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## Re: [TenTec] NEC, ground, grounds, and radials.

 To: tentec@contesting.com Re: [TenTec] NEC, ground, grounds, and radials. Dave Kelley Discussion of Ten-Tec Equipment Sat, 8 Jan 2011 19:14:06 -0500 mailto:tentec@contesting.com>
 ```Dear Steve and Jerry, I think I might not have been clear in my original post about the 3 dB gain "error" in NEC. The moment method codes like NEC, EZNEC, Mininec, and all of their variants are calculating gain in dBi correctly. I cannot access the "IEEE Standard Definitions of Terms for Antennas" right now, but I am sure that it states or implies that gain measured in dBi is always referenced to an isotropic radiator in free space whether the antenna being modeled is over a ground plane or not. It has to be this way; otherwise, it could create untold confusion. What I was saying in my original post (and I think Jerry agrees) is that to be "fair," one should compare an antenna mounted over ground to an isotropic radiator radiating into a half-space (a "hemi-isotropic" radiator?). However, there is no standard gain unit defined this way. However one chooses to define the terms, NEC and its variants do calculate the correct fields, and they corroborate with the standard definition of the dBi unit. A concrete example might help: Imagine a 1-kW transmitter feeding a 1/4-wave monopole over perfect ground. The transmitter is perfectly matched to the antenna, so all of the 1 kW of power is radiated. EZNEC predicts that the electric field right at the horizon (0 degrees of elevation) at a distance of 1 km from the antenna is 313 mV/m (rms). That translates to a maximum power density (S_max) of 260 microwatts per square meter, using S_max = E^2/eta, where E is the rms value of the electric field, and eta is the intrinsic impedance of free space (377 ohms). The gain of the monopole reported by EZNEC is 5.14 dBi (a multiplying factor of 3.27), which means that in the direction of maximum radiation, the power density should be 3.27 times that of an isotropic radiator for the same input power. For a 1-kW input power (Pin) and a distance (r) of 1 km, using S_iso = Pin/(4*pi*r^2) [for a full sphere], the power density from an isotropic radiator would be 79.6 microwatts per square meter. If we multiply S_iso by the 3.27 (5.14 dBi) gain of the monopole reported by EZNEC, we get (79.6 microwatts/m^2)*(3.27) = 260 microwatts/m^2, which is exactly the maximum power density that EZNEC predicts for the monopole as derived from the calculated electric field data. It doesn't seem "fair" to compare the fields radiated by an antenna over a ground plane to a free space isotropic radiator, but that is the standard definition as adopted by the IEEE (with the guidance of the Antennas and Propagation Society), and so that is what all of the antenna analysis software uses. The advantage of the standard definition is that one can predict the actual power density or electric field at a given distance from an antenna knowing nothing more than the antenna's gain and the input power. 73, Dave ND3K > Jerry, > > I can understand that the interpretation of dBi which appears in most > engineering texts (and possibly the IEEE definition?) throws up > conundrums with which you are uncomfortable. But what I think is unfair > is to claim that EZNEC is in error, simply because it adopts the > "industry standard" interpretation rather than an alternative that you > might prefer. > > I make widespread use of EZNEC, and when I saw someone as authoritative > as yourself state that it is 3dB in error I felt it needed further > investigation! > > 73, > Steve G3TXQ > > On 08/01/2011 00:20, Dr. Gerald N. Johnson wrote: >> My conundrum is that I expect equal power intensity at the the measuring >> point from the isotropic source whether a ground plane is involved or >> not and that I also expect equal intensity from a vertical dipole in >> free space and a quarter wave vertical on the ground plane (except for >> the ground absorption at the real ground plane). >> > _______________________________________________ TenTec mailing list TenTec@contesting.com http://lists.contesting.com/mailman/listinfo/tentec ```
 Current Thread Re: [TenTec] NEC, ground, grounds, and radials., (continued) Re: [TenTec] NEC, ground, grounds, and radials., Dr. Gerald N. Johnson Re: [TenTec] NEC, ground, grounds, and radials., Steve Hunt Re: [TenTec] NEC, ground, grounds, and radials., Rsoifer Re: [TenTec] NEC, ground, grounds, and radials., Dr. Gerald N. Johnson Re: [TenTec] NEC, ground, grounds, and radials., James Duffer Re: [TenTec] NEC, ground, grounds, and radials., Dr. Gerald N. Johnson Re: [TenTec] NEC, ground, grounds, and radials., Steve Hunt Re: [TenTec] NEC, ground, grounds, and radials., Rsoifer Re: [TenTec] NEC, ground, grounds, and radials., Rsoifer Re: [TenTec] NEC, ground, grounds, and radials., Dave Kelley <= Re: [TenTec] NEC, ground, grounds, and radials., Steve Hunt