Cecil Moore wrote:
Ian White G/GM3SEK wrote:
All agreed. Along with the math that Cecil has retrieved and quoted
again, everything points towards the distance in question being a
function of coax diameter only; and not wavelength.

Please forgive my previous senior moment.
It was ~2% of a wavelength at 10 MHz for RG-213.
It appears that one foot of coax on each side of
a Bird wattmeter is enough to establish Z0 at
50 ohms which forces Vfor/Ifor=Vref/Iref=50,
the necessary Bird boundary conditions.

The Bird doesn't require any upstream and downstream boundary
conditions. You can insert the instrument between any source impedance
and any load impedance, and what it reports is entirely about the load
impedance, unaffected by the source impedance.

However, it was scaled and calibrated assuming a 50 ohm system reference
impedance, so in order to read correctly, it requires you to agree that
your system reference impedance is 50 ohms too.

The element is trying to sample the voltage and current at a single
point along the instrument's internal line. Because that line is
physically quite long, it is built as an accurate 50-ohm line so that
the instrument will cause minimal disturbance when inserted somewhere
along a 50-ohm cable.


--
73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek