The tower is a slightly different situation. There you are not between the
antenna and ground - the ground is between the antenna and you....
|/
| 0
| -+-
| /\
==================================================
If the element is at the top of the mast at left, the path from it to your
phone (e.g.) is not close to the ground,
so the losses along the path from you to the element are small. But as the
path gets closer to the ground, the electric field tends to be dragged toward
zero. So the higher the antenna the better, because if it were close to the
ground, the energy along the path would tend to be absorbed by the ground
because it is closer to the antenna than you.
But if the situation looks like this:
__ [antenna]
\
\
\
0
-+-
/\
=====================================================
There is no ground between you and the antenna, but the field is forced to zero
at the earth-ground, and also reflected away from you back towards the sky as
the slanted "ray" between you and the antenna will reflect off the ground to
the right. So you will find that since you are close to the ground, the field
is zero near your head at the spot there, too. There is no absorption of
energy by the air/wood between you and the antenna, but the zero boundary
condition at the ground makes the field weaker around you by attenuating the
field and reflecting it away from you.
The differential equation solutions that describe the time varying EM fields in
both situations are, of course, far more complicated (Maxwell's equation with
fixed boundary conditions). But what I'm saying is a rough characterization of
the fields' energy structure, given the earth-ground being a roughly flat
surface that is conductive enough to hold a near constant zero voltage.
Hope this helps.
On Thursday, December 19, 2013 7:50pm, "Theodore Ts'o" <[email protected]> said:
> Thanks for the detailed explanation about antenna issues. One
> question, if I might:
>
> > (and don't put your AP in the attic and expect a good signal near
> > the ground.... or in the basement. Physics will make sure that the
> > signal is zero at any ground, so being closer to the ground than the
> > antenna weakens the signal a lot!)
>
> I thought the opposite was true? That is, ground loses go up when the
> antenna is closer to the ground, so it was good to have bigger, taller
> atenna towers? If what you say is correct, how do antennas on cell
> towers mitigate this particular issue?
>
> - Ted
>
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