On Wed, 28 Feb 2007 08:11:30 GMT, Owen Duffy wrote:

Now, are you prepared to post your solution?

Hi Owen,


Your quick computation of 3.3 dB is suitably close to my reference's
first pass solution (3.27 dB), but it neglects the contribution of the
source's resistance.

The solution is 4.9 dB.

If we were to revisit your 1 meter long cable used in the 80M band and
force the transmitter to be a voltage source through the common
mechanism of adding a substantial resistor, and mismatch the other end
of that 1 meter long cable to the same degree (each end seeing 10K Ohm
for the purpose of this statistical curiosity); then that same cable
will heat up with its contribution of at least 3dB of ADDITIONAL loss.

The 10K Ohm specification is a forced one, but it responds in kind to
the original forced solution too. In fact, it comes close to the
source resistance found in a tube amplifier (a common voltage source)
driving a halfwave element (a common application for such a source)
and demonstrates the common futility of using coax (that I have
already expressed) to accomplish this.

However, we don't have voltage sources to conveniently solve either of
these statistical curiosities. Both the tube transmitter, and the
solid state transmitter employ impedance matching to either draw down,
or pull up the native source resistance to a level suitable for
applying to a transmission line.

I would again point out that reverse power suitably accounts for the
1.6 dB difference between your answer and the solution, it also
accounts for the 3 dB difference between your short cable's example,
and my twist in its application.

All such differences have been described and used in design for quite
a few decades, and they have been couched in exactly the terms I've
used here.

If anyone wants to challenge the 4.9 dB solution, they can impeach my
reference "Reference Data for Radio Engineers," (various editions). I
can supply other references that have been named in this group too,
but I would suggest with tackling one authority at a time.

73's
Richard Clark, KB7QHC