To answer questions I fabricated a foam shape similar to what I believe Robert displayed in the first post to test the effects on antenna performance.
The test consisted of a TED monopole transponder antenna mounted on a 8? dia ground plane and 1? thick blue "Home Depot" type foam. The foam shape did not touch the antenna element.
Data collected with Anritsu antenna measurement equipment and calibrated with traceable standards.
Results:
The antenna resonate frequency was pulled down but the VSWR was still within reasonable limits. Based on what I tested I would not be hesitant to attach this antenna system to my transponder.
Suggestions:
Minimize material on the exterior of the foam shape, construct a thin FG layup to form the hard shell.
Do not use the antenna element to attach the foam shape, the results improved when the foam did not touch the element.
A base plate could be constructed from a material that will adhere well with epoxy. A wood paint mixing stick may work well in this application.
Studs or nut plates could be bonded on the base plate to attach the foam shape on the airframe.
Position the mounting studs or screws with the hole layout shown below as it will allow mounting a Delta Pop or Comant CI-105 transponder antenna in the future without drilling any additional holes.
As always, use caution when modifying antennas, the proper test equipment should be used to insure the modified antenna meets the system requirements before attaching to your expensive avionics.
Depiction of test antenna