Hello group,
it seems to be generally accepted that a small vertical antenna has a
gain of 3 (4.77dBi / 2.62dBd), due to the radiation pattern in the
vertical plane.
But in very few cases this gain seems to be measured when doing ERP
measurements.
As far as I understand this gain of 3 is based on a perfect ground
(endless conductivity) that acts as a (perfect) mirror and thus
creates the "mirror image" needed for radiation.
But what about a short vertical antenna above a not-so-perfect ground?
I have done some simulations with MMANA-GAL (15m high vertical at
500kHz) and over a perfect ground I get the expected 4.77dBi and a
take-off angle of 0 degrees.
If the conductivity is not endless the gain drops and the take-off angle rises:
1000mS/m (salt water) = 4.5dBi / 9 degrees
500mS/m (clay) = 4.4dBi / 10 degrees
50mS/m (marsh) = 3.9dBi / 14 degrees
15mS/m (loam) = 3.5dBi / 17 degrees
5mS/m (wet sand) = 2.9dBi / 21 degrees
0.05mS/m (dry sand) = -1.5dBi / 32 degrees
Could this explain (a part of) the missing dB's ?
73, Rik ON7YD - OR7T
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