The plots show 10 MHz ground constants at eleven locations in Great Britain from a paper by R. L. Smith-Rose. At each location the soil was sampled at several depths to 10′ and then analyzed in a lab. ESU ∕ 108 = 11.1 mS/m.
Rudy Severns, N6LF, used a 12″ ground probe and NanoVNA-H4 to measure ground constants in a newly dug trench in Oregon bottomland. He inserted the probe horizontally in the trench wall. The soil was loam down to the bottom at 6′ where it became gravel. He inserted the probe vertically at 6′ and at the surface near the trench.
It's tempting to calculate effective ground constants by averaging the values at each depth weighted by antenna-induced current, which decays exponentially at rates the ground constants determine. But this simple model does not account for power reflected at soil layers due to ground impedance differences. Reflections and re-reflections alter the current. It's like cascading transmission lines of different characteristic impedance. Now consider an example with two soil layers. Reduce the ground constants of the upper layer until they become those of air. This increases antenna height by the depth of the first layer. But no set of effective ground constants can do this in NEC. Evidently multiple ground layers require a more complex ground model.
For total reflection at a subsurface soil layer one skin depth deep, reflected surface current will be 14% of incident current. For realistic partial reflection, it will be less. This suggests that an antenna is mainly affected by soil within one skin depth of the surface.
Location Surface Soil Skin Depth ■ Rugby 1 2.5′ ■ Tatsfield 2.8 ■ Moorside Edge 3.2 ■ Washford Cross 4.5 ■ Teddington 4.6 ■ Brookmans Park 5.3 ■ Brendon Hills 5.4 ■ Daventry 6.0 ■ Baldock 6.1 ■ Westerglen 7.1 ■ Rugby 2 7.7 N6LF 5.5
Subsurface soil effects are most likely for low-conductivity soil at lower frequencies where skin depth is greatest. Use this program to calculate skin depth.
This article explains how to determine ground constants with a low dipole. Unlike a ground probe, the method excites relevant ground. However, it is restricted to a single band, needs a large, flat, open area free of interfering wires, structures, vegetation, and ground conductors, requires meticulous construction, measurement, and modeling, and must be left in place or rebuilt to measure at different seasons. Despite these limitations, the method may be useful if you suspect your ground is inhomogeneous. Use bare copper wire. At each end use the smallest possible loop through a tiny polystyrene insulator attached to nonconductive line. Minimize wire sag and model average height. Measure and model the excess shunt capacitance at the center insulator. Attach the VNA directly using a male-male adapter. Record measurements without touching the VNA. Increase model segmentation until impedance converges. Recalculate the Sommerfeld-Norton parameters whenever you alter ground constants. Since so many things can go wrong, do a sanity check against ground probe measurements.
A one-wavelength square loop eliminates sensitive wire ends and may better fit the space available. Support with tiny polystyrene insulators without looping the wire. Attach the VNA at the middle of one side.
To overcome the single-band limitation, use this program to extrapolate ground constants to other bands.
Smith-Rose, R. L., "Electrical Measurements on Soil with Alternating Currents," Proc. IEE, Vol. 75, pp. 221-237, 1934. I skipped the two locations with indefinite depths and the location with no surface measurement.
88–108 MHz