To All from PA0SE
Walter Blanchard quoted the RACAL expert who has in the past designed the
Decca antennas as follows:
(...) If the small one has an assymetric horizontal component (flat-top)
this will complicate matters considerably since it will radiate both
vertical and
horizontally-polarised and there will be interaction between them that
will affect the radiation pattern. So it would be unlikely to have a
circular
polar pattern and this might account for some of the observed variations.
This is not confirmed by computer simulation.
Computer program AO by Brian Beezley, K6STI, shows for the L-antenna over
medium ground an almost perfect omnidirectional radiation pattern. The
deviation
from a circle is no more than about 0.1dB.
The program also shows that the maximum of the horizontally polarised wave
is straight up and 21.1dB down on the maximum of the vertically polarised
wave.
The latter maximum occurs under an angle of 12 degrees (direct + reflected
wave).
In the region where the inverse distance law is applicable the space in the
vertical radiation pattern between zero degrees and the lobe under 12
degrees is filled by the surface wave.
