# Measuring a transmission line

#### Gibson486

Joined Jul 20, 2012
317
How does one make a measurement on a transmission line? If I use a scope, the impedance of the scope would change the load, correct? Is there a method that is used to ensure that you do not change the load?

#### crutschow

Joined Mar 14, 2008
23,725
There likely is.
What measurements are you trying to make?
What's the maximum frequency?

#### Papabravo

Joined Feb 24, 2006
12,684
What kind of transmission line: twisted pair, coax, hardline?

#### Gibson486

Joined Jul 20, 2012
317
I am trying to measure the voltage at the output. The signal is a 230kHz sine wave. The cable is a dual conductor eco cable by Alphawire. The cable is approx 3 feet long.

#### Papabravo

Joined Feb 24, 2006
12,684
I am trying to measure the voltage at the output. The signal is a 230kHz sine wave. The cable is a dual conductor eco cable by Alphawire. The cable is approx 3 feet long.
At that frequency cable length there won't be much in the way of attenuation. Are the "dual conductors" twisted or are they just side by side like speaker wire? The total delay for a round trip is likely to be less than 10 nsec. so the reflection situation will be hard to see on a scope. How were you planning to measure the voltage?

#### Gibson486

Joined Jul 20, 2012
317
It is twisted. I tried to check for a reflection situation, but yeah, you cannot see it. I took the simplest approach at first...put a scope probe at the output and measure away. The measurement I took had a drop, but I am guessing that it is due to the out impedance of the frequency generator in the input impedance of the scope and probes causing a divider?

#### Papabravo

Joined Feb 24, 2006
12,684
If I had to swag an impedance for twisted pair I'd choose 120 Ohms. Terminate it with a resistor between 60 and 120 Ohms on one end only and see what you get. The value is not that critical because the line is so short and you are simulating the characteristic impedance at both ends. Now take your measurement again.

#### EM Fields

Joined Jun 8, 2016
583
It is twisted. I tried to check for a reflection situation, but yeah, you cannot see it. I took the simplest approach at first...put a scope probe at the output and measure away. The measurement I took had a drop, but I am guessing that it is due to the out impedance of the frequency generator in the input impedance of the scope and probes causing a divider?
1. Alpha part number?
2. Signal generator output Z?
3. 'Scope probe input Z?

#### DickCappels

Joined Aug 21, 2008
6,003
For all practical purposes 3 feet is not a transmission line for a 230 kHz sine wave .

#### Gibson486

Joined Jul 20, 2012
317
1. Alpha part number?
2. Signal generator output Z?
3. 'Scope probe input Z?
1. 78342
2. 50 Ohms
3. 12 pF and 1Mohm

#### Gibson486

Joined Jul 20, 2012
317
For all practical purposes 3 feet is not a transmission line for a 230 kHz sine wave .
true...I think at this point, the capacitance of the cable probably has the biggest impact....

#### crutschow

Joined Mar 14, 2008
23,725
3. 12 pF and 1Mohm
That's too low a resistance for a 10:1 probe and too low a capacitance for a 1:1 probe.