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NEC Evaluations
NEC in its various forms is an excellent means to evaluate the
radiation from various structures. But without due care it still can lead to incorrect conclusions about system performance when propagation effects are present -- as in the recent thread here about the elevation patterns of MW monopoles. Here is another situation... http://i62.photobucket.com/albums/h8...PatCompare.jpg RF |
NEC Evaluations
"Richard Fry" wrote in message ... NEC in its various forms is an excellent means to evaluate the radiation from various structures. But without due care it still can lead to incorrect conclusions about system performance when propagation effects are present -- as in the recent thread here about the elevation patterns of MW monopoles. Here is another situation... http://i62.photobucket.com/albums/h8...PatCompare.jpg RF The example above does not compute the surface wave. The following card computes "Space Wave plus SurfaceWave": CM 144 MHz Vertical Dipole CE GW 1 13 0 0 33.15 0 0 30 0.010417 GS 0 0 0.304800 GE 1 -1 0 GN 2 0 0 0 13.0000 0.0050 FR 0 1 0 0 147.3 0.1 EX 0 1 7 0 65.66981572 0 LD 5 0 0 0 3.801E7 RP 1 11 1 0000 0 90 1.00000 1.00000 50000 EN In this example the vertical half wave dipole, with the base 30 ft above an average ground, on 147.3 MHz, shows a field strength at ground level of: 0.418 uV/m from 30 W into the antenna. Frank |
NEC Evaluations
In this example the vertical half wave dipole, with the base 30 ft above an average ground, on 147.3 MHz, shows a field strength at ground level of: 0.418 uV/m from 30 W into the antenna. Frank And, obviously, at 50 km. |
NEC Evaluations
Richard Fry wrote:
NEC in its various forms is an excellent means to evaluate the radiation from various structures. But without due care it still can lead to incorrect conclusions about system performance when propagation effects are present -- as in the recent thread here about the elevation patterns of MW monopoles. Here is another situation... http://i62.photobucket.com/albums/h8...PatCompare.jpg RF Your statement is one of the most correct I have ever seen offered in this group. However, it, NEC, still remains one of the best available "antenna prediction" offering of software obtainable to "normal" people ... and provides some sort of an income for those who write the GUIs' who feed the text file to it ... Regards, JS |
NEC Evaluations
On Dec 22, 11:13*am, "Frank" wrote:
In this example the vertical half wave dipole, with the base 30 ft above an average ground, on 147.3 MHz, shows a field strength at ground level of: 0.418 uV/m from 30 W into the antenna. And, obviously, at 50 km. ________________ Here is another method (Longley-Rice) for calculating the field intensity produced at the receive site by your model. But the NEC approach is less accurate than L-R for long path lengths (due to earth curvature), and for specific terrain contours. In your model the path loss calculated using L-R is about 68.8 dB more than the free space loss. The peak, free space field produced by a 1/2-wave, linear dipole radiating 30 watts over a 50 km path is about 770 uV/m. This voltage reduction of 68.8 dB is a field multiplier of about 0.00036, so the 770 uV/m field is reduced to about 0.28 uV/m -- a bit less than your NEC model predicts. Agreement probably would be better over shorter paths (as long as no specific terrain profile needed to be applied), and worse for longer paths. In the L-R example I set the path over the middle of Lake Michigan in order to get a smooth earth contour, such as used in NEC models. This all just illustrates that analyses made using NEC and any other method need to consider the limits inherent in their algorithms with respect to the physical reality being analyzed. http://i62.photobucket.com/albums/h8...strialPath.gif RF |
propagation models NEC Evaluations
Richard Fry wrote:
On Dec 22, 11:13 am, "Frank" wrote: In this example the vertical half wave dipole, with the base 30 ft above an average ground, on 147.3 MHz, shows a field strength at ground level of: 0.418 uV/m from 30 W into the antenna. And, obviously, at 50 km. ________________ Here is another method (Longley-Rice) for calculating the field intensity produced at the receive site by your model. But the NEC approach is less accurate than L-R for long path lengths (due to earth curvature), and for specific terrain contours. Once you get away from the near field, there's tons of models and modeling approaches available, depending on the kind of path you're interested in, and what you're looking to find out. For instance, ioncap and its ilk (VOACAP,etc.) model skywave paths in a statistical sense. Other models do raytracing for a more "point solution" type model. Yet others are good for things like forests or terrain. Since nobody has a full up computed electromagnetics finite model of everything at fine resolution, all those models basically trade off computational resources against some approximations. Whether it's approximating the earth as flat surface of a uniform dielectric (NEC) or whatever.. |
