Hi Jim and LF group
I think the mathematical theory of transmission lines evolved with the
appearance of long-distance telegraph lines, but measuring standing waves
could only have become practical with the appearance of HF radio. In some
pre-war papers I have seen descriptions of little trolleys carrying
thermocouple ammeters that could be pulled along an open-wire line (there
is
a rather vague description in Terman's "Radio Engineering")
That rang a bell!. I looked through my older radio books but could't find
anything in my copies of Termans. Eventually I found it in Air Publications
1093, which must have been the RAF's bible on radio engineering.
The device for measuring SWR is a length of wood about 10inches long
supporting a triangular loop of wire connected to a 0-120mA thermo-ampmeter.
Two hooks on the top of the wood support allowed the device to be towed
along the transmission line with a cord looking for minimum and maximum
standing current due to missmatch. The antenna/transmission line matching is
adjusted for minimum SWR. From this it would appear early measurements were
ISWR not VSWR.
There is a couple of pages on how to make and use this device. It also
answerd a further question - why were old timers very keen to ensure
balanced currents on the feeder and no one seems to bother now. The reason
"...it is advisable to change the ammeter over to the other conductor to
ensure the maxima and minima are in the same positions on the line.......it
is hopeless to attempt to match an unbalanced line".
and I guess this is what led to Mr Smith producing his chart around the
1930s. This
really came into its own with the development of VHF, UHF and microwaves,
when bridge impedance measuring techniques became impractical and it
became
possible to fit a long enough piece of transmission line on a bench top -
hence the slotted line, in coaxial and waveguide forms. With the slotted
line, you can measure complex impedances quite accurately by measuring SWR
and the position of the maxima or minima, using effectively just a piece
of
transmission line with a moving probe and a diode voltmeter. Of course,
the
Smith chart was the ideal device for doing the calculations in
pre-computer
days, so I suppose that is the importance of the SWR scale on the chart.
I will look into this one
I sometimes supervise
students doing an experiment with an ancient Rohde and Schwarz slotted
line,
the main point of which is to show that standing waves exist, although the
Smith chart usually stumps them completely!
Most Smith charts cover the whole universe of impedance values. It can be
made far easier to use and understand by restricting the range (making the
edge SWR 4 or 5:1) and using far less R and X lines. I have one which makes
a simple Smith calculator - I can let you have a copy if you wish.
Many thanks for your input Jim.
Peter, G3LDO
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