Ok,

 Thanks guys; I was not sure about the electrical properties of carbon fibre, but I’ve had a look on google as well now. 

If the carbon fibre is a good ground plane, then it may not be necessary to grind the resin away to make a physical connection. What ought to work is to use a disc of a specific radius flat against the skin but electrically isolated. The centre of the disc will then appear to be connected to the carbon fibre at the specific frequency. 

The radius needs to be a quarter wavelength, but corrected for the presence of the resin. In practice going about 15% lower than a quarter wavelength should work out.

 The disc can go on the inside of the skin. Other shapes than a disc are also possible. The key is that a quarter wave transmission line open at the far end appears as a short circuit at the near end. 

Nick

 

Yes “carbon” is a good conductor. Adhesive and e-glass are not a good conductor.  I painted on my ground planes and checked the conductivity of the conductive paint.  The conductive paint is .002" thick and was painted on over deglazed carbon.  Here are the conductivity test results using a very expensive ohmmeter.

 

Biddle DLRO Meter (Digital Low Resistance Ohmmeter)

Full scale of lowest setting is 6000 micro ohms (setting for this test was 6 ohm full scale)

The Com ground planes are 36” diameter and the TCAD/Transponder are 12” diameter.

 

Probes at Center of circle (antenna mounting area)

Transponder  =  .040 ohm

Com1           =  .035 ohm

TCAD           =  .045 ohm

Com 2          =  .037 ohm

 

At far extreme of circle

Transponder  =  .106 ohm

Com1           =  .134 ohm

TCAD           =  .118 ohm

Com 2          =  .096 ohm

 

From one ground plane to another (probe at centers) (This shows how conductive carbon is)

Transponder to Com1  =  .130 ohm

Com1 to TCAD           =  .063 ohm

TCAD to Com2           =  .112 ohm

 

Craig Berland

N7VG