NEC Evaluations
Richard Fry wrote:
This all just illustrates that analyses made using NEC and any other method need to consider the limits inherent in their algorithms with respect to the physical reality being analyzed. Absolutely true. http://radiomagonline.com/fcc/radio_fcc_clamps_down/. Roy Lewallen, W7EL |
NEC Evaluations
"Richard Fry" wrote in message ... On Dec 22, 11:13 am, "Frank" wrote: In this example the vertical half wave dipole, with the base 30 ft above an average ground, on 147.3 MHz, shows a field strength at ground level of: 0.418 uV/m from 30 W into the antenna. And, obviously, at 50 km. ________________ Here is another method (Longley-Rice) for calculating the field intensity produced at the receive site by your model. But the NEC approach is less accurate than L-R for long path lengths (due to earth curvature), and for specific terrain contours. In your model the path loss calculated using L-R is about 68.8 dB more than the free space loss. The peak, free space field produced by a 1/2-wave, linear dipole radiating 30 watts over a 50 km path is about 770 uV/m. This voltage reduction of 68.8 dB is a field multiplier of about 0.00036, so the 770 uV/m field is reduced to about 0.28 uV/m -- a bit less than your NEC model predicts. Agreement probably would be better over shorter paths (as long as no specific terrain profile needed to be applied), and worse for longer paths. In the L-R example I set the path over the middle of Lake Michigan in order to get a smooth earth contour, such as used in NEC models. This all just illustrates that analyses made using NEC and any other method need to consider the limits inherent in their algorithms with respect to the physical reality being analyzed. http://i62.photobucket.com/albums/h8...strialPath.gif RF Interesting comparison between methods at VHF frequencies. For curiosity I had done a comparison between the FCC predicted curves, for an AM broadcast station on 1655 kHz, and NEC. It seems that at the lower frequencies NEC has greater accuracy. Of course NEC was never intended as a propagation tool, but still appears to be reasonably useful. I had cut and pasted an Excel spread sheet below, so not sure if it will retain the formatting when posted. Frank Field Strength Comparison at 1655 kHz.. Antenna Description: 45.3 m ground mounted monopole. 120 X 45.3 m radials, 15 cm below ground. All conductors copper. Input power 100 W Source: http://www.fcc.gov/mb/audio/73184/index.html per 47 CFR Sections 73.183 and 73.184 Nittany Scientific GNEC Version 1.1a. Ground parameters: Conductivity 5 mS/m, permittivity 13 (Average ground) Field strength RMS V/m. Distance FCC GNEC Difference Difference (km) (mV/m) (mV/m) (%) (db) 0.10 950.000 960.000 1.0 -0.09 0.50 170.000 168.000 1.2 0.10 1.00 77.000 75.000 2.6 0.23 5.00 8.500 8.110 4.7 0.41 10.00 2.400 2.270 5.6 0.48 50.00 0.068 0.067 2.1 0.18 100.00 0.014 0.015 7.0 -0.61 200.00 0.002 0.004 62.1 -5.58 |
NEC Evaluations
"Frank" wrote in message news:EcU3l.65$z%.25@edtnps82... "Richard Fry" wrote in message ... On Dec 22, 11:13 am, "Frank" wrote: In this example the vertical half wave dipole, with the base 30 ft above an average ground, on 147.3 MHz, shows a field strength at ground level of: 0.418 uV/m from 30 W into the antenna. And, obviously, at 50 km. ________________ Here is another method (Longley-Rice) for calculating the field intensity produced at the receive site by your model. But the NEC approach is less accurate than L-R for long path lengths (due to earth curvature), and for specific terrain contours. In your model the path loss calculated using L-R is about 68.8 dB more than the free space loss. The peak, free space field produced by a 1/2-wave, linear dipole radiating 30 watts over a 50 km path is about 770 uV/m. This voltage reduction of 68.8 dB is a field multiplier of about 0.00036, so the 770 uV/m field is reduced to about 0.28 uV/m -- a bit less than your NEC model predicts. Agreement probably would be better over shorter paths (as long as no specific terrain profile needed to be applied), and worse for longer paths. In the L-R example I set the path over the middle of Lake Michigan in order to get a smooth earth contour, such as used in NEC models. This all just illustrates that analyses made using NEC and any other method need to consider the limits inherent in their algorithms with respect to the physical reality being analyzed. http://i62.photobucket.com/albums/h8...strialPath.gif RF Interesting comparison between methods at VHF frequencies. For curiosity I had done a comparison between the FCC predicted curves, for an AM broadcast station on 1655 kHz, and NEC. It seems that at the lower frequencies NEC has greater accuracy. Of course NEC was never intended as a propagation tool, but still appears to be reasonably useful. I had cut and pasted an Excel spread sheet below, so not sure if it will retain the formatting when posted. Frank Field Strength Comparison at 1655 kHz.. Antenna Description: 45.3 m ground mounted monopole. 