Not just a quarter wave of coax in a mismatch. Any additional length of
coax or transmission line with SWR (mismatch) will produce modification of
impedance. Play with SimSmith using nothing other than additional series
lines in a mismatched condition. It's a good educational tool as well as a
design and evaluation tool.
Installation of ferrite beads and/or choke balun at the feedpoint of a
balanced load will affect the impedance as, without those, the outside of
the coax shield is part of the radiating structure.
Dave - W?LEV
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On Sat, Jul 24, 2021 at 8:04 AM Warren Allgyer <allgyer@...> wrote:
There is not much mystery here. Transmission lines that are mismatched at
all create impedance transformations at every odd multiple of a quarter
wavelength. They become transparent, meaning the impedance at the input is
exactly the impedance at the output, at even multiples of a half wave.
Calibration at the input allows you to see the same load your transmitter
sees. Calibration at the output will show the actual impedance of the
radiator.
Ferrite beads and baluns do not affect either measurement unless they
introduce loss or an impedance transformation. Baluns especially are
misused as matching devices because, when not properly deployed, they
introduce loss. Loss reduces reflected power by a 2X factor and lead to
grossly misleading VSWR readings at the input side of the coax.
The nanoVNA is a wonderful tool for both.
Warren Allgyer - WA8TOD
--
*Dave - W?LEV*
*Just Let Darwin Work*
