Baron wrote:

Radio propagation through the earth is one of the most challenging
problems you could attempt to solve. You seem to be concentrating on
very low frequencies, fine if you can tolerate very very slow data
flows. I'm sure there are frequencies that will propagate through the
earth fairly easily, but getting to a point where a usable signal
occurs with reliable results isn't easy.
. . .

Attenuation through the ground depends on the soil conductivity and
dielectric constant, and the frequency. Here's the attenuation in dB/ft
for two ground types and a number of frequencies:

Freq MHz Avg soil(1) Vy good soil(2)
0.01 0.037 0.091
0.1 0.12 0.29
1 0.35 0.90
10 0.66 2.4
100 0.69 3.3
10,000 0.69 3.4

(1) Conductivity = 0.005 S/m, dielectric constant = 13
(2) Conductivity = 0.03 S/m, dielectric constant = 20

So the distance you can communicate depends on these factors as well as
antenna efficiency, power, and receiver sensitivity.

Attenuation in salt water is very much higher, which is why submerged
submarines have to communicate at VLF.

Roy Lewallen, W7EL

Roy Lewallen wrote:
Attenuation through the ground depends on the soil conductivity and
dielectric constant, and the frequency. Here's the attenuation in dB/ft
for two ground types and a number of frequencies:

Freq MHz Avg soil(1) Vy good soil(2)
0.01 0.037 0.091
0.1 0.12 0.29
1 0.35 0.90
10 0.66 2.4
100 0.69 3.3
10,000 0.69 3.4

So if I get this right, a 2.4gHz signal in normal soil would be anttenuated
6.9 db (less than 2 "S units). In very good soil, it would be anttenuated
around 34 db, which would make it difficult to receive.

A Pringles can antenna has a gain of about 18db, that would certainly be
enough for average soil, and might be good enough for very good soil.

It also has the advantage of possibly being a 2 way link.

Geoff.
--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM
New word I coined 12/13/09, "Sub-Wikipedia" adj, describing knowledge or
understanding, as in he has a sub-wikipedia understanding of the situation.
i.e possessing less facts or information than can be found in the Wikipedia.

Geoffrey S. Mendelson wrote:
Roy Lewallen wrote:
Attenuation through the ground depends on the soil conductivity and
dielectric constant, and the frequency. Here's the attenuation in dB/ft
for two ground types and a number of frequencies:

Freq MHz Avg soil(1) Vy good soil(2)
0.01 0.037 0.091
0.1 0.12 0.29
1 0.35 0.90
10 0.66 2.4
100 0.69 3.3
10,000 0.69 3.4

So if I get this right, a 2.4gHz signal in normal soil would be anttenuated
6.9 db (less than 2 "S units). In very good soil, it would be anttenuated
around 34 db, which would make it difficult to receive.

Those dB values are correct for 10 feet of homogeneous soil. Real soil
is stratified, and reflections from layer boundaries could have some
pretty profound effects. 6.9 dB is from 2 to 4 "S-Units" on my Icom,
depending on where on the S meter scale it is.

A Pringles can antenna has a gain of about 18db, that would certainly be
enough for average soil, and might be good enough for very good soil.

Relative to what? According to this site
http://www.turnpoint.net/wireless/has.html, a Pringles can antenna
showed about the same gain as a Lucent omnidirectional antenna. Of
course, an 18 dB error is only a mis-estimation of power density by a
factor of 1,000,000,000,000,000,000. Pah, piddly nit-picking details.
You could make it up by increasing the power by the same factor.

It also has the advantage of possibly being a 2 way link.

A reliable one would take some calculation, planning, and a realistic
idea of antenna performance.

Roy Lewallen, W7EL