Perhaps a better definition of far field is the point of increasing distance where the shape of a pattern calculated by a near field process quits changing at angles and azimuths of concern.
Let's test this idea using the NEC4 near field table generator. We will not change processes, using the near field table generator exclusively. We find that the ultimate shape, particularly the notch, is not even close to being formed at 3 km. With increasing distance, the shape very gradually morphs into the familiar deep notch at 0.2 degrees, finally equaling the far field plot value out around 50 km. We will run NEC4 near field calculations on a 1/4 wave radiator with 120 buried 0.4 wavelength radials at 1.825 MHz, soil char of (5, .13). Even at 30 (thirty) km the depth of the notch near ground is still increasing. The notch relative to pattern max (highest reading at that distance and any elevation) is -23.71 dB. At 31 km this figure is -24.04 dB. At 3 km, seemingly considered far field by listening to posts here, this "notch", really a gentle bow at this point, is a mere -3.46 dB, only a hint of the depth of the ultimate notch. At 10 km the notch is much deeper at -12.13 dB, but still far away from the ultimate value. Remember this is the NEAR field process generating these figures. At 50 km out the minimum at 100m height is -28.69 dB below the max value at 17 km height, with the pattern of values up to elevation 50 km looking very much like the familiar FAR field process pattern plot which coincidentally has a value of -28.93 dB below max at 0.2 degrees elevation. So it will make no sense to say that near field generated values should be added to far field values at very low angles because far field doesn't compute them. Near field doesn't either. If given enough room to work, the NEAR field generation will show the same notch. The problem all along has been 3 km is nowhere near far enough away to complete whatever accounts for the notching. OR perhaps the NEC4 near field process has a really serious unpublished flaw or weakness? Or ??? Maybe the equations in the books were done for ground wave and standing-man-with-meter, and are in fact approximations themselves, and only really apply to sky wave in special circumstances which hams mostly don't care about? Or maybe the equations are right and no one tried them out 50 km, and just assumed that 3 km is far enough? Measurements of actual reality? Done at distances and altitude to confirm at least the progressive deepening of the notch? Sitting-man-with-meter-in helicopter would have to remove to a distance of 50 km and descend from an altitude of 17 km to confirm the presence of the notch at that ratio to maximum which matches the far field plot notch depth. Practically he would have to come in quite a bit so the altitude with predicted max signal was within the normal achievable altitude of the helicopter. Not sure there is anything around that hovers at a particular GPS point up at flight level 56 Far field is the point of increasing distance where the shape of a pattern calculated by a near field process quits changing at elevations and azimuths of concern. 73, Guy. On Thu, Oct 11, 2012 at 8:11 AM, Tom W8JI <[email protected]> wrote: > According to antenna engineering textbooks (Kraus, Balanis. Johnson & >> Jasik etc), the free space, far-field radiation pattern is not a function >> of the distance from the radiator, as it is in the near field. The >> near-field/far-field boundary conventionally is defined as equal to >> 2L^2/lambda, where L = the greatest physical dimension of the radiator. So >> for a 1/4-wave monopole on 160 meters, that boundary would be located about >> 20.25 meters from the radiator. >> > > The boundary is around 1 wavelength minimum even for a very small antenna, > like a "magnetic" loop. As a matter of fact a small magnetic loop is > electric field response dominant at a distance of about 1/8th wave from the > antenna outward to about 1 wavelength. > > http://www.w8ji.com/images/**emfield.gif<http://www.w8ji.com/images/emfield.gif> > > Page 341 of Jordon and Balmain ( as with other EM radiator texts) define > farfield boundary as a distance "large with respect to a wavelength and > also large with respect to the largest dimension of the source". A > wavelength is safe for a small antenna, but not a large system. My 4 square > on 160 still behaves slightly like nearfield at a distance of two > wavelengths, but my 200 foot omni vertical is settled down about 1 > wavelength away. > > 73 Tom > > > > > > ______________________________**_________________ > Remember the PreStew coming on October 20th. http://www.kkn.net/stew for > more info. > _______________________________________________ Remember the PreStew coming on October 20th. http://www.kkn.net/stew for more info.
