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