Dear Peter, 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"), 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 used a coaxial slotted line in one job I had (1980s), mainly just to
measure SWR though (at about 1GHz). There was also network analysers
available, but that was regarded as overkill with an expensive bit of kit
just to measure SWR. At the university where I work, 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!
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
From: g3ldo <[email protected]>
To: <[email protected]>
Sent: Saturday, January 15, 2005 3:04 PM
Subject: LF: SWR-off topic
I am currently updating some of the antenna material in the RSGB Radio
Communications Handbook.
I have a question about SWR meters...
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