|Subject:||[TowerTalk] Modeling, ground, etc.|
|From:||Jim Lux <firstname.lastname@example.org>|
|Date:||Fri, 22 Oct 2004 15:06:10 -0700|
Folks interested in how NEC was validated might find the following LLNL report interesting:
G.J. Burke, "Recent Advances to NEC: Applications and Validation", 3 March 1989, UCRL-100651
This one is particularly interesting because it gives some history (that NEC-3 was partly developed to address things like ground rods and EMP, and structures that are small in comparison to wavelength (NEC-3VLF)).
It also makes the telling comment (page 3-20 (22 in the pdf)):
"... The most difficulty has been encountered in modeling ground screens floating above the ground. ..." and goes on to how to construct your model to deal with it.
Another LLNL report, particularly relevant to wires close to (or in) the ground, etc. is:
E.K. Miller & F.J. Deadrick, "Analysis of Wire Antennas in the Presence of a Conducting Half Space: Part III - The Buried Antenna", 25 Feb 1977, UCRL-5228
One take home message from the above is that if you're within, say, 0.2 wavelength of the ground, you'd better be using the Sommerfeld ground analysis.
Another useful work (not online, unfortuately) is Hagn's paper on measuring ground constants:
G.H. Hagn, "HF Ground Constant Measurements at the Lawrence Livermore National Laboratory (LLNL) Field Site", Applied Computational Electromagnetics Society (ACES)(I think), 1988
Of some interest is that the relative epsilon (permittivity) varied with frequency. At one test station, the best estimate ranged from 182 at 2MHz to 17 at 30 MHz. The measurement uncertainty is quite large for the lower frequencies. At 7MHz, the best estimate was 62, with a lower bound of 38 and an upper bound of 217.
Quite a ways from assuming 13/.005, isn't it?
They also assessed the horizontal homogeneity (which is of interest to us, when considering how to model ground radials and ground screens). "The small scale variations seemed rather large, but the variation seemed to decrease with increasing measurement frequency." Very interestingly, (or not so suprising, given the crew doing the measurements, most of whom, if not all, were hams) they did some surveys at 7,14, and 30 MHz.
In general, they concluded that variations were about an order of magnitude (10:1) for conductivity, and half an order of magnitude (3.16:1?) for epsilon.
One of the comments from this paper is that the "uniform slab" model cannot be used if the water table is within one skin depth of the surface. (The Livermore ground constants work out to a skin depth on the order of a meter, and the water table was some 80 feet down).
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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