Roy Lewallen wrote:
Wes and Ian can probably point you to more possible causes for this
measurement result -- they're the real experts on VNA measurements.
Not I... I'm just trying to think acurately about it...
I'd be very surprised if the VF really varies with frequency -- in
theory, it can't, in an ideal coaxial cable.
Greg had written:
adjusted
the VF on the distance to fault tool until I obtained the
same physical length. The far end of the cable was
terminated with an open circuit.
I happened to start with the measurement frequency set
to 1 MHz. Lo and behold, the VF needed to compute the
same physical length was 72%, as you suggested.
My own understanding is that VF should be constant with
respect to frequency, so I decided to vary the test frequency.
I should have left well enough alone.
I picked 1, 2, 4, 8, 16, and 32 MHz. For those frequencies,
I measured the following VFs:
1 MHz = 72%
2 MHz = 73%
4 MHz = 75%
8 MHz = 80%
16 MHz = 79%
32 MHz = 79%
I'm rounding the VF to integer values, since I don't think that any
more accuracy
can be claimed in this setup.
Since that result was a little surprising, I grabbed some mini 8
(8X)
that was nearby, about 51.25 feet. The published VF is 78%, and
I measured the following, at the same test frequencies:
1 MHz = 78%
2 MHz = 78%
4 MHz = 79%
8 MHz = 79%
16 MHz = 80%
32 MHz = 80%
With this cable, the VF appeared much more constant across the
1 to 32 MHz range.
Is there an explanation that fits with my measurements?
Since Greg's test setup measured a much more constant VF for the mini-8,
it doesn't seem that the open-circuit termination is causing much error
(note also that the largest deviation for the BF is at the lowest
frequency, where fringing C would have the least effect).
This seems to bring us back to Greg's method of computing "distance to
fault" from measurements of R and X at one end. Does that method involve
any assumptions about idealized cable behavior that could create an
*apparent* change in the computed VF?
--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek