>
>> 2a. For vertical antennas, it is said that there is no reflection
>> (radiation) from
>> the ground system. It isn't very clear why, horizontal or vertical
>> polarization or ?
>
> well, there can be radiation from the ground system. using the term
> 'reflection' will
> lead to lots of heated discussion as that implies certain things that
> normally either
> aren't true or are hard to visualize. the ground system normally can't be
> said to
> reflect anything, it can provide currents that may be visualized as images of
> the
> vertical element. in a well balanced ground system the currents on opposite
> sides
> cancel out so there may be no far field contribution from the ground system,
> though
> there may be near field effects.
>
One has to be careful when using terms like "reflection" because it
sometimes is just a nice conceptual model for a much more complex behavior.
For instance, if one is calculating the electric field between an object
and a infinitely conductive plane, you can replace the plane by a copy
of the object spaced appropriately, if that makes the calculation more
tractable.
Particularly in the days when we used slide rules and pencil and paper,
such approximations and simplifications made things feasible to analyze,
but that doesn't mean that the conceptual model necessarily represents a
real behavior.
These approximations persist even in modern programs like NEC (e.g.
reflection coefficient approximation vs Somerfield-Norton ground),
because until very recently, they were computationally faster, and work
adequately for a lot of modeling purposes.
In the near field, magnetic fields are as important as electric fields
(in fact, one of the three classical definitions for where the near
field ends is where the ratio of E to H field becomes 377 ohms: free
space). The "stuff" under an antenna (of any polarization) is part of
the system.
The buried ground radials have lots of different functions: They serve
as a low impedance connection from base of antenna to soil currents;
they reduce the apparent soil resistivity. and so on. An odd
observation from some modeling with NEC 4 is that if you model a buried
radial field with lots of long radials that you can "break" the radials
a short distance from the antenna, and the performance doesn't change
all that much. That is, the "conductivity improving" aspect of the
radials seems to be as important as the "low impedance connection to the
soil"
In the far field (a long way from the antenna), a reflection coefficient
model works fairly well. The problem is that you really need the
terrain shape to do it. HFTA does a nice job assuming horizontal
polarization and that the terrain is not tilted cross ways to the
direction of propagation, but that's because waves polarized parallel to
the surface have almost perfectly reflectivity, regardless of the soil
properties, so it make the calculations simple: all you have to worry
about is diffraction.
As soon as the polarization is tilted at all, it gets a LOT more
complex. And, in fact, so much more complex that it's probably not
worth modeling: not only would you need a very detailed terrain model
(on a scale better than 0.1 wavelength) but you'd need equivalently
detailed information on the dielectric properties. Unless you happen to
live in the middle of a flat dry lake or salt flat, or maybe among a
bunch of sand-dunes, that just isn't going to happen.
So, we make some approximations, make a qualitative assessment that
something is better or worse in the general case (e.g. is higher
better?) or identify a specific case where it's really good or bad
(verticals on the beach = very good, regardless)
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