Topband: Inverted L Dimensions

[K4SAV]

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." ....

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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