Thanks for the explanation, Ed!
73,
Scott K9MA
On 4/17/2020 14:06, Hare, Ed W1RFI wrote:
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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Scott K9MA
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