In message <000301bfcc87$d6d381a0$9dd699d4@w8k3f0>, Dick Rollema
<[email protected]> writes

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.

And yet did not Marconi use a vertical antenna with a long horizontal
wire to obtain directivity from Cape Race? When the original aerials
came down in a gale he found equal or stronger signal strengths using an
inverted 'L' with the arm pointing the right way.

Mike
--
M.J.Powell