Let's go back to basics and use only the basic behavior of an open and
shorted 1/4-wavelength of coax (in the cable, not physical length:
SHORTED END measures as an open
OPEN END measures as a short
With the end of your coax open, determine the frequency of the lowest short
indication. Repeat with the end of your coax shorted and determine the
lowest frequency of an open indication. The two should be very close but
take the numerical average of the two. If there is no short or open
indicated for the above tests, the coax is faulty. If there is a distinct
short/open indication, calculate the length of the coax using:
Wavelength X Frequency = speed of light (all in basic units)
This will be slightly less than the physical length as the measurements
include the velocity of propagation in the coax.
Dave - W0LEV
On Mon, Jun 19, 2023 at 2:15?PM RJOLLIS via groups.io <rjollis=
[email protected]> wrote:
Thanks for your suggestion, Stan. I will attempt to reset the sweep range
to a lower stop frequency, but it won¡¯t happen until tomorrow.
I did make return loss and characteristic impedance measurements and they
look nominal for this coax. Zo=48.2 ohms tested at 9.8Mhz +/- 90¡ã and the
return loss at 10Mhz is .99dB and 1.3dB at 30Mhz. Both of these figures
seem reasonable for a 100ft chunk of RG-8X. But that big spike at 14.65m
has me scratching my head. There are only minimal artifacts apparent on the
display around 120nS which you can see in my original post. So maybe there
is something hiding between sampling points.
By the way, I really love this new toy. I am trying to consume as much as
I can from the various postings, discussions and videos to be able to use
it to its fullest. What a fantastic invention! The guys who developed this
tool should get an award for one of the most useful inventions for Radio
Communications since the oscilloscope.
On Jun 19, 2023, at 1:32 PM, Stan Dye <standye@...> wrote:
?Also, for a cable that long using the 101pt scan, you need to set your
stop
frequency to about 70MHz to see the reflection from the whole length of
the
cable. So what you are seeing now may be due to aliasing or something.
(I just experimented with the Transform settings with a 100ft length of
good cable on my -H4. When I had it at 400pt scan, 125MHz let me see
much
longer than 100ft, but at 100pts I needed to set the frequency down to
see
the reflection from the end of the cable. This is due to the "bin size"
used in the FFT.)
On Mon, Jun 19, 2023, 10:13 AM Roger Need via groups.io <sailtamarack=
[email protected]> wrote:
On Mon, Jun 19, 2023 at 08:23 AM, RJOLLIS wrote:
Procedure. I have set up the VNA on Trace 0 in the Linear mode. VF is
set to
a nominal 80% for the 100 ft (30.5m) RG-8X coil of coax. The Transform
function is selected and Low Pass mode is on. I sweep from 500Khz to
125Mhz. I
get a spike at 14.79m. When I swap the inputs, the number doesn¡¯t
change. I
would expect that if there was a short in the coax, I would observe
15.21m
when I flip the ends.
Question. Am I doing something wrong here or do I have a defective
nano?
I
haven¡¯t yet characterized impedance or line loss. Those come next.
Looking at your photo your yellow trace shows S21 linear instead of S11
Linear.
I don't know what revision of firmware you have but recent firmware has
a
Cable Measure function built in. Have you tried that?
Roger
--
*Dave - W?LEV*
--
Dave - W?LEV
