Comparing these calculated field strengths with the measured
values
shows that the actual field stength is about 7.5dB down on
the calculated
field strength. Increasing the height of the antenna
gives an increase in
field strength of 2.8dB, close to that predicted
by the radiation
resistance formula.
My experience is the same, calculating
power into the aerial from the radiation resistance of the aerial (found
by computer modelling) and the aerial current and measuring the field strength
at a distance of more than 3 km. I cannot remember whether the discrepancy was
also 7.5 dB but it was considerable.
The other possibility is the vertical directional pattern of the
antenna. An ideal short monopole over perfect ground has field
proportional to the cosine of the elevation angle, ie. maximum at
ground level and zero straight up. As others have discussed,
imperfect
ground results in a null in the radiation pattern at ground
level. If the
maximum radiation occurs at 20 degrees elevation, you
would have to
position the measuring antenna at a height of 1400
metres at 4.2km
distance in order to measure it.
As I understand it (but I am not an expert on
propagation matters) the power at low angles not contained in the lower part
of the vertical radiation pattern is the power that goes into the surface
wave. Over a perfect ground the surface wave could not exist.
I checked this by computer modelling the field
at a distance where the near field would be negligible, so what remains
in the total field is the sum of the direct wave, the wave reflected against
the earth (these two almost cancel each other due to the 180° phase change at
the reflection point and the almost equal path lengths at grazing angles) and
the surface wave. The field remained of the same strength up to at least 2
km.
Vaino, OH2LX, by measuring field strength
using an aircraft, also found that field strength hardly changes with
height.
He also gave as his opinion that finding
ERP by measuring field strength is almost impossible.
I wonder how the authorities do it in
countries where ERP is limited and not transmitter output, as in the
Netherlands?
It seems that The Mystery of the Missing
Decibels remains unsolved for the time being.
73, Dick, PA0SE
JO22GD
