[Antennaware] Testing the theory..
k9ay at k9ay.com
Thu Mar 29 11:02:01 EST 2007
>> ...snip... In addition I want to measure the field
>> strength with the different ground system components as well. (also
>> using the LP-100). My question is how far away from the antenna should
>> I measure the field strength?
> I recall reading somewhere that you should be at least 10 wavelengths
> distant from the antenna. This is to ensure you are out of the near
> field when taking the measurement.
> I've always wondered however, if the near field wouldn't also show a
> similar change in field measurements as the far field.
> Erik n0ew
Ultimately, the far field is where it is impossible to distinguish the
radiation due to distribution of current along the antenna from that of a
point source. This can be quite different for various antennas and arrays,
which is why we have rules-of-thumb instead of definite formulae.
In the near-field, A/B comparison measurements on a simple antenna like a
vertical would normally be valid, but not necessarily so with changes to the
ground system. The change in the surface wave (ground wave) can be different
from the change in skywave.
However, as long as the area of the radial system is the same (e.g. same
diameter, but adding more radials), I can't think of a case where an
improvement in ground wave does not have a corresponding improvement in
skywave radiation. It just might be a different amount.
More information about the Antennaware