Topband: How to determine impedance
dm4im at t-online.de
Fri Jan 26 06:27:47 EST 2018
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
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.
Ohne CW ist es nur CB..
73, Martin DM4iM
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