|To:||W2RU - Bud Hippisley <W2RU@frontiernet.net>,<firstname.lastname@example.org>|
|Subject:||Re: [TowerTalk] Re: radials for shunt fed tower|
|From:||Jim Lux <email@example.com>|
|Date:||Wed, 15 Dec 2004 11:55:45 -0800|
At 09:31 AM 12/15/2004, W2RU - Bud Hippisley wrote:
At 17:57 2004-12-05, firstname.lastname@example.org wrote:
As you're probably aware, modeling of ground systems is a pretty dicey operation, even using the NEC4 modeling engine. NEC4 has been validated pretty extensively and does model a system quite accurately, provided you've described what you're modeling sufficiently accurately.
So, why do you get seemingly weird effects? A couple things spring to mind (although I haven't rigorously looked into it, for reasons described below):
1) Al modeled the vertical as a AWG12 copper wire, which is probably more lossy than the usual large diameter aluminum tube or large diameter tower. (Anyone got some good modeling parameters for towers?) The increased loss changes the current distribution a bit along the antenna, which changes the induced current distribution in the ground system.
2) The ground system losses are a combination of two things: the radial current flow to form the "image" of the monopole; and the dissipative loss of the E and H fields in the earth surface. The former is essentially a resistive kind of thing, and the copper ground wires reduce the sheet resistance (this is what all those other models of ground systems (including George Brown's) analyze to death at around 1 MHz). The latter is strongly influenced by the skin depth of the earth and it's lossiness. Looking over some charts derived from George Hagn's work at SRI, skin depth varies from a meter or so in "Very Good" soil to 10-15 meters in Very Poor. However, skin depth needs to be combined with the resistivity to determine loss. So what if skin depth is 1/2, if the conductivity is 5 times greater. There's also whether the conductivity is greater because of the water content, which changes the epsilon (which also changes the skin depth).
3) The same wire size was used in all ground screens, and a real tradeoff shouldn't be with a fixed length of fixed size wire, but on the cost of the wire (which pretty much scales with mass). Are you better off with 1000 feet of AWG 16 or 250 feet of AWG 10 or 10,000 feet of AWG 26? The labor cost is another aspect.
In the case of the models, one has to be very careful about things like skin effect, when the skin depth is a fraction of the wire diameter. In air, skin depth at 1MHz in copper is about 0.003 inches. Compare to AWG 16 wire, which is 0.05 inches in diameter, approximately. The skin depth might be different if the conductor is immersed in a medium other than air (I assume NEC4 takes this into account, though). However, the change in skin depth means that the resistivity of the ground wires (and their loss) changes with frequency: to a first order, as 1/sqrt(frequency), so the same ground wire will have twice the loss on 40m as it does on 160m. Depending on the relative ratio of the grounding network resistivity and the soil resistivity, this might change the optimizing tradeoff.
Now to the practical aspects... real soil is generally not particularly homogeneous or consistent. You could easily have variations of an order of magnitude over distances of a few feet (in my own case, it depends on how close the rocks are to the surface and how recently and where the watering happened). These variations can account for performance variations on the order of a dB or so, which is the same magnitude as the variations for the different ground radial lengths, etc.
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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