Several different techniques are being used on composite aircraft to produce the antenna ground plane required by a 'spherics (Stormscope) and a TCAS-type traffic alert.  I started an investigation into the use of a metallized graphite scrim that could be easily applied.  At this point that application is questionable because the Ryan TAS operating frequency is 1 GHz and the wavelength is so short that the open spaces in the scrim might become re-transmitting antennae.  Jury's still out on that one.   However the engineers at Ryan like Bob Schofield have been helpful beyond measure, and here is what I have gleaned from them so far: 


The ground plane must be VERY conductive.  The resistance from center antenna attach point to aircraft ground must be less than 10 milliOhm - 0.01 Ohm.  Don't try to measure that with the VOM that you bought at NAPA.  The plane must be symmetrical along 2 axis fore-aft and port-starbord.  Curved to match the fuselage shape is OK. 


A ground plane of solid metal foil seems to work best.  Conductive paints based on Ni or Ag plated copper will NOT work over time.  Tests on composite helicopters have shown that the painted plane is effective initially but after one year begins to degrade.  The engineers theorize that the paint becomes brittle with age and begins to crack.  That leaves slots that re-transmit and isolated islands that are not grounded.  The situation would be worse with a pressurized aircraft where the skin walls flex out and back with every flight, so a definite no-no for the Lancair IV-P and ES-P. 


One material recommended is a thin Al foil available from McMaster-Carr, p/n 9060K16.  It is dead soft, 0.005" thick, and comes in rolls 36" wide by 100 feet long.  The full rolls cost $145 - hopefully your avionics shop will sell you smaller quantities.  Typical installations require two pieces about 3 ft by 3 ft each.  The material is soft enough to conform to curves and bumps on the cabin deck and overhead, and could be adhered with a contact spray like 3M 7700.  The ground plane must be connected to the aircraft ground with at least two 12 to 14 ga. wires - do not rely on the coax cable shield to ground the plane.


The external antenna should be mounted over a thin Al sheet cut the exact same footprint as the antenna.  That sheet should be affixed to the fuselage with two 6/32 countersunk machine screws that go through the Al sheet, then the fuselage and then the internal Al ground plane where the screws are fastened with two self-locking nuts.  The external antenna is then attached with its six screws to the fuselage, trapping the Al plate beneath, and the external sandwich edge is sealed with RTV. 


The coax cable to use is 50 Ohm RG-400 cut 16 feet long.  Not about "15 feet" - cut it 16 feet long.  The extra could be looped into a large coil at least 2 feet in diameter.  You will need 4 of the leads, they each must be 16 feet long, and they should measure 2.5 dB to 3.5dB at 1 GHz. 


More details when and if I get 'em. 


Robert M. Simon,  GlaStar N161GS and Lancair ES-P(xl) N301ES.