There are actually at least two different "near field" regions. The first is
the reactive near-field region, where the electric and magnetic fields can
decay rapidly, are not necessarily in the far-field phase relationship of 90
degrees and generally not in the same E/H ratio of 377 ohms as they would be in
the far field. For physically small radiators, E dominates for a short dipole
and H dominates for a small loop. For small radiators, the dominant field
decays at a 1/R^3 rate, 60 db/distance decade, to a point approximately a
distance of wavelength/2pi, or approximately 1/6 wavelength. Beyond that, the
fields are orthogonal, at a ratio of approximately E/H = 377 ohms and decay at
a 1/R rate, where R is the distance from source. Within this region, power is
actually flowing outward from the antenna, but, as in any reactance, some part
of the levels of the dominant field are reactive, meaning that energy is being
put into th field, adding to its value, but then is being taken back by the
source, not being real power at all, much like the circulating currents and
voltages in a resonant circuit.
There is also the radiating near field region. This applies to larger antennas.
There are several rules of thumb for what constitutes the radiating
near-field/far-field boundary. 2*D^2/wavelength is the one I generally use,
where D is the largest physical dimension of the radiating antenna. This falls
apart a little for a power-line as a radiator, because that can be miles long
and as power flows down the line, more of it is radiated, so the formula really
is an approximation. For a 10 mile line on 28 MHz, this formula would put the
far-field boundary out to 30,000 miles, a bit larger than it really is. 🙂
In this radiating near field region, the fields are acutally radiating, but any
one point is not equidistant from all parts of the radiating antenna, so all
parts of the antenna do not contribute equally to the field at a particular
point. The result is a very complex pattern of both E and H, forming a moire
pattern in which E and H vary up and down with distance. In this region, you
might find E at a minimum node 30 feet from the antenna, with H at a maximum,
but at 50 feet away, E could be maximum and H lower, so the measured or
calculated E field could actually increase with distance.
Within the near-field boundaries, and antenna such as a Yagi might not exhibit
the same directional characteristics as it would to a far-field wave, so having
an HF Yagi close to the source can result in some pretty whacky reading that
are not always easy to interpret.
Now, there is a pretty sharp knee for small radiators at the
near-field/far-field boundary of wavelength/2pi. But for large radiators, the
boundary to the far-field region is not sharp. That pattern of ups and downs
with distance, often at angles not perpendicular to the radiator, just shows a
smaller and smaller wave, approaching true far-field conditions. Essentially,
the far-field begins at the point that one can accept the amount of error. So,
a Yagi antenna might have a forward gain far-field boundary of 2D^2/wavelength
but to get an accurate measurement of F/B ratio, where a smaller error could
throw things off a lot, one might have to be much farther away.
This explanation may be a bit simplified and takes a minor liberty or two, but
it gets the points across and helps explain why we care.
Ed
There is also what is known as the radi
________________________________
From: RFI <rfi-bounces+w1rfi=arrl.org@contesting.com> on behalf of K9MA
<k9ma@sdellington.us>
Sent: Friday, April 17, 2020 12:45 PM
To: Eddie Edwards <eddieedwards@centurylink.net>; rfi@contesting.com
<rfi@contesting.com>
Subject: Re: [RFI] Power Line Noise
The 1/6 wavelength is just a rough rule of thumb. For a yagi with a 24
foot boom, the calculator below says the far field starts at 15 feet.
1/6 wavelength is 11 feet. Both are much smaller than the distance to my
nearest power line, about 130 feet.
73,
Scott K9MA
On 4/17/2020 10:24, Eddie Edwards wrote:
> Scott,
>
> I think this calculator will be a bit more accurate than your 1/6th
> calculation.
> https://www.everythingrf.com/rf-calculators/antenna-near-field-distance-calculator
>
> Also don't forget to use the full size (either dimeter or length) of the
> entire yagi array as Jim Brown pointed out. When I do this for typical 20
> meter yagi, please correct me if I'm wrong, but I get a far field distance of
> 9.5 meters or about 31 ft for a typical 20 meter yagi. It still might be
> this far away if across the street.
>
> 73, de ed -K0iL
>
> -----Original Message-----
> From: RFI On Behalf Of K9MA
>
> I believe near field for antennas is generally considered to be about
> 1/6 of a wavelength. That's only 11 feet on 20 meters.
>
--
Scott K9MA
k9ma@sdellington.us
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