On 3/9/2015 3:29 AM, Jeff wrote:

I've been lurking in this thread and it reminded me of a time many years
ago when I was working on a receiver setup. A colleague gave me a book
with an equation for signal strength of a signal in the cell phone
frequency range in various terrestrial environments. I had a little
trouble accepting an arbitrary equation that wasn't at least close to
the typical 1/r^2 formula in free space. I seem to recall there was no
1/r^2 term at all rather it was more like a linear or maybe had a
rlog(r) term.

In any event, no one could explain where the equation came from. I
suppose it was an empirical equation rather than something derived from
theory. Ignoring waves bounced off the upper atmosphere, I assume the
earth acts to help focus the signal and strengthen it close to the
ground?


You are correct, most of those formulas are empirical, base on actual
observations. Look up papers by Egli and by Hatta, they will five you
some idea on how theses formulas were derived.

Jeff

As are basically all formulas. Even Ohm's Law was derived from actual
observations.

Although Einstein's equations such as E=mc^2 wasn't derived from actual
observation, it did come by projection of existing knowledge by an
exceptional mind.

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

On 3/9/2015 10:11 AM, Jeff wrote:


As are basically all formulas. Even Ohm's Law was derived from actual
observations.


That is certainly not correct in a lot of cases. The inverse square law
for free space path loss, for example, is derived intuitively and simply
from the transmitted power being equally distributed in all directions,
not from observations.

S= P*(1/(4piD^2))

Jeff

Jeff,

Actually, not. It was observed first back in the 1700's-1800's when the
link between electricity and magnetism was being investigated. And
hundreds of years before that, it was a know property of magnets.

The equations didn't come until later.

--
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Remove the "x" from my email address
Jerry, AI0K

==================

On 09/03/15 15:43, Jerry Stuckle wrote:
On 3/9/2015 10:11 AM, Jeff wrote:


As are basically all formulas. Even Ohm's Law was derived from actual
observations.


That is certainly not correct in a lot of cases. The inverse square law
for free space path loss, for example, is derived intuitively and simply
from the transmitted power being equally distributed in all directions,
not from observations.

S= P*(1/(4piD^2))

Jeff

Jeff,

Actually, not. It was observed first back in the 1700's-1800's when the
link between electricity and magnetism was being investigated. And
hundreds of years before that, it was a know property of magnets.

The equations didn't come until later.


You are confusing a magnetic field with an EM field. You can have a
magnetic field with no E field- eg from a bar magnet. It will have a
magnet field which exhibits the inverse square law but no E field.

On 3/9/2015 12:54 PM, Brian Reay wrote:
On 09/03/15 15:43, Jerry Stuckle wrote:
On 3/9/2015 10:11 AM, Jeff wrote:


As are basically all formulas. Even Ohm's Law was derived from actual
observations.


That is certainly not correct in a lot of cases. The inverse square law
for free space path loss, for example, is derived intuitively and simply
from the transmitted power being equally distributed in all directions,
not from observations.

S= P*(1/(4piD^2))

Jeff

Jeff,

Actually, not. It was observed first back in the 1700's-1800's when the
link between electricity and magnetism was being investigated. And
hundreds of years before that, it was a know property of magnets.

The equations didn't come until later.


You are confusing a magnetic field with an EM field. You can have a
magnetic field with no E field- eg from a bar magnet. It will have a
magnet field which exhibits the inverse square law but no E field.



Brian,

No, I'm not confusing the two. But my point is that one led to the
other. The equations didn't appear out of mid air - measurements
preceded them.

The observations I was talking about in the 1700's-1800's were for EM
fields. And my point was their loss with distance is the same as with M
fields - which had been known for a much longer time.

And E fields were also measured back in the days of Leyden jars and the
like.

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

On 3/9/2015 12:54 PM, Brian Reay wrote:
On 09/03/15 15:43, Jerry Stuckle wrote:
On 3/9/2015 10:11 AM, Jeff wrote:


As are basically all formulas. Even Ohm's Law was derived from actual
observations.


That is certainly not correct in a lot of cases. The inverse square law
for free space path loss, for example, is derived intuitively and simply
from the transmitted power being equally distributed in all directions,
not from observations.

S= P*(1/(4piD^2))

Jeff

Jeff,

Actually, not. It was observed first back in the 1700's-1800's when the
link between electricity and magnetism was being investigated. And
hundreds of years before that, it was a know property of magnets.

The equations didn't come until later.


You are confusing a magnetic field with an EM field. You can have a
magnetic field with no E field- eg from a bar magnet. It will have a
magnet field which exhibits the inverse square law but no E field.

The problem would seem to be that there is confusion with an equation
being preceded by measurements (pretty much *every* equation known) with
equations that were crafted in the absence of derivation solely to fit
data. Even Einstein's equations had measurements that preceded them and
were essential to their formulation. Michelson and Morley made the
measurements that set the stage for E=Mc^2. I would hardly call that an
empirical equation.

Not much point in trying to discuss this. It will be impossible to find
any common ground I am sure.

--

Rick

rickman wrote:

On 3/9/2015 12:54 PM, Brian Reay wrote:
On 09/03/15 15:43, Jerry Stuckle wrote:
On 3/9/2015 10:11 AM, Jeff wrote:


As are basically all formulas. Even Ohm's Law was derived from actual
observations.


That is certainly not correct in a lot of cases. The inverse square law
for free space path loss, for example, is derived intuitively and simply
from the transmitted power being equally distributed in all directions,
not from observations.

S= P*(1/(4piD^2))

Jeff

Jeff,

Actually, not. It was observed first back in the 1700's-1800's when the
link between electricity and magnetism was being investigated. And
hundreds of years before that, it was a know property of magnets.

The equations didn't come until later.


You are confusing a magnetic field with an EM field. You can have a
magnetic field with no E field- eg from a bar magnet. It will have a
magnet field which exhibits the inverse square law but no E field.

The problem would seem to be that there is confusion with an equation
being preceded by measurements (pretty much *every* equation known) with
equations that were crafted in the absence of derivation solely to fit
data. Even Einstein's equations had measurements that preceded them and
were essential to their formulation. Michelson and Morley made the
measurements that set the stage for E=Mc^2. I would hardly call that an
empirical equation.

Not much point in trying to discuss this. It will be impossible to find
any common ground I am sure.

OK, lets not treat it as aerial question. Though this is an aerial
group, I would have thought propagation was on topic. Can I ask if
there is any information around which would give us some guidance on
what power one would need in a dampish country about 200 by 800 miles
across to intercommunicate by ground wave at 1.8MHZ? I think this is
actually the gist of Spike's question, assuming everyone uses decent
vertical aerials (a big assumption, of course).

--
Roger Hayter

Roger Hayter wrote:

snip

OK, lets not treat it as aerial question. Though this is an aerial
group, I would have thought propagation was on topic. Can I ask if
there is any information around which would give us some guidance on
what power one would need in a dampish country about 200 by 800 miles
across to intercommunicate by ground wave at 1.8MHZ? I think this is
actually the gist of Spike's question, assuming everyone uses decent
vertical aerials (a big assumption, of course).

http://www.itu.int/rec/R-REC-P.368/en

Follow the link to the latest version, language, and format desired.

Covers the frequency range of 10 kHz to 30 MHz.


--
Jim Pennino