At 01:55 AM 4/26/2006, K8RI on Tower talk wrote:
> > At 10:45 AM 4/25/2006, Peter Sundberg wrote:
> >>You can download a program that does all calculations for you, be it
> >>yagis,
> >>dipoles, verticals or what..
> >
> > Very slick... and makes it easy to check across bands
> >
> > Does it take into account nearfield effects for low gain antennas?
> >
> > e.g. for a dipole or inverted V close to the ground, it's the ends that
> > present the RF safety problem, not the far field radiation
>
>Generally if you are talking inverted V you are taling 150, 75, or 40. Evne
>when running the full legal limit, key down on those bands you can get
>within a couple of feet of the ends of the antennas. If they are high
>enough off the ground that no one can reach up and touch it they will
>probably pass the test.
Hmm.. I ran a quick model of a 40 m inverted V, suspended 30 feet off the
ground, with the ends about 9 ft off the ground (i.e. high enough so you
can't touch it).
With just 100 Watts into the antenna, the peak field at 2 m off the ground
(head height) is about 88 V/m, about 9m from the center support. The peak
field at 3m off the ground is somewhat higher, about 1000V/m.
The ANSI C95.1 limit for controlled environments at 7 MHz is about 270 V/m
In practice, you're not going to be keydown for 6 minutes, so you get all
those averaging factors to work with, but certainly, one can bust the
limits pretty easily, particularly on 20m and higher, where the limit is
lower (61.4 V/m at 30MHz), with remarkably low power.
One might want to be careful if you're doing, for instance PSK31, RTTY, or
Pactor.. pretty constant envelope, long duration.
> >
> > Another problem I ran into was with a phased array, where there's a lot of
> > circulating power among the elements. I suspect that for any
>
>Even then, don't you have to get pretty close to any particular element even
>in the hear field as inside a phased array?
yes.. but "close" is a relative term. Think of someone putting up a multi
element Yagi for, say, 20m, on a temporary pole at field day, say 20ft off
the ground, and then putting 100W into it.
A Yagi with decent gain is going to have power circulating in the near
field that is 4 or 5 times the power being radiated. So, take numbers like
the ones for the inverted V, above, and multiply them by 2 or 3 to account
for the circulating power that's stored in the antenna, and the fields can
get pretty high, even at some distance (say a few meters).
Look at Emerson's article in the antenna compendium, or his graphics here:
http://ourworld.compuserve.com/homepages/demerson/endfire.htm
Emerson gives an interesting way to think about the extent of the high
field area: It's comparable to the size of a nonsuperdirective antenna
with the same gain. A dipole has a gain of 2.15 dBi (G=1.64) To a first
order, antenna gain (as a number) is the number of
Directivity = 4*pi*Aperture (in square wavelengths)
for a dipole, the aperture is about 0.13 square wavelengths (a lot of
people assume it's close to a half wavelength by a quarter wavelength,
which is 0.125 square wavelengths.. close enough)
A 10 dBi antenna will have an effective aperture of about 0.8 square
wavelengths. If we say that it's like a circle the radius of that circle
would be about 0.5 wavelengths. So, for that 20m Yagi, with 10dBi gain,
you'd want to stay at least 10m away to stay out of the high field area.
Jim
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