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Richard Clark · April 14th 07, 06:02 PM posted to rec.radio.amateur.antenna

On Sat, 14 Apr 2007 10:59:17 -0500, (Richard
Harrison) wrote:

That is why King, Mimno, and Wing on page 30 of Transmission Lines,
Antennas and Wave Guides say:
"Since the input impedance of a short-circuited quarter-wavelength
section of transmission line is a very high resistance, short-circuited
stubs may be used to support the line."

Hi Richard,

One example begs another, symmetrical one; and here is the example of
my asking you why you would struggle through building an open line
reflected as a short (the quarterwave section).

For argument's sake, let's amend KM&W to say:
"Since the input impedance of a open-circuited quarter-wavelength
section of transmission line is a very low resistance, open-circuited
stubs connected in series with the line elements may be used to
support the line."

This statement is equivalent in the transmission line mechanic's tool
kit, certainly. However, it brings with it the difficulties of
maintaining a "good" open (especially when that open is in close
proximity to earth, or at least to the press of humanity). Casting
those problems into the line obviously presents loss issues of a poor
virtual short.

This conforms to my experience with many plumbing designs on the
microwave bench. The absence of open quarterwave sections was nearly
universal due to the problems of their implementation. It takes
little effort to realize why KM&W did not offer this alternative. If
you browse Terman, he also avoids such constructions.

73's
Richard Clark, KB7QHC

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