On Wed, 7 Jan 2004 00:32:17 -0500, "Craig Buck" wrote:
But what happens where the line hits the antenna? If the line is 50 or 450
ohm and the antenna is exhibiting 2 ohms, isn't there a big mismatch and a
lot of lost power? How much? Would the 450:2 mismatch lose more power than
a 50:2 mismatch and thereby give up the advantage gained by the low loss in
the 450 ohm ladder line. I don't see this quantified in the antenna
modeling programs.

Hi Craig,

SWR is not inherently lossy, it contributes to the existing loss which
can only be found in ohmic resistance. That ohmic resistance is part
of the finals' bulk resistance, transmission line wire, and metal
construction of the antenna. If all such characteristics were 0 Ohms,
then any amount of SWR would exhibit no loss whatever.

However, the antenna also exhibits a "radiation resistance" in series
which is part of the mix. This resistance does not transform power to
heat, and thus does not qualify as loss in the conventional sense. In
other words, radiation resistance (Rr) does not diminish efficiency.
Even if we had 0 Ohms ohmic resistance, there would still be the
radiation resistance to eventually consume all the power.

Now backing up and then considering that there is some ohmic
resistance in these elements, in comparison to that radiation
resistance (Rr) we find an efficiency (which IS quantified in antenna
modeling programs). If you tried to drive a 2 Ohm (Rr) antenna with 2
Ohms of ohmic loss, you can guess where this leads. If you tried to
drive a 200 Ohm (Rr) antenna with 2 Ohms of ohmic loss, you can see
you've improved your DX. BOTH resistances consume power, but the
radiation resistance is a productive use.

These are extremes and ignore SWR which takes that ohmic loss and
multiplies it. Hence it is preferable to present a 50 Ohm source
(your transmitter is characterized as such at full power) to a 50 Ohm
transmission line, feeding a 50 Ohm Antenna. Within this scenario,
there still remains the ohmic loss that is strictly a matter of local
variables (poor connections, small diameter wire, iron wire instead of
copper, proximity of earth...).

One last conundrum: there are very few 50 Ohm antennas in nature or
off the shelf. What you see characterized as a 50 Ohm antenna is
derived through matching connections at the feed point that transform
the actual radiation resistance (Rr) to the anticipated transmission
line Z. The standard quarterwave vertical whip exhibits an Rr of 30
to 35 Ohms for the best of conditions, and the standard halfwave
dipole exhibits an Rr of 70 to 75 Ohms for its best of conditions.

Those native Rr's are often left untransformed because the SWR they
present are in fact meager (even 2:1 is fairly trivial). If you were
to examine such plumber's delights like the series available from GAP,
you would find that they have conspired to arrange elements to present
as nearly 50 Ohms at the different bands as they could; however, the
native Rr varies from 10 - 40 Ohms and is transformed to 50 Ohms by
the time it reaches the connector dangling below.

In this case, the SWR resides on the antenna itself, and it is the
ohmic loss of the antenna structure that determines how efficient the
system might ultimately be. GAP uses large metal tubes that in
comparison to #32 wire wrap wire is a slam dunk in comparison; and
thus the efficiency concerns are cast back into the losses of the
transmission line and source that are suitably matched (use a bigger
diameter coax, and a more expensive rig to do better).

73's
Richard Clark, KB7QHC