120 X 45.3 m radials, 15 cm below ground. All conductors copper. Input power 100 W Source: http://www.fcc.gov/mb/audio/73184/index.html per 47 CFR Sections 73.183 and 73.184 Nittany Scientific GNEC Version 1.1a. Ground parameters: Conductivity 5 mS/m, permittivity 13 (Average ground) Field strength RMS V/m. Distance FCC GNEC Difference Difference (km) (mV/m) (mV/m) (%) (db) 0.10 950.000 960.000 1.0 -0.09 0.50 170.000 168.000 1.2 0.10 1.00 77.000 75.000 2.6 0.23 5.00 8.500 8.110 4.7 0.41 10.00 2.400 2.270 5.6 0.48 50.00 0.068 0.067 2.1 0.18 100.00 0.014 0.015 7.0 -0.61 200.00 0.002 0.004 62.1 -5.58 Rats, loused up the formatting. Here is a 2nd attempt. Distance FCC GNEC Difference Difference (km) (mV/m) (mV/m) (%) (db) 0.10 950.000 960.000 1.0 -0.09 0.50 170.000 168.000 1.2 0.10 1.00 77.000 75.000 2.6 0.23 5.00 8.500 8.110 4.7 0.41 10.00 2.400 2.270 5.6 0.48 50.00 0.068 0.067 2.1 0.18 100.00 0.014 0.015 7.0 -0.61 200.00 0.002 0.004 62.1 -5.58 |
NEC Evaluations
Dear Group: What a delight it is to see a computer doing the calculations
for VHF propagation. Almost fifty years ago, I led a team who measured field strengths in the 100 to 250 MHz range (FM and TV broadcast transmitters) to verify (qualify) the propagation model. Of course, I used a slide rule and log tables to perform the calculations and manually extracted path profiles from topo. maps. The goal was to place confidence in the model for estimating expected interference levels at a radio-astronomy site located in a valley. The result from extensive filed measurements and data reduction was that we could be confident in the model. I recall also doing some comparisons of predicted and measured strengths involving scattering (over quite long distances) in the VHF range with good correlation. IONCAP, and its predecessors and successors, I have used to good effect for almost as many years. In short, the developed propagation methods have been proven by me, and many others, to provide reasonably small uncertainties. Of course, the critical element is knowing which tool to use. That, I believe, is part of the point brought forward by Richard Fry and others. But put yet another way, any dam fool can (now) put numbers into a computer and get numbers back out of the computer - experience and judgment is needed to have significance accrue to the results of such calculations. Central to all of the propagation models is the need to understand what the antenna and its environment actually does. I am also delighted that several of you are providing the education to the silent so that they do not fall into the traps that are always present. Warm regards and season's greetings, Mac N8TT -- J. McLaughlin; Michigan, USA Home: "Richard Fry" wrote in message ... On Dec 22, 11:13 am, "Frank" wrote: In this example the vertical half wave dipole, with the base 30 ft above an average ground, on 147.3 MHz, shows a field strength at ground level of: 0.418 uV/m from 30 W into the antenna. And, obviously, at 50 km. ________________ Here is another method (Longley-Rice) for calculating the field intensity produced at the receive site by your model. But the NEC approach is less accurate than L-R for long path lengths (due to earth curvature), and for specific terrain contours. In your model the path loss calculated using L-R is about 68.8 dB more than the free space loss. The peak, free space field produced by a 1/2-wave, linear dipole radiating 30 watts over a 50 km path is about 770 uV/m. This voltage reduction of 68.8 dB is a field multiplier of about 0.00036, so the 770 uV/m field is reduced to about 0.28 uV/m -- a bit less than your NEC model predicts. Agreement probably would be better over shorter paths (as long as no specific terrain profile needed to be applied), and worse for longer paths. In the L-R example I set the path over the middle of Lake Michigan in order to get a smooth earth contour, such as used in NEC models. This all just illustrates that analyses made using NEC and any other method need to consider the limits inherent in their algorithms with respect to the physical reality being analyzed. http://i62.photobucket.com/albums/h8...strialPath.gif RF |
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