Design a quarter-wave monopole. Enclose the antenna in a fiberglass hollow rod or Sullivan pushrod sleeve. Have the antenna flush against the body when retracted, via a servo mechanism that allows you to deploy it vertically after take-off (and retract before landing). The autopilot should auto-deploy and retract the antenna based on height AGL at the "take off" and "landing" locations.
There are two viable methods for retracting the antenna. One is by employing a standard servo with 90 degrees of motion to change from vertical (deployed) to horizontal (stowed against the body); the other is by using a winch servo (VSD-22YMB, which can do up to 6 turns) with a kevlar line attached to the end of the antenna, effectively "reeling in" the end like a big fish. This would not place the antenna flat against the body, but it would "bow it in" like a fishing rod.
Thin-wire radials (of one-half wavelength at the lowest freqency in use) should be impregnated in the wings and fuselage for use as a ground plane, rather than trying to secure an electrical connection to the epoxy-impregnated carbon fiber body/wings.
Another alternative would be to create a similar retract mechanism using a coaxial dipole instead (which would obviate the need for a ground plane - a requirement for the end-fed monopole). In most cases, several ferrite beads (type 61 material) should be used where the coax meets the load, to prevent the feed line from radiating.
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