Dear Brian, LF Group,
Very interesting to see the field strength measurements spreadsheet from
CT1DRP. At M0BMU, the results of similar measurements show comparable
variations in calculated ERP - a range of a few dB. However, there is no
obvious trend in variations in ERP with distance or azimuth angle. The
terrain around here is fairly flat, with height variations of only a few 10s
of metres in the measuring area, so there is little variation in elevation.
The variations that occur between different locations are quite consistent
if the measurement is repeated over a period of months, so it seems likely
variations are due to some feature of the receiving sites, rather than just
being "random errors" in measurement.
At CT1DRP, the results show a rather convincing trend to a higher apparent
ERP at higher elevations. I am no statistician, but there are enough data
points to make it rather far-fetched that this could be some random
coincidence, so I think it must be a real effect. I am not sure about how
this might occur. If it were a case of simply raising the measuring antenna
to different elevation angles, then it would seem reasonable that the
results show part of the radiation pattern in elevation. But of course, the
ground plane underneath the measuring antenna is also being raised up by the
same amount. If the raised ground underneath the measring antenna was
totally transparent to radio waves, or was a perfect absorber, then the
apparent radiation pattern of the transmitting antenna should be unaffected.
But since the ground is neither of these things, I'm sure it must have an
effect on the field at the measuring antenna. Since the scale of the
distances and heights of the sloping ground "obstacle" involved are
comparable to the wavelength of the signal, I think the processes involved
must be a mixture of reflection and diffraction. Intuitively, it might be
somewhat like water running over a bump in a river bed - there is a local
increase in the current where the obstructed water is forced out of its
original path to join the flow above and around the bump. In an analagous
way, might the RF energy being diverted up the hill lead to an increased
field intensity at the top?
Cheers, Jim Moritz
73 de M0BMU
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