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## Re: Topband: Inverted L Dimensions

 To: Guy Olinger K2AV Re: Topband: Inverted L Dimensions K4SAV Thu, 30 Sep 2010 23:36:03 -0500 mailto:topband@contesting.com>
 ```K2AV: "Ground resistance is a CONSTANT in a given setting, the same as an ordinary resistor. IT DOES NOT VARY depending on induced current short of loss heat converting ground moisture to steam or enhancing evaporation, thus changing a physical characteristic. The induction of current usually varies positionally as antenna elements are changed. The degree of LOSS attributed to ground is a complex sum of the instant current times resistance at points in the ground. This is a power value, which, given the current at the radial feed, can be expressed as a SUMMARY resistance. This is the useful term when combined with radiation resistance, allows us to express the apportionment of our hard-bought power to worm-warming and chasing DX." .... --- No one is claiming that ground resistance varies depending on antenna length or anything else associated with the antenna. The question is "what is the equivalent resistance that would have to be added at the feedpoint to equal the ground loss, if the real near field ground loss in the model was set to zero". That is the thing that varies with antenna length. There are some charts floating around compiled by data from the broadcast industry that shows equivalent series resistance for ground loss, but that data was compiled for 1/4 wave verticals. There aren't any charts for longer antennas or for inverted Ls. Consider for a moment using a Mininec ground. If you do any analysis using a Mininec ground you have to add that resistor to get something realistic. Those charts should give something close when analyzing a quarter wave vertical, but not necessarily close for longer antennas, because there is no data available for that antenna. For example if you have a 1/2 wave vertical, the point of maximum current density in the earth moves out to about 0.35 from the base of the antenna. So if you had 1/4 wavelength radials with that antenna, then there would be no radials present in the area where the current density was maximum. Calculating the feedpoint resistance and subtracting the radiation resistance doesn't give you a value of resistance that would represent ground loss for this antenna. If you could somehow refer that loss resistance that happened at considerable distance from the feedpoint, back to the feedpoint, then it might be OK. I don't know how to do that. If NEC2 or NEC4 modeled the ground correctly, we wouldn't have to use Mininec. (Besides a Mininec ground gives a significant error for inverted Ls.) All we would have to do is model the radials and put it over a real ground. We would have the right antenna gain, and by creating a close to zero ground loss radial system and then adding a resistor at the feedpoint and jugging that resistor until we got the same gain as before, we would have an equivalent ground loss resistor referred to the feedpoint. However I have no confidence that NEC will give an accurate gain answer in this case. Several people have noticed large discrepancies between NEC calculations and measured data for very low wires. A dipole doesn't act like a radial system but it is an antenna where there is some experimental data available to which NEC comparisons can be made. Consider the data generated by Hagn-Baker at this link: http://www.w8ji.com/nvis_n_v_i_s_antenna.htm That data agrees reasonably close with NEC calculations until the antenna reaches somewhere around 0.02 wavelengths above ground. At 0.005 wavelengths NEC is in error by about 6 dB. So when you are modeling radials close to the ground how much error do you expect and why? Jerry, K4SAV _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK ```
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