I'm a little late, but you're fine. The amount of RF danger to humans depends mostly on frequency and power levels. Transponders do operate in a frequency range that is potentially dangerous for humans (pretty much everything aviation related is), but the power levels they operate at don't pose a significant danger. Plus, as Carl noted, transponder antennas are vertically polarized, meaning they radiate primarily on a horizontal plane out to the sides. Add to that the fact that a metal airplane will add significant major protection, and you have no worries. That's still true in 'plastic' planes. The protection is at a lower level, but still plenty to be safe. Having said all that, the warning you mention isn't about the antenna at all. The "antenna feeder" is the cable going to the antenna, not the antenna itself, which makes sense when you consider the antenna isn't inside the aircraft. The antenna cable should be shielded, so it doesn't radiate RF energy like an antenna does. What minute amounts of RF may emerge from the cable is more of a concern for potential interference with other equipment rather than a problem for occupants. So I suspect that warning is much more due to possible interference issues with other equipment rather than due to a potential danger to humans. If you used waveguides rather than cables, it could potentially be a different story. But using waveguides in a Zenith aircraft would be akin to putting an ILS system on a powered parachute. Ain't gonna happen.
If you really, really, really want to look at the science involved, and you really, really, really love doing complicated mathematics, you can go here:
