"Roy Lewallen" wrote in message
...
Imagine that you put 100 watts into an isotropic radiator in free space
and measure its field strength at some distance. Then you create a ground
plane like the ones used by most modeling programs, perfectly flat and
infinite in extent, and make it perfectly conductive. Now distribute that
100 watts evenly in all directions, above the ground plane -- imagine a
sort of hemi-isotropic radiator which does this. Measure the field
strength at the same distance from the radiator as before.

find that the field strength is 3 dB higher, and the
power density twice as great,

That's interesting! That part regarding change of "density"
How does one determine the density changes in various parts of a radiation
pattern?
Does the Laws of Partial Pressures apply?
Does high density radiation penetrate the earths layers more than low
density
R.F. radiation before it returns to earth? If R.F. has density then it must
have mass
thus gravity can return radiation to earth at odd places on earth. Grin
Regards
Art:



as for the isotropic source in free space.
It's simply because you're distributing the same amount of power in half
the amount of space.

That illustrates the first principle, that putting an antenna over ground
gives you an automatic 3 dB increase in average field strength compared to
free space. If you compare the field strength of an antenna over ground
with a free space isotropic source (i.e., give its gain in dBi), you'll
find it has a gain of 3 dBi when averaged in all directions. (EZNEC's
"Average Gain" calculation automatically takes this into account and
normalizes to 0 dB when a ground is present; NEC-2's average gain
calculation doesn't.)

You can also pick up about 3 dB additional gain because of the increased
directivity you get when putting an antenna over ground. You get the same
pattern you'd have if there were another antenna below the ground (an
"image" antenna) and with out of phase current relative to the actual
antenna -- it's like having a two antenna beam. (Pattern gain won't
usually be exactly 3 dB because of the impedance change of the antenna
caused by interaction with the ground -- when the ground is perfect, it's
exactly the same as mutual coupling to the "image" antenna.) So typically
a dipole over ground has a gain somewhere between about 3 and 6 dBi --
around 4 or 5 is common for most typical grounds and heights.

Comparing the gain of an antenna to that of a dipole over ground requires
calculating or measuring the gain of the dipole over ground --
you can't just assume some gain. The most universally used way is to get
both gains in dBi, since dBi is universally defined and understood, then
subtract to find the difference.

Roy Lewallen, W7EL

Cecil Moore wrote:

In *free space* the gain of a dipole over isotropic is about 2.14 dB.
Over ground, the reflections from the ground add another few dB.
dBi is *always* referenced to free space. An isotropic over perfect
ground would probably have a gain of about 3 dBi assuming no destructive
interference. A dipole over perfect ground has a gain of about 7.5 dBi.
Rule of thumb: A dipole over average ground has a gain of about 6 dBi.