[TowerTalk] Open wire measurments
Wes
wes_n7ws at triconet.org
Tue Sep 3 18:34:10 EDT 2019
Paul,
I have a number of analyzers, the gold standard DG8SAQ, the FA-VA5, the
RigExpert AA-55 Zoom, the EU1KY FAA-450 and a coax bridge used with my HP 8405
Vector Voltmeter, but I do not have the CIA, so can't speak to it.
Having written about measuring window line a number of years ago when I had
access to an HP-8510, which didn't cover HF, I decided to remeasure some line
with HF equipment that I now own and with some hindsight and new computational
tools. Since we tend to quote things (in the colonies) in dB/100 feet, I
decided to actually measure 100' of line as I mentioned previously. As I don't
have 100+ feet of air-conditioned space I was doing this outside and the WX was
simply too toasty to do extensive tests at that time. (I intend to revisit this
when it cools off, November maybe.) Nevertheless, my preliminary measurements
showed that, even with the AA-55, completely battery-powered analyzer, some
common-mode choking stabilized the results.
I have no doubt that less reflective loads can be adequately measured without
common-mode suppression, but my task has to handle 100% reflections and do it
pretty accurately.
Wes N7WS
On 9/3/2019 1:30 PM, Paul Christensen wrote:
> Wes,
>
> "As has been suggested elsewhere in this thread, you might be tempted to try connecting the balanced line to the unbalanced test port of the analyzer, in the belief that since it is a "floating" device it's also "balanced". This has not worked for me."... "So the line radiates; it's part transmission line and part antenna."
>
> No argument there, but that doesn’t mean the CIA cannot report back a valid impedance measurement. I have yet to find an instance where it hasn’t worked with a complex Z on a balanced line. I routinely reverse the balanced termination on the unbalanced reflection port and achieve the same result. Moreover, I obtain the same (or nearly the same) result when using a 1:1 Mini-Circuits RF transformer, then OSL calibrate after the transformer. By contrast, I often cannot obtain identical results when changing line polarity when: (1) using a "bench" type VNA device where powering comes from electrical mains; and (2) a transformer isn’t used.
>
> Paul, W9AC
>
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