Topband: How to determine impedance

[Grant Saviers]

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.
>