Another line Z measurement technique is with a scope and reasonably fast
risetime (few nsec) pulse generator. Terminate about a 20' length of
line with a variable non-inductive pot or carbon comp resistor selection
and drive a pulse at the other end and adjust the resistance for minimum
reflection at the drive point looking at the scope. At ~1 nsec per foot
for the transmission line that should be about 40nsec from the pulse
output. On a few hundred nsec long pulse the reflection will be a shelf
higher or lower on the original pulse. With no shelf, the resistance
value is the line Z. Maybe not as accurate as a network analyzer
(didn't have one) but worked well enough in the digital data
transmission world for me.
Grant KZ1W
On 1/26/2018 3:27 AM, Martin wrote:
Am 26.01.2018 um 02:42 schrieb Richard (Rick) Karlquist:
You don't need to divide by length.
You are darn right. I just verified it.
I found Zo=sqrt(L/C) on a forum and thought this can't be right
because coax like rg58 has C about 100 pF PER METER. So i modified the
formula, verified it and got feasible results. My fault. At least it
does no harm.
With SI units, there is no coefficient of "1000".
You are right IF you insert L in Henry (not uH) and C in Farad (not
uF) in the formula. I should have highlighted this in my post.
This is fine for coax.
Not so good for balanced line. It is actually quite difficult
to measure the correct capacitance and inductance of balanced
line, because you would need a balanced capacitance meter and
a balanced inductance meter. Also, the values to measure
might be difficult ones to do accurately.
A better method is to connect the line to a network analyzer
via a balun and terminate it with a small pot, set to the
estimated Zo. Adjust the pot until the display on the s11 Smith
chart collapses to a dot. Then read out the pot resistance
with an ohmmeter = Zo.
This technique has around for decades.
Rick N6RK
Thanks for this. I actually failed when i tried to measure L and C of
a 220m long beverage. The result was way off of what you'd expect.
But then we have a AM station only 10km NW with the beverage pointing
exactly on it. This or the balance problem you describe above may have
caused the AADE L/C meter to display weird results. Fortunately the
local club owns a network analyzer. Will re-do it all . Now wind that
balun....
Aside of all this, i'm far from being an expert. My life outside of
ham radio was repairing cars, running a bakery, catering and recently
being a homemaker -minus giving birth ;-)
No mathematics, no electronics, no RF.
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