"Roy Lewallen" wrote in message
...
wrote:
I am curious as to whether RF exposure concerns are greater for a
small transmitting loop [like the MFJ tuned loop] compared to a dipole
radiating the same power. It would seem that close to the loop, the
RF power density may be greater [than it would be at the same distance
from the dipole apex] since the radiating volume is smaller. Can I
just assume that the power is evenly distributed on the surface of a
sphere having a radius equal to my distance from the loop antenna,
calculate the power density on the sphere surface, and use that number
for evaluation - or are there some near-field considerations not
captured using this approach?
Thanks,
-JJ
The method you describe is valid only in the far field. There are higher
order terms to the field strength (field relative to distance) in the near
field, and they're strongly a function of the distance and the antenna
geometry. Using the method you propose can produce very erroneous results
close to the antenna.
Roy Lewallen, W7EL
A good question and an interesting, but not very helpful response.
It seems to me that there are elements of truth in both the original
proposition and in the response comment, but that each is only a partial
truth. The essential aspect is surely the separation distance relative to
the size of the loop antenna.
However, the obvious comment is that the small physical size of a loop is
likely to lead to use in a situation (for example, indoors and close to the
operating point) that would/could lead to excessive levels of RF exposure.
For an electrically small loop (the typical loaded loop less than 0.1
wavelength), then it is probably fair to assume that all of the input power
is radiated through the sphere surrounding the loop provided that the
separation is reasonably large, however, for a large loop (eg half-wave or
larger) its probably best to approach the RF exposure issue as you would
with any other antenna such as a dipole or vertical.
Keith G Malcolm
VK1ZKM
28 July 2007