In calibration for measurement and in measurement at RF frequencies, high-Z
is far more difficult than a short or proper 50-ohm termination. Take 144
MHz and assume you are attempting to measure 1 kohm. There will always be
present roughly 0.5 to 1 pf in the measurement. At 144 MHz, 0.5 pF =
-j2210 ohms. This is over twice the real part even at 0.5 pF - a very
small capacitor.
True, the output impedance (if you will, resistance plus a small amount of
several pF of capacitance) of an HF tube amplifier is very high, between
1000 and 4000 ohms (+/-). The pi-L network between the tube plates and the
output to the antenna feedline matches that high-Z (and renders pure
resistance) to 50-ohms. That impedance is not measured with a VNA but
rather but by using standard Ohm's Law based calculations specifically
tailored for determining the values of the matching network to enable
maximum power transfer.
450 ohm ladder line is a real differential transmission line as is 300-ohm
TV twin lead as are long parallel close-spaced ribbons of copper strip as
are parallel copper traces on opposing layers of a PC board. I use
parallel open wire transmission line between my home brew L-network
matching network (a.k.a., "antenna tuner" - it does NOT 'tune' the
antenna!!) and the feed point of my 450-foot long doublet. Works great as
a relatively lossless transmission line and withstands high SWR with
insignificant additional loss.
Dave - W?LEV
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On Mon, Feb 17, 2020 at 3:47 PM Nels Nelsen <nels.nelsen@...> wrote:
My tube amplifier takes exception to your statement:
"High Z is hard to obtain at RF. "
The amp wants to know who do you think has been talking on 40 meters for
the last 30 years?
And the 450 ohm ladder line does not agree with your concepts of
transmission line either.
But I told them that I would talk to you on their behalf. So please be more
open to all the work they do, OK ? :)
All done now, I hope they settle back down. (-:
Nels
NE7LS
On Mon, Feb 17, 2020, 6:52 AM W1RS <Deflatermaus@...> wrote:
High Z is hard to obtain at RF. For parallel twin lead with #16 wire,
the
spacing is 0.3 inches for 300 Ohms. The spacing for 1000 Ohms is 105
inches!
--
*Dave - W?LEV*
*Just Let Darwin Work*
*Just Think*
