impedance
I've just calculated trace parameters so that it would have 50 Ohm impedance on PCB.
Then I've noticed, that its impedance does not change when I make line longer or shorter as long as I leave trace width & distance to ground plane intact.
Is that correct and why is that?
Let's look at the formula and equivalent circuit for a transmission line.
(1) Impedance rather than reactance.
Reactance refers to the opposition to the change in current (of an inductor) or voltage (for a capacitor) - single components. The transmission line has \$R,L\$ and \$C\$ components - impedance is the ratio of voltage phasor to current phasor.
(2) It is \$50\Omega\$ because the ratio of inductance to capacitance per unit length produces that value. As \$R << j\omega L\$ and \$G \to 0\$, these values can be ignored and so the expression reduces to \$\sqrt{L/C}\$ (frequency independent).
(3) Nope, but it's generally a good idea to keep things as standard as possible. You may find it difficult to find a suitable connector for your \$167\Omega\$ transmission line. There's also a lot of information available for designing standard transmission lines on PCBs, etc. The magic number in my book is 376.73031... the impedance of free space. Now without that one we'd live in a different universe.
(4) Going back to the formula. At low frequencies \$R\$ may be significant as the reactance of the inductor will be small). At very high frequencies the dielectric losses may become significant.
When you send a signal along a trace, if the length of the trace is of the same order as the wavelength of the signal, then you have to match impedances or, you will get a "reflection". What this could mean in extremes is that the power actually emitted by the antenna is only a fraction of the power being pumped out by the chip - in other words, you are not efficiently coupling the chip to the antenna and you get "reflections" - this can cause your chip to overheat and maybe it might damage it.
So, if you are transmitting 3 GHz (say), it has a wavelength of 100mm and, a rule of thumb is that if your PCB track is longer than 10mm (one tenth the wavelength), you need to ensure its characteristic impedance matches the antenna's impedance.
Best Answer
That is correct. The 50 ohm impedance refers to the "characteristic impedance" of the "transmission line". This comes from electromagnetic theory and is usually applicable to RF and high frequency applications. At DC your trace will still be very low impedance (resistance). If you were to take an ohmmeter to it you would probably measure maybe 1 ohm or 0.5 ohm, and that is only because the probe resistances would dominate over the actual trace resistance (which should be in maybe the tens of milliohms range).
Characteristic impedance has (mostly) to do with the capacitance and inductance per unit length of the transmission line. The capacitance and inductance are not that important at low frequencies, but as your signal frequency increases they will cause effects that can no longer be ignored. This is why you often will see coaxial cables advertised as 50 ohm or 75 ohm. This refers to the characteristic impedance (applicable to higher frequencies, like upper MHz range and GHz), not the DC resistance.
Since the characteristic impedance depends on C and L per unit length, as long as you do not change the distance between trace and ground plane (affects capacitance) nor do you change the trace width (affects inductance) your characteristic impedance should not change, no matter how long the trace is (Note: this is a simplified explanation, but captures the basic idea).
Note that often the word "characteristic" is dropped from the term, and it's just called "impedance". It sounds like this is the case in your layout program.