Timeline for Do Far-Field Radiation Patterns/Gains of NEC Software Accurately and Always Show the E-M Fields "Launched" by Vertical Monopoles?
Current License: CC BY-SA 4.0
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| Nov 29, 2019 at 12:50 | history | edited | Richard Fry | CC BY-SA 4.0 |
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| Nov 7, 2019 at 19:47 | history | edited | Richard Fry | CC BY-SA 4.0 |
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| Nov 7, 2019 at 19:35 | history | edited | Richard Fry | CC BY-SA 4.0 |
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| Nov 7, 2019 at 15:29 | history | edited | Richard Fry | CC BY-SA 4.0 |
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| Nov 7, 2019 at 13:37 | history | edited | Richard Fry | CC BY-SA 4.0 |
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| Nov 6, 2019 at 14:11 | comment | added | Richard Fry | RE: There's no "close to the antenna site" in the definition of gain,<stop> ... — Agree with that part. Other things equal, the gain of a given antenna system configuration is not a function of distance from that antenna system. | |
| Nov 6, 2019 at 6:48 | comment | added | tomnexus | There is no way of analysing a real-ground antenna in isolation - a monopole must have ground. If you assume infinite, perfect ground, then you get 5 dBi on the horizon as above. If you mount it on real ground, earth stake or radials, there's no radiation along the ground, apart from the fast-decaying ground wave. There's no "close to the antenna site" in the definition of gain, it's at infinity, only the 1/r component of the fields. The linked image shows this, blue and red curves will converge. So this is a question of definition, not a truth about antennas that can be found by simulation. | |
| Nov 5, 2019 at 9:29 | comment | added | Richard Fry | RE: You would need to ask NEC to calculate near fields, these will correctly show all propagation components at a particular distance. In both cases though, you need to request the real ground. — Yes, but this query deals with the __gain__of a transmit antenna system, not the fields it produces. The following link leads to another paper I wrote dealing with fields existing after propagation over real Earth: i.postimg.cc/J04g7kJq/GW-vs-Space-Wave-at-1-km.jpg | |
| Nov 5, 2019 at 2:17 | comment | added | tomnexus | True, but you need to define Low Angle Radiation precisely. On real ground a surface wave decays completely in the far field. For HF this happens reasonably fast, compared to the 1000+ km skip distances of interest. For MW and 100 km it hasn't decayed yet, so the far field approximation doesn't hold. You would need to ask NEC to calculate near fields, these will correctly show all propagation components at a particular distance. In both cases though, you need to request the real ground. | |
| Nov 4, 2019 at 21:50 | comment | added | Richard Fry | However the "expected null" that appears in the NEC far-field analysis does not appear fairly close to the antenna site when the surface wave is included in that analysis. This leads some observers to believe that no low-angle radiation ever exists from a vertical monopole antenna system. If that was true, MW AM broadcast stations would have fewer daytime listeners than they have now. | |
| Nov 4, 2019 at 19:33 | comment | added | tomnexus | Your text is correct but as you say the simulation is only of a perfect, infinite ground. Try the Sommerfeld-Norton ground in NEC, GN type 2. It more accurately models the currents in the antenna itself (near field interaction with the ground, more important with horizontal antennas) and also the radiation patterns by far-field approximation using reflection. If you use this you'll see expected null on the horizon, and a low angle of radiation. | |
| Nov 4, 2019 at 14:58 | history | answered | Richard Fry | CC BY-SA 4.0 |
