I'd like to display the attenuation versus the frequency of a coax cable connected to port 1 of a network analyzer. The coax cable has a shortede end (an open end is also possible but not recommended). The measured reflection value in "dBmag" is twice the cable attenuation. I need to divide the measured trace by two to directly read out the cable's attenuation. However, the trace is available in dB. What equation do I use to divide the shown trace by two.
Example
The wave travels once through the cable and is attenuated by 5dB. The wave is reflected at the shorted end of the cable and travels back to the analyzer. Now the wave is attenuated by an additional 5dB. The instrument shows 10dB attenuation. The cable, however, has 5dB attenuation. We want the instrument to show 5dB.
Instruction to define the User Mathematic
This is the step by step instruction to divide the trace by two. Press the following buttons:
PRESET- key
S11- softkey
TRACE FUNCTION- key
User Def Math- softkey
Enter the equation as shown in the picture: Trc1 (0.5) *
OK- softkey
Math = User Def- softkey
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* The way to get the equation in topic 5
We need half of the displayed value.
a = 20 log(U) (general equation to calculate the attenuation in dB [volts])
a = 10 log(P) (general equation to calculate the attenuation in dB [power])
The display trace for Trc1 is calculated as:
dBmag(Trc1) = 20 * log(Trc1)
To halve the display value, we have to multiply by 0.5:
0.5 * dBmag(Trc1) = 0.5 * 20 * log(Trc1)
As a general rule we have: r * log(x) = log(xr)
Applying the general rule, we get:
0.5 * dBmag(Trc1) = 20 * log(Trc1 0.5)
The term "20 * log" is done inside the analyzer. So we only have to write the following term in the "User Def Math for Trc1":
(Trc10.5)
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Hint:
The attenuation can also be measured using the "Auto Length and Loss" function located under the "OFFSET" hardkey.
