[K6BW] Antenna Q

[Bill Smith]

Borrowed from QRP-L newsgroup...  Antenna Q relates to the bandwidth of a
resonant antenna, in general how far you can tune away from the antenna's
resonant point and still work with a reasonably low SWR.

Hue,

The true Q of an antenna is based on the amount of
"stored" (near field) power, divided by the radiated
power (and losses).  The former depends on a parameter
called volumetric "divergence", but to understand at
that level, you need to study the theory of
electromagnetic waves - a hefty undertaking.

At the practical level, one can observe that it
is impossible to get (usually desirable) low Q,
small size and high power-gain all in the same
structure.  This is due to the physics of both the
electrical and mechanical parameters involved.  So
normally if you want a low-Q antenna, you must be
willing to give up some smallness of size to get it.

The Q of a full-length half-wave dipole does depend
on the size of the conductors, because close to the
surface of each conductor there is higher volumetric
divergence for small conductors, and vice versa.  One
practical way to avoid some of the problem is to
employ multiple conductors, as has been done for many
years.  But that, of course, leads to larger size.
(It is very hard to fool Mother Nature :)

Calculating the exact Q for a practical HF antenna
is not easy.  But for practical ham antennas, and the
typical bandwidths needed, you are still best off
using a good-sized wire or tube, or multiple such.

I know this is only a general explanation of what
Q of an antenna is, but it is a complicated issue
to get specific in detail about without a lot of
math.

A good forum for further investigation is the
antenna-discussion group at antennex.com.

73 - Bob, K6KL