I am talking about modern laptops.
When I am replacing the PCIe wireless card do I have to check that the antenna(s) match(es) the new card? I mean properties like the length and the gain (dB value).
If yes which values do I need to compare and where do I get the values from?
I don't want to replace a 1x1 card with a 2x2 card and only connect one antenna as asked here.
Instead I want to know if the length of the existing antenna(s) (and dB gain value) have to match the new card.
Yes, for best results, you need your antennas to be good matches for the radios you're connecting them to, but not on the factors you asked about.
The main factor to watch out for is what frequency bands the antennas were optimized for. If your card has a tri-band radio (2.4, 5, and 6GHz) behind a given antenna connector, you need to connect a tri-band antenna to that connector. If your factory-installed card didn't have tri-band radios on every antenna, then you might not have all tri-band antennas. Unfortunately there's no easy way for a consumer to know whether or not a given built-in laptop antenna has good performance characteristics in all three bands.
You need as many antennas as your card has connectors. If you leave one of your card's radios without an antenna, you'll be cutting your Wi-Fi bandwidth in half at the very least, or possibly fouling up Wi-Fi all together, or leaving the Bluetooth radio without an antenna, thus breaking Bluetooth.
Cable length (length of the antenna leads) tends to not be a problem. Connectors have become standardized on Hirose u.fl connectors. Impedance is always 50Ω. Directional gain (dBi) doesn't matter; they're all going to be in the low single digits, and that's appropriate because you never know which direction the nearest AP is going to be.
If you were asking about the length of the antenna elements themselves and not the antenna leads (cables), that's related to what frequency bands the antenna is optimized for. For example, it's good for a dipole antenna to be exactly one wavelength (or exactly one-half or one-quarter of a wavelength) for the frequency in question. So dipoles for 2.4GHz generally need to be longer than dipoles for 5 or 6GHz, because lower frequencies have longer wavelengths. Quarter-wavelength dipoles are very common in the Wi-Fi world. But dipoles are not the only antenna designs out there, and different antenna designs have different ways to be optimized for different frequencies (although as a general rule of thumb, lower frequencies require larger antennas).
Most antennas are designed with the same impedance, about 50 Ω. In a laptop, the prime requirement is that it fits the space available, without being blocked (e.g, by a large metal part.)
If the current antenna's connector fits the connector on the card, odds are it would be fine. However, if the signal is poor, you might be better served getting an inexpensive USB Wi-Fi adapter with external antenna.
