Hi Craig,

The 200:1 mismatch is "significant", but it does not directly cause any loss. In
handwaving fashion, this is how things work.

A mismatch allows some fraction of the power to pass through the connection
point, with the remainder reflected. (You can substitute voltage or current for
power. The numbers are different, but the principle is the same.)

Assume the transmitter supplies some level of power, say 100 W, to the
transmission line in a perfectly matched manner. A tuner will generally be
required. For purposes of this discussion, nothing passes from the transmission
line back to the transmitter.

The energy supplied by the transmitter has to go somewhere, and the only two
choices are to the antenna or to losses in the transmission line. If the line is
lossless then all of the energy goes into the antenna.

How does this happen when the junction between the line and the antenna reflects
most of the power?

The power level in the line increases so that even the small percentage
transferred to the antenna equals the same 100 W supplied by the transmitter.
There are typically long and loud arguments in this newsgroup on the exact
mechanism for this buildup in the transmission line, but it does happen within a
few cycles of RF. The resulting voltages and currents will be much higher than
those found in a fully matched system.

So far all is good. The antenna receives the full transmitter output, and there
are no added losses.

The problem comes from the higher losses that occur in even the "lossless"
transmission line when operating at high voltages and currents. In the case of
ladder line these losses may still remain quite small, but in the case of RG-58
they can become quite large. The transmission line may fail at lower power
levels than expected.

Soooo, the mismatch at the antenna junction cannot really be ignored, but its
impact is in the transmission line, not the junction itself. Unless the mismatch
is extreme the ladder line solution takes care of the loss problem.

73,
Gene, W4SZ





Craig Buck wrote:
I am focusing on the issue of power transfer at the junction of the line and
the antenna. Specifically, in a multiband dipole where the figures (all of
them) will vary wildly from band to band. Maximum power transfers at
resonance (oh no, let's not get into a war defining that). But I think it
is safe to say by anyone's definition, a multiband dipole usually not
operated at it's resonant frequencies. If the line is 450 ohm and the
antenna is 2 ohm or 20 ohm or 2000 ohm, there is not resonance.
Intuitively, I have got to think a 200:1 mismatch is significant.

So what is the loss at the antenna/line junction? I understand matching at
the transmitter end. I understand using low loss line. I don't understand
why the mismatch at the antenna junction is ignored.