electromagneticelectromagnetismmicrowaveRFwave
I have done a measurement with a vector analyzer in which I have connected a certain coaxial cable to the two port of the instrument.
Precisely, I have got a measure of the phase of S21 of this type:
You may see that the phase of S21 is a periodic signal with respect to frequency. Why?
I know that in a transmission line S21 = exp(-jβl), and so there is a linear dependence of it with respect to the frequency, not a periodic one,
A Smith Chart is not so much an attenuator design aid
as a means of evaluating and adjusting a design.
So - see attenuator articles below and then Smith chart articles.
Carbon resistors may be carbon film or carbon composition?
Carbon film are not suitable for UHF work as they are formed by cutting a spiral track in a carbon film cylinder, so have very substantial inductance.
Carbon composition have a solid carbon body and may be suitable for UHF work depending on other factors.
UHF Attenuators:
Attenuator design tutorial - looks good.
Interest - Commercial products
What is a Smith Chart?
Wikipedia gives a better than average concise summary:
From here
Somewhat gentle introduction - 27 page powerpoint intro - still gets deep quite quickly BUT a Smith Chart can be very very useful with almost no maths or numerics involved.
Superb Smith Chart resource - essentially an index of indexes - breaks subject up into sections and provides many references for each.
Another good list of references
Smith Chart tutorial from Maxim - reasonable "dense" but looks understandable.
You'll understand this once you've read it :-)
When you're using a network analyser to measure S21, it generates a signal on port 1, and receives a signal on port 2. S21 is a measurement of the ratio of those signals.
While a network analyser does much more than a signal source and power meter can (S21 is the complex ratio), for this cable loss application, if you display simply the magnitude of S21, you can regard it as a signal source and power meter in a single box.
Best Answer
What you're seeing is called "wrapped phase". The phase angle wraps around a circle, like a clock, and the plot only shows the angle as a value between -180 and 180 degrees. This makes the graph much more compact, especially if many multiples of 360 degrees would be shown. It could have chosen to show the angles between 0 and 360 degrees instead, or any other pair of numbers 360 degrees apart.
You can manipulate the data to unwrap the phase and see a linear plot in your graph by tracking when it crosses the -180 to 180 threshold and adding or subtracting 360 degrees as necessary.
But be careful with unwrapping, since all phase measurements are relative to some reference point. As an example, your chart shows 1GHz at about 125 degrees, and at approximately 1.4GHz it's about 125 - 360 = -235 degrees. But that's just relative to the first measurement of your graph. In this case, since you know you measured a transmission line, you can use the slope of the phase change to extrapolate down to DC and unwrap from there to get absolute phase angles for your plot. But in general, you should avoid extrapolating unless you have a model/equation for how the phase will behave outside of the graph.
See this page for more information: https://en.wikipedia.org/wiki/Instantaneous_phase_and_frequency