[RFI] Power Line Noise

K9MA k9ma at sdellington.us
Fri Apr 17 15:54:50 EDT 2020

Thanks for the explanation, Ed!

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 at contesting.com> on behalf of 
> K9MA <k9ma at sdellington.us>
> *Sent:* Friday, April 17, 2020 12:45 PM
> *To:* Eddie Edwards <eddieedwards at centurylink.net>; rfi at contesting.com 
> <rfi at 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 at sdellington.us
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Scott  K9MA

k9ma at sdellington.us

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