In message , Roy Lewallen
writes
On 8/31/2010 3:09 PM, Richard Fry wrote:
On Aug 30, 5:52 pm, Roy wrote:

In my example, the antenna is not matched to the transmission line. Nor,
for that matter, is the transmitter matched to the transmission line. My
point is that power transfer doesn't depend on either of these points
being matched.

Roy:

My post showing very high input SWR at the base of an unloaded, base-
driven, 10 foot vertical on 3.8 MHz described an UNMATCHED system
resulting from its connection to transmission lines of typical
impedance values. It did not include matching networks, whether
located at the base of the vertical radiator, the output connector of
the transmitter, or wherever.

Then you posted, "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. ... The
transmitter will see 50 + j0 ohms, the antenna will see an impedance
of 0.6 + j1250 ohms, and full power will be transferred."

That configuration you posted is a MATCHED system, and its performance
does not disprove the accuracy of my post.

RF

So you're saying that the mismatch between the impedance of an antenna
and the transmission line connected to it doesn't inhibit power flow
when there's a matching network anywhere in the system. But it does
interfere with power flow when there's no matching network?

What if the antenna is 50 ohms and the transmission line is a half
wavelength of 600 ohms, for a 12:1 mismatch? There's no matching
network. The transmitter sees 50 ohms. The antenna sees 50 ohms. What
will interfere with the power flow?


In my simplistic world, this is how I understand things:

Provided the TX is followed by a tuner/matcher, which matches whatever
is attached to the tuner output to 50 ohms at the tuner input, the TX
will be happy.

The power loss in the feeder is essentially a function of its inherent
loss (when matched) per unit length, and the SWR on it.

The SWR is a function of the mismatch between the load on the antenna
end of the feeder, and the feeder characteristic impedance. The greater
the SWR and the longer the feeder, the higher will be the loss on the
feeder. 'Lossless' feeder have no loss, regardless of length and SWR.
However, with real-world feeders, the losses rise increasingly rapidly
with increasing SWR.

The impedance looking into the tuner end of the feeder is the impedance
of the load, transformed by length of the feeder, and is also modified
by the loss of the feeder.

The higher the feeder loss, the closer the impedance seen at the tuner
end will approach the characteristic impedance of the feeder. [Long
lengths of lossy feeder - maybe with a nominal termination on the far
end - can make good dummy loads at VHF and UHF.]
--
Ian