Modeling a free space dipole made from a lossless conductor, 100 ft in
length, at 1.8 MHz shows an input impedance of 6.694 - j1621 Ohms. As
expected the radiation efficiency is 100%.
Adding 300 ft of 600 Ohm, 6" spaced, copper open wire transmission line
degrades the radiation efficiency to 16.75 %. The result, therefore
indicates a transmission line loss of 7.76 dB. The input impedance is
calculated as 11 - j619.7 Ohms.
The ARRL, DOS based program, "TL" computes, for 300 ft of 600 Ohm line
terminated with 6.694 - j1621 Ohms, a loss of 8.19 dB, and an input
impedance of 18.35 - j805 Ohms.
Realizing that 6" spaced, #14 AWG, is not exactly 600 Ohms, and NEC's
computation of parallel wire transmission lines is not 100% accurate; the
results do seem to confirm the validity of the ARRL's program.
Another interesting experiment with the ARRL's program also seems to verify
its accuracy:
RG8, 1000 ft, frequency 100 MHz. Matched line loss = 24.82 dB.
Load impedance 1 - j1000 Ohms. Total line loss = 61.82 dB.
The program computes the input impedance to by: 50.3 - j0.2 Ohms.
73,
Frank
"George, W5YR" wrote in message
...
Keep in mind that real ohmic and dielectric losses measured in watts
depend
upon sqrt(SWR). Thus, the higher the SWR (load mismatch) the greater the
I^2R losses in the conductors and similarly in the dielectric.
So, to me, a non-unity SWR connotes real power loss measurable in watts
and
attributable to well-known loss mechanisms.
Of course, any real power lost in the line materials represents power not
delivered to the load, so this fits somewhat with the viewpoint that
Line Loss is in fact the magnitude of power undelivered to the load due to
the mismatch. But, I think that we are looking at real watts of loss here.
Another confusing factor is that one is usually interested in the total
loss
attributable to the use of a mismatched line and not especially in how
that
loss is distributed along the line from load to source. But there are
applications where the loss distribution with line length is of concern.
An
example is the case of a complex Zo with rho unity in which the majority
of the power loss occurs in the section of the line nearest the load and
decreases toward the source. In that case of probably limited application,
the line nearest the load might be required to handle more power than that
further toward the source.
A somewhat related example concerns the W2DU balun in which is it observed
that the beads nearest the mismatched load endure the largest heat
dissipation and are commonly larger that the remainder further toward the
source.
However, since complex Zo is an issue of magnitude usually only at low r-f
and more so at audio frequencies, this is seldom a practical
consideration.
Thanks for bringing this topic to light, Bob. Like most engineers, I have
been guilty of looking at "line loss" as a monolithic phenomenon and not
being concerned with the micro-structure of its distribution.
--
73, George W5YR
Fairview, TX
http://www.w5yr.com
"Robert Lay W9DMK" wrote in message
...
On Thu, 25 Nov 2004 20:51:16 -0700, Wes Stewart
wrote:
On Thu, 25 Nov 2004 20:20:32 GMT, (Robert Lay
W9DMK) wrote:
Bob,
You might want to look at this paper:
http://users.triconet.org/wesandlinda/AIEE_High_Swr.pdf
Dear Wes,
I have downloaded the pdf file and printed it out. It's tough reading.
I hope that MacAlpine agrees with what Dave and Richard are telling
me, because their responses seem to be correct and are exactly what I
was afraid of - that I've been sucked into another example of the
strange terminology used to describe "losses".
I have always thought of "loss" as a conversion to another form of
energy (typically heat energy) which is lost from the system.
Apparently, the kind of "loss" being described in the example that I
gave is not a loss at all. It's more like "return loss", which is also
not a true "loss" in my thinking. In other words, it seems that the
"Additional Losses Due to SWR" are not losses at all, but are simply a
measure of the power that "could" have been delivered to the load were
it not for the mis-match.
Bob, W9DMK, Dahlgren, VA
http://www.qsl.net/w9dmk