Reg:
[snip]
Perhaps after the festivities you are feeling too lazy to
satisfy your own curiosity by exercising your brain cells. ;o)
[snip]
Ahem... well I do admit to imbibing during the Holiday, but I feel that at
least
20% of my brain cells are still intact which should enable me to pass the
next generation of ham radio exams with no problem. :-)
[snip]
But I guess this is the first occasion on which you have been confronted
with the *distributed* variety and have been brought to a sudden dead
stop.
[snip]
Well yes and no!
Even tho,. for professional reasons, I have extensive transmission line
modelling
software [self-developed] which supports extremes of complex Zo and
distributed
losses with various loss distributions along the lines, I have never used
these
computer codes/algorithms to simulate antennas.
[My professional applications of these codes, written in Fortran, have been
for broadband
digital subscriber loop, DSL, BRA ISDN and cable modem transmissions over
telco local
loops. i.e. upwards of 1000 to18,000 feet of twisted pairs of mixed guages
and dielectrics,
with bridged taps etc. These codes allow for empirical fits to primary
parameters, R, L, C and G
as functions of frequency and other effects, etc... I had posted on this NG
some of the models
developed by several contributors to the ANSI T1E1.4 Standards Committee
over the past
few years sometime in the last year or so if you recall.]
Clearly such software/algorithms which are sort of like finite element
analysis methods breaking
the lines into incremental sections and summing the results, etc... and can
also be used to simulate
the driving point impedances and losses, both disipative and radiative, of
antennas as you suggest.
Until your posting I had never fully thought through what the distribution
of radiative losses
on antenna structures should be...
[snip]
Or 2, leave the wire resistance where it is and distribute the radiation
resistance along the wire. We have no choice about the type of
istribution - it must be the same as the wire resistance is
istributed -
i.e., uniformly.
:
:
Whatever we do we cannot avoid transforming from a lumped to distributed
resistance value, or vice-versa. Electrical engineers do it all the time.
In the case of a dipole there are several ways. But its a simple process
and the result is amazingly even more simple.
:
:
The equivalent lump of resistance located at the centre (where 1 amp
flows)
turns out to be exactly half of uniformly distributed end-to-end
resistance
of the wire. In fact, that's exactly how the radiation resistance of the
usual 70-ohm lump got itself into a dipole's feedpoint. It is exactly
half
of 140 ohms. If radiation resistance itself had any say in the matter I
am
sure it would prefer to be nicely spread along the length of the wire
instead of being stuck in a lump next to the feedpoint.
If the end-to-end wire loss resistance is R ohms then the ficticious
equivalent lump at the centre feedpoint is exactly R/2 ohms. So easy to
remember, eh?
[snip]
Yes it sure is!
[snip]
In fact, it is the pair of 1/4-wave, open-circuit, single-wire lines
constituting the dipole which transform the uniformly distributed wire
loss
resistance to the equivalent lumped 1/2-value input resistances as
measured
at the dipole centre. And, of course, the antenna performs exactly the
same
transformation on an antenna's uniformly distributed radiation resistance.
I sometimes feel sorry for things which find themselves securely locked
in,
constrained for ever to obey the irresistible laws of nature, helpless to
do
othewise, for ever.
See how the interlocking bits of the jig-saw puzzle now fit very nicely
together.
[snip]
Linear distribution...
Yes, now with your simple, yet very clear explanation, I now see that,
thanks!
[snip]
\ use - it would never correspond to an actual antenna. When calculating
efficiency of wire antennas it seems only a uniform distribution of
resistance is of any use. An investigator has no choice in the matter.
[snip]
Hmmm... I'm just thinking... that may not always be the case!
What about certain kinds of travelling wave antennas. i.e. a V-beam,
or a rhombic, etc... which are transmission lines with an ever changing
spacing between the elements. Surely the radiation resistance along such an
antenna/transmission line is not distributed uniformly even tho the
dissipative
losses are!
Thanks again for your lucid reply, I am indebted to you for refreshing
some of my *besotted* brain cells... hmmm, I wonder is it the
reds or the whites that cause most of the brain cell damage?
I'm gonna go try some of my homebrew transmission line software on
some antenna problems and see how it does...
Best Regards for the New Year.
--
Peter K1PO
Indialantic By-the-Sea, FL
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