I was not aware that I have to build a ground plane for the transponder antenna until I started reading the installation manual for my Trig TT31 transponder. I also noted from the Cozy archives that the many Cozy fliers archived best result when their transponder antenna sticks out of the bottom fuselage skin. Since my TT31 unit is mounted in the instrument panel, it would be best if I can place the transponder antenna somewhere up front. For me, the one logical location is forward of the co-pilot's rudder pedals - on the fuselage floor.
I cut up an octagonal aluminum plate per installation manual with a hole at the center to mount the antenna post (left). A bit difficult to see in this picture, I carved a step in the foam for the aluminum plate to embed flush to the top of the foam. It is because I plan to cover up the antenna with 2 layers of BID later on.
One of the criteria for the ground plane is that it should be level to the ground. Unfortunately, our fuselage is curved from aft to front and side to side. That means I have to shave the bottom of the foam to accommodate. That required some patience because I do not have a good way of measuring the curvatures of the fuselage floor. I double taped two miniature bubble levels on top of the ground plane to guide me along.
The total height of the foam is also an important consideration because it has to be thin enough to allow the antenna post to protrude out the bottom of the fuselage. Based on the length of the antenna post and the estimated thickness of the fuselage floor (~3/8") + support foam, I should have ~1" protrusion...That was WRONG! I forgot that the fuselage bottom at the nose area is much thicker than that.
Not realizing the error (above), I went ahead and glass the sides and top of the foam in place. I first put packing tape on the floor, weighed down the plate/foam, micro fillet and applied 2 BID around the foam, leaving a footing around the perimeter for mounting later on. The miniature bubble levels are there to assure the BID skirts will keep the ground plane are level.
Once cured, I popped the glassed skirt off and trim to shape.
Subsequently, I applied 2 layers of BID onto the top of the ground plane (top cover) as shown. Weighing the BID down with another aluminum plate and weight.
Both the side skirts and top cover are trimmed and mounted with a small metal screws.
Final hole drilled through the fuselage, antenna and coax cable installed. The round tube shown at the bottom left of the picture is a cover for protecting the coax connector - just in case.
As mentioned above, I miscalculated the thickness of the fuselage bottom. It was thicker than expected. As a result, the antenna post protrusion was much shorted than planned. As you can see, the protrusion is ~3/4" and it was bit too late to correct. I decided to just wait and see.
As most of the Cozy builders/fliers before me, I mounted my GPS antenna for my Garmin 650 right under the nose cover such that there is no interference or obstruction above it (i.e. line of sight to the satellites). There were some discussions as to the size of the ground plane under the GPS antenna. I had a couple conversations with Garmin tech support and was told that the manual's instruction of 6" ground plane under the antenna is really not necessary. Fine by me!
[Hindsight] My GPS antenna works fine without the large ground plane per Garmin's installation manual.