Signal Absorbtion

[Matt]

Hey Guys

One of my friends was having trouble with his GPS system the other day, we think that the carbonfibre airframe absorbs the signal that the gps sytem getting from the satellites Etc

does anyone have any clues to why this, and maybe how to fix this problem


thanks heaps

 

[Justy]

Yep... don't use carbon fiber. Or, more precisely, get the antenna outside the carbon fiber. My best solution is to use a wire antenna and string it up the parachute or drogue shock cord, but I haven't tested this yet.

Alternately, you could put the antenna in a section of airframe made of another material, like phenolic or filament would fiberglass, or a hollow plastic nosecone.

But as for a way of making the carbon fiber more radio-transparent, nothing I know of.

For the record, black ABS plastic has the same problem. I'm told it has to do with the carbon filaments resonating with the radio signal.

 

[Newtons 3rd]

Because CF is highly conductive (the C is after all Carbon) it acts as a very good absorber of RF. Phenolic isnt the best either. With our GPS and video systems we use a FG payload section on birds that are CF. An easy way to test for RF absorbtion is to put a small sample of the material in the microwave with a small cup of water for 30 seconds. If the material heats up it is absorbing RF and is not appropriate for RF applications. Our newest payload uses a 3" phenolic tube wrapped with SGlass. At the point where the GPS antenna is placed we have cut away the phenolic to make a "window" that is the FG only, this minimizes absorption and will give you much more reliable locks.