On 11 Jul 2005 19:28:01 -0700, "JArthur"
wrote:

+If a longer antenna is better then why do they sometimes have to be cut
+shorter when tuning?
********

First off there is physical length and then there is electrtical
length. The two can be the same or they can be different.

Electrical length is the length that at some frequency the antenna is
self resonant. This is found for a half wave antenna by the following
formula

(299.8 meters / frequency in MHz.)/2 = resonant length of a half wave
antenna.

Other things can affect tuning of the antenna like nearby buildings,
how close the antenna is to the Earth, and many other variables. These
variables will alter the above formula from as little as 2% to as much
as 10%.

There are techniques that can actually shorten the physical length
while maintaining proper electrical length. One method is linear
loading of an antenna. Another is to introduce some reactance to alter
the physcal length while maintaining proper electrtical length. This
reactance is often in the form of a coil. This coil can be located
pretty much anywhere in the antenna. Coils are generally not
reccommended at the far ends of the antenna from the feed point.

Ideally one should have an antenna that has the physical and
electrical length the same. This is not always possible but the most
desirable. Reactances in the antenna alter radiation patterns off the
antenna and in most cases reducing overall efficiency of the antenna.

Antennas can be somewhat simple to very complex.

A good refereence book to pick up would be the ARRL Antenna Handbook.
There are many out there that range from introductory to very
technical.

james