transmission line simulation

I did some simulations of the ideal transmission line in LT spice using LC chain with 2.5nH and 1pF which results in 50Ω transmission line.
The transient analysis gives result that i expected - no reflections if chain is properly matched with 50Ω.
But in AC analysis the amplitude greatly decrease at 6GHz, which is logical because i used chain of LC low-pass filters.

But what should I add to this model to be frequency independent as the ideal transmission line should be?

(Hmm - shooting from the hip here... :)

My first thought is that the LC "ladder" transmission line 'unit cell' approximation describes an incremental section of line, spanning distance dx (where the line extends in the x direction). If higher frequencies are to be accurately described, then the physical size of the unit cell must be decreased (as the wavelength contracts).

In short, this means your discretised model must be adjusted to consist of more cascaded LC unit cells, where the L and C values are reduced such that the "capacitance per unit length" and "inductance per unit length" remains constant. I suspect this would result in your simulations showing the same effects you're seeing now, just at a higher frequency, which would be remedied by (again) increasing the number of unit cells, resulting in.... etc etc etc. ...until you arrive at the assumption underpinning the traditional calculus analysis where dx -> 0 (which is a *lot* of discretely drawn unit cells ;)

Ignorant question, why are you using the lumped approximation instead of an ideal tline component? It's particularly saving a lot of processor time in AC and should have no disadvantage in transient analysis.

As you already guessed, to get infinite frequency range, you need infinite number of elements...

Purpose is not the simulation itself, but it is to understand the transmission line better.

You could use for example qucs http://qucs.sourceforge.net/. Is a free simulator but very powerful and easy to use for both low and high frequency circuits. There you can find a lot of models for transmission lines like RLCG where you can actually put the parameters of each lumped component in the transmission line.