton Abbot Racecourse.

 These are only recommendations because the testing of all makes of transmitter, receiver and model combinations would be an impossible task. The growing popularity and use of 2.4 Ghz equipment avoids frequency clashes.

 Nigel Rollason  (Hon.Secretary Exeter & District Radio Control Club)

13th February 2011

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