" The radiation toward an elevation angle of 5 degrees shown in the surface wave plot continues in essentially a straight line, to reach the ionosphere."
I'm still puzzled by these statements. What is NEC calculating when one selects the surface field? I am under the impression that it finds the TOTAL vector field, with no distinction between parts that are radiated, ground wave, etc. If so, how can you say anything about radiation continuing in a straight line? Rather than finding the fields at a single point (x = 100m, z = 9m) for your 5 degree angle, why don't you find them at the series of points constant*(100, 9) and plot the radial dependence? Better still, do that for the Poynting vector. Jack WS3N -----Original Message----- From: Topband [mailto:topband-boun...@contesting.com] On Behalf Of Richard Fry Sent: Saturday, September 28, 2013 14:59 To: topband@contesting.com Subject: Topband: 5/8 wavelength vertical is mo betta than shorter versions?? Niko Cimbur wrote: >Sure, over perfect ground the 5/8 wl has 3db more gain at 0 degrees >elevation. You are talking about theoretical gain. I am talking about >real world experience. ___________ Here are a couple of reposts of mine on this subject. http://s24.postimg.org/6nchfpt1h/NEC_FF_vs_NF_Calcs.jpg 1. The NEC far-field pattern for 0.1 km linked abovew shows a maximum field intensity of 590 mV/m at an elevation angle of 23 degrees (the assumed "takeoff angle"). It also shows that the field at an elevation angle of 5 degrees is 348 mV/m. The NEC surface-wave pattern for 0.1 km shows that the maximum field lies in the horizontal plane rather than at 23 degrees, and is about 890 mV/m rather than 590 mV/m. The surface wave analysis also shows that the field radiated toward 5-degree elevation is about 850 mV/m, rather than the 348 mV/m shown by the far-field analysis. Of course, the ratios of these fields are even greater for elevation angles below 5 degrees, and infinite in the horizontal plane. It is true that at great distances from a vertical monopole, the radiation present at low vertical angles is much less than at higher angles. But that does not mean that the greater radiation directed at low elevations __as launched by the monopole__ no longer exists. The radiation toward an elevation angle of 5 degrees shown in the surface wave plot continues in essentially a straight line, to reach the ionosphere. It is the radiation launched at these low elevation angles that can provide the greatest single-hop range and fields for skywaves reaching that range, even though its existence might be unrecognized, or disregarded. 2. Here is an earlier post of mine on Topband, comparing measured, real-world fields at the bottom of the page linked below with the fields calculated by NEC for that same scenario, at the top of the page. The agreement between the measured groundwave field and the NEC-calculated groundwave field for this 8 km, 6 mS/m groundwave path is quite good. It certainly proves that this monopole is not radiating zero or near zero fields in/at very low elevation angles, even for earth of rather poor conductivity. A common use for correctly defined NEC models shows the electrical characteristics of the radiator system itself. But NEC also will show the field intensities that system will produce at a given distance for a given applied power, frequency, and earth characteristics -- and do so quite accurately. The link below leads to a comparison of the groundwave field measured from a real-world AM broadcast system by a broadcast consulting engineering firm vs. the NEC-2D output using those same system parameters. Included in the NEC analysis is the value of the space wave field at an elevation of 100 meters above the (level) ground plane, for this installation -- which should help better understand the points of the opening post in this thread. Note that the value of the space wave at 100m elevation and 100m downrange is lower than the groundwave 100m downrange, and increases as the range increases. This is as expected, because the relative field (E/Emax) of the elevation pattern "launched" by this 1/4-wave monopole reduces as the elevation angle increases. Hope this graphic and explanation are useful. http://i62.photobucket.com/albums/h85/rfry-100/Measured_vs_NEC2D_Fields2.jpg RF _________________ Topband Reflector _________________ Topband Reflector