Paul Moody <paulmoody_at_onaustralia.com.au> wrote:
>
> How near is the near field? If a 1/20 wavelength CFA is one
> wavelength from any conducting structure ( the ground ). If the
> supporting structure is PVC ( ie non conducting ) and the
> transmitter and battery supply is fitted to the base of the
> groundplane. ( to remove any possibility of feedline radiation
> ...there is no feedline ). Modulation and control is via IR link.
In free space, the near field is usually regarded as extending to a
radius of roughly lambda over two pi, but bear in mind that there is
no sharp cutoff. The strength of the reactive component of the total
E or H field decreases as the inverse cube of the radius as the radius
goes to infinity, whereas the radiation field strength decreases as
the inverse first power of the radius.
> ...The bandwidth of the antenna is 3.5 Mhz at 29Mhz centre. ( SWR
> less than 1.3 : 1 across this range ). I cannot believe that
> re-radiation can account for field strength measurements ( reference
> 1/4 wave standard ).
One-twentieth wavelength is not so small; the efficiency of an antenna
this size could be pretty good. I believe that the MF CFAs that
people are so skeptical about are much smaller in terms of the
wavelength.
73 -Chuck W1HIS
Received on Mon May 24 1999 - 19:42:14 EDT
This archive was generated by hypermail 2.2.0 : Sat Oct 02 2010 - 00:10:39 EDT