[BC] RF field intensity at a transmitter site

[Richard Fry]

RichardBJohnson wrote about his belief that fields near antennas always are 
related to their inverse distance from the antenna, regardless of that 
distance:

>There was a response from one reader that it was not correct and that it 
>seemed to be derived from some incidental field intensity measured 
>somewhere.  This is not correct.

The equation for field intensity posted by Mr Johnson is correct only in the 
far field of the antenna.  This can be seen in the graphic from John Kraus I 
linked in that post, repeated below.

http://i62.photobucket.com/albums/h85/rfry-100/KrausGraphicFields-.jpg

For those who may have missed that point, and to spare the added research 
that may be needed, below is a clip on this topic from RADIO ENGINEERS' 
HANDBOOK by F. E. Terman (1st Edition, page 771).

Note that Terman's Eq. (2) in the first paragraph applies to the radiation 
field (far field) of the antenna.

Both Kraus' graphic, and especially the last paragraph in the quote below 
support my prior statement that EM fields close to a radiating antenna 
increase faster with decreasing distance than the inverse distance value.

 \\  FIELDS IN THE VICINITY OF AN ANTENNA - INDUCTION FIELDS

The electric and magnetic fields in the immediate vicinity of an antenna are 
greater in magnitude, and differ in phase, from the radiation field as 
calculated with the aid of Eq. (2). The electric and magnetic fields that 
must be added to the radiation field in order to give the fields actually 
present are termed induction fields.

These induction fields diminish in strength more rapidly than inversely 
proportionally to distance. Thus the induction magnetic field from a doublet 
is inversely proportional to the square of the distance, and the induction 
electric field from a doublet has one component that is inversely 
proportional to the square of the distance and another one that is inversely 
proportional to the cube of the distance.

Inasmuch as the radiation field is inversely proportional to the distance, 
the induction fields die away much more rapidly with distance than do the 
radiation fields, and at distance of a few wave lengths become negligible in 
comparison with the radiation field.

However, at distances from the antenna that are small compared with a wave 
length (or small compared with antenna dimensions if the antenna is large), 
the induction electric and magnetic fields will be much greater than the 
radiation field of the antenna. //

One reader