In article , "Reg Edwards"
writes:

Antennas have directional properties.

It the total available power is concentrated in one direction then, as far
as the receiver is concerned, the APPARENT power of the transmitter has
increased.

But receivers in less-favoured directions from the transmitter will
experience an APPARENT reduction in the transmitter's output power.

Quite true, Reg.

To get even more basic for new folks to radio theory, assume the
ideal isotropic antenna, one that radiates equally in all directions.
It creates an EM field of the same density of RF energy per square
area all around an ideal sphere enclosing the isotropic antenna.

A half-wave dipole that is very high (elevated above ground) has an
antenna pattern (of RF energy per square area) that is maximum
perpendicular to the axis of the wires. That RF energy is minimum
along the wire axes.

An ideal dipole has a "gain" of about 2.4 db over the ideal isotropic
antenna and such gain is referred to as "2.4 dbi" with the little "i"
indicating the reference to the isotropic.

Because it is difficult to build a reference antenna that can perform
like the ideal isotropic, many more complex antennas reference their
gain to the half-wave dipole and those gains, in db, are labeled as
"dbd" with the little "d" suffix refering to a dipole.

FM and TV broadcast antennas are usually designed for antenna
patterns that are almost omnidirectional in the horizontal plane and
have very little RF energy at elevations above or below horizontal.
The term "ERP" for Effective Radiated Power was first used with
FM and TV broadcasting to indicate the basic power output of the
transmitter multiplied by the antenna gain. For broadcast listeners
they would "hear" a signal as if the station's power output was as
strong as the ERP value.

Directional antennas simply focus the RF energy in certain
directions. What would be an equal value of RF in all directions
with an ideal isotropic antenna now increases above ideal in the
direction of maximum RF energy. That is the "gain."

If one were to plot RF energy density per square whatever in a 3-D
graph (almost always the case in antenna analysis programs or on
antenna range receiver plotters), the "shape" formed is what everyone
intuitively describes as the "pattern." [it could be called an "isopower"
plot, I would guess]

For an ideal isotropic antenna, the "pattern" is a sphere. For a good
half-wave dipole very high above ground, the pattern looks like a torus
or doughnut shape. A typical FM or TV antenna pattern looks like
a fat pizza. A very high gain parabolic reflector radar antenna pattern
looks sort of like a long breadstick. Complex wire antennas start
looking like the outline of the contents of a spaghetti bowl.

It's about 10:30 PM local time here and I'm getting hungry...bye...:-)

Len Anderson
retired (from regular hours) electronic engineer person