On 8/30/2010 3:44 AM, Richard Fry wrote:
On Aug 29, 10:38 pm, Roy wrote:

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. /etc

Rather than using an example of a balanced antenna having reasonably
high radiation resistance and zero or low reactance at its input
terminals, let us consider a base-fed 10 foot whip at 3.8 MHz -- which
is more along the lines of this thread.

Without using a loading coil, the input Z of that whip is about 0.6 -j
1250 ohms. The SWR that this antenna input Z presents to unmatched 50
to 600 ohm transmission line ranges from 52,167:1 to 5,340:1.

Not much power will be transferred through such a match, which is the
reason for the statements in my quote which you referred to.

RF

Power will indeed be transferred through such a match. Using your
antenna as an example, suppose that a transmitter with output Z of 50
ohms is connected to a tuner that transforms its output impedance to 0.6
+ j1250 ohms. Connect the output of the tuner to a half wavelength 600
ohm transmission line to the antenna. The transmitter will see 50 + j0
ohms, the antenna will see an impedance of 0.6 + j1250 ohms, and full
power will be transferred. Power transfer has nothing to do with the SWR
on the line or the match between the line and antenna.

In practice, the line loss will increase some due to the very high SWR,
but the loss increase won't be much if the matched line loss is low.

I chose a half wavelength for simplicity, but it's not necessary. Other
lengths of line will transform the antenna impedance to different
values. All that's necessary is to readjust the tuner accordingly to
match the different impedance.

Amateurs have successfully been using this method to feed nonresonant
and multi-band antennas for decades.

Roy Lewallen, W7EL