On 8/29/2010 1:04 PM, Richard Fry wrote:

The purpose and function of a loading coil used with an electrically
short antenna is to offset the capacitive reactance of the short
radiating section. Otherwise it will not accept much power from a
transmitter or deliver much power to a receiver, due to a very high
mismatch to common types of transmission line connected to its
terminals.
. . .

Difficulty in getting power to an antenna is due to the mismatch between
the transmitter and the impedance it sees, rather than between the
transmission line and antenna.

As a simple example, consider a 75 ohm dipole connected to a transmitter
through a half wavelength of 600 ohm transmission line. The transmitter
sees 75 ohms. Most transmitters will deliver full power to a load of
that impedance and, except for line loss, all that power is delivered to
the antenna in spite of a 12:1 mismatch between the transmitter and
transmission line (assuming a 50 ohm output transmitter) and 8:1
mismatch between the transmission line and antenna. If you change the
transmission line impedance to 75 ohms, the transmitter can't tell the
difference -- it still sees 75 ohms and delivers the same amount of
power, even though the line and antenna are now perfectly matched.

Roy Lewallen, W7EL