Originally posted by Kris^
Crash . .if the secondary "sparkplug side" of the ignition is putting out enough interference to saturate your igntiions battery leads and battery with voltage spikes strong enough to cause interference, the receiver and radio wiring is already taking quite a beating from it.
Think of it this way .. the BATTERY and switches on an ignition handle about 6 volts, with on-off "spikes that range from 30-100 volts usually. These are VERY tame compared to the actual firing voltage of teh sparkplugs, which can exceed 20,000 volts easily. That's 200-600 times stronger of a voltge spike (with attendant RF interference).
I routinely have my receiver and ignition batteries withing 6" of eachother, as well as having other radio related items, such as servos, in the firebox or within 4-5" of the ignition itself. I've never experienced interference from this. . EXCEPT when one of the sparkplug leads or plugs develops a problem. Then, it does not matter if all my radio gear is 3 feet away. . I get glitches.
Case in point. .for 125 or so flights my 2.6m Fiberclassics Extra flew fine. . then one day I took off and had NOTHING on the radio. .I managed to get the engine throttled back and regain control (obvious ignition interference problem there) and landed. My post landing range check was less than 25 feet, with antenna collapsed, even though it had jsut passed a 200' range check when I first ran the engine. the problem? One of the spark plug terminal ends developed a problem and had an air gap of at least .020" between the terminal and plug. . caused so much interference you could hear the engine running on my van's radio, on the AM band, 40 feet away!!
Quick rule of thumb. . at least 6" and 18" is a better idea. . but if you get a SERIOUS RF problem from an ignition, you can have all the radio components in the tail, and it won't matter a bit. Make sure the ignition leads are good and undamaged, the plug caps make good clean contact with the plug, and all the wirign for the battery and switches is in good shape. . after that, there's almost no problem with interference.
Please keep a copy of this post and repeat it whenever one of our "experts" claims you must have several inches separation from the ignition system.