Way too many unknowns.
Where did he calibrate the VNWA? Was the balun/impedance transformer in the
calibration path? If so, what standards were used?
Standard practice would be to make both an open and short measurement of the
line. Half wavelength multiples are actually to be avoided since a network
analyzer doing S11 measurements is least accurate at impedance extremes. (See
Chipman, "Theory and Problems of Transmission Lines" Section 7.5)
The VNWA-3 is an excellent instrument and when used with AC6LA's "Z-plots"
greatly simplifies the process of transmission line analysis. These two tools,
which I can afford to have in my shack (especially since Z-plots is amazingly
free) provide capability better than I had with a $200,000 network analyzer in
the past.
Wes N7WS
On 4/13/2019 9:05 AM, jimlux wrote:
On 4/13/19 8:44 AM, John Pieszcynski wrote:
See "A Novel Approach to Using Window Line" by W6NBC, QST August 2018.
The article is on his website @
http://www.w6nbc.com/articles/2018-08QSTladderline.pdf
73,
John W2FV
Fascinating data..
I would have liked to see one more test run, with the window line loss without
any foam around it.
The lower loss on the metal roof may be a measurement artifact - I'd not be
trusting 0.01 dB kinds of measurements. The metal roof is "mostly" farther
away than the thickness of the foam, except where the roof ribs stick up.
'NBC didn't give the length of his line, but if we work from the 19.8 MHz half
wave, that's about 7 meters (25 feet?), so he's taking the measurement and
multiplying by 4 to get the "per 100 ft" - so 0.01 dB on the graph is really
0.0025 dB in the measurement. (the picture over the chairs is consistent with
25 feet)
I'd say that metal and free air are "the same within experiment uncertainty"
One cool thing here is that measurement tools like the VNA are common enough
now that we can actually *measure* the losses, rather than speculating or
using dialectic (aka arguing on forums, letters to the editor, and email lists).
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