RadioBanter - Where does high gain antenna get the gain from?

Wireless routers come with small antennas. There are high gain,
non-directional antennas you can buy , like this:
http://www.amazon.com/Linksys-High-A.../dp/B0002F3E9M

If the antenna is directional, I can understand how it could achieve high
gain (by concentrating the radio wave in one direction). But it is
non-directional, so where does the extra gain come from?

In this case, "non-directional" is misleading... they're more commonly
referred to as "omnidirectional".

These antennas are non-directional in one plane (perpendicular to the
axis of the antenna). Normally, the antenna is oriented vertically.

They are *quite* directional in their elevation, though. That is to
say, they're most sensitive (highest gain) in the directions which are
perpendicular to the antenna. As soon as you start getting "above" or
"below" this perpendicular angle (at higher or lower elevation angles)
the gain drops away... and directly above or below the antenna there
is a deep "null" in the gain pattern and the antenna is *extremely*
insensitive.

The three-dimensional gain pattern of a simple half-wave dipole
antenna looks somewhat like a donut (or, in cross-section, like a
figure-8).

For these higher-gain "omnidirectional" antennas, the donut is
"squashed flatter" - there's more gain out in a horizontal direction
(compared to a half-wave dipole) but the gain falls off more rapidly
as soon as you start getting much above or below the horizontal plane.
It's concentrating the power (and receive sensitivity) just as you
say, but in the vertical (elevation) direction rather than by picking
a sector of the horizontal plane.

As a result, these higher-gain omni antennas can be a good solution
for increasing your signal coverage throughout a single-floor building
space. However, if you're trying to provide coverage for multiple
floors, you may very well find that such an antenna makes signal
coverage *worse* for users on any floor other than the one with the
wireless router.

In short, you pay Peter by robbing Paul... not unlike every system of
taxation ever tried by humanity :-)

A truly non-directional antenna (technically referred to as an
"isotropic" antenna) would put out precisely equal amounts of power in
every direction in the sphere. It can be proven that no such antenna
can actually exist - it's impossible to create such a unform EM field
without the presence of singularities (i.e. holes) or one sort or
another. However, the isotropic antenna is a useful point of
reference (even if it doesn't really exist) and you'll often see
antenna gain figures given as a "dBi" number (that is, antenna gain in
decibels, compared to what you'd get from an isotropic antenna if one
actually existed).

--
Dave Platt AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
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