Graham S,
There isn't a proven health risk from the radiation from a transponder antenna
but there is also a lot of concerned research. Now the qualifier, all none
ionising radiation causes some gentle joggling of water molecules within the
body and will cause some local heating. For the extent and result you would have
to ask a medical expert, except to say there is frequency dependancy and of
course power dependancy as well as the period of exposure etc. I know of
research which attempts to show a link between eye cataracts and hand held
cellphones. So treating RF with caution irrespective of power level and
frequency is perhaps a good starting point.
The RF level from the transponder is actually tiny although they may be quoted
as having typical output powers of 150 watts or more the duty cycle is only a
few percent, so low in fact it will hardly warm a 50ohm dummy load. Of course
the duty also varies with interrogation rate.
Reasonably though you should never take risks and positioning the antenna on
a ground plane (1/4 wave monopole) helps in this respect in providing some self
shielding, something that comes for free with a tin plane. At minimum a 140mm
diameter disc bonded to the inner skin but larger will not impair performance.
As far as the half wave dipole you refer to, the ARINC specs on antennas show
implied preference to a monopole so that there is a slight downwards tilt to the
field which is probably why the vast majority of commercial antennae are in
fact monopoles even though they may be dressed up in a streamlined blade to
confuse the issue.
This is mainly to ensure the main lobe points towards the near horizon where
the Secondary head is more likely to be located than wasting energy that
will otherwise travel spacewards above the horizon. The ARINC specs deal very
specifically with MTLs (Minimum Triggering Levels) for all modes of operation,
more especially for Mode S, and some of these levels may not be as achievable
with other antenna patterns. Ensuring true omnidirectional field patterns with
dipoles that are not in free space is fraught with problems especially at higher
frequencies.
The drag associated with a half wave will in theory be at least twice that for
a monopole, something which the Europa's design sets out to minimise from the
outset.
--
Graham Edward Laucht
Birmingham UK
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