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Hello, finally i bought a skystar2 card, but i have a problem. On the old card the level and the quality was between 80% and 95% on hotbird, but when i replace the old one with Skystar2 the level and quality decrease to 30%. Is there any requirment on LNB type for this card ? i have a KU LNB.

No, normally all standard LNBs should work with all tuners.

Basically, the LNBs have far enough gain, to keep far away from tuner sensitivity limits and also tolerate some cable loss.

But if losing very much signal on very long or low quality LNB cables, some differences may occur. In most cases, the output stages' tolerances / gain variations of the LNBs do have much more effect then, than tuner inputs would.
Nevertheless, there are several different construction principles of tuner packs on the market, mainly to be sorted into discrete and monolithic concepts. Monolithic / fully integrated chip tuners in many cases have less sensitivity and some worse selectivity, compared with the old style discrete units, that usually come in separate tin boxes.
If in doubt, check the data sheets / technical specifications, like for noise factor or treshold level.

However, with a properly aligned dish of sufficient size, and with low loss cable, quality adequate to it's length, all this should not matter at all.
I'm using cable runs of > 100ft / 30m here, with a specific loss of < 25 dB / 100m at 2 GHz, no noticeable differences found between several tuners.

Level readings normally are NOT calibrated, so they simply can't be compared directly. Even a driver may change the scaling without a difference in receiving quality.
Most important is to get good quality readings (if available), as that reading should refer to bit error rate, more or less.
The closer to the lower limits you are, the more side effects do have a real influence on your results. Like, with a bigger dish (with so0me more gain), cable quality or length would matter far less.
With a high grade LNB, you may get some fractions of a dB of better S/N ratio, but a bigger dish can give several extra dBs.
Cable loss normally is less important, if not eating up all the the gain of the LNB output driver. Even 20 dB or more of cable loss won't matter there, normally. But once you are getting far to much cable loss, you may indeed lose transponders or find different results on whatever component you may change.

Perfect aligning is a must in any case. With a Chinese wok, tied to a broom-stick and tangling in the wind, even the best tuner, LNB and cable won't help a lot...

One more for the road...
L and Q readings - and even resulting real quality - may also depend on proper LNB tilt angle, as this effects polarisation decoupling, for the bird you're on and against cross talk from neighbour positions as well, as in Europe those would typically use quite similar transponder lists but inverted polarisation.
So it's not always the true gain and level but also cross talk of any kind, that may lead to (partly) bad results.