plusmax1 wrote

Does anyone know if I can replace my 3dBi antennas with, say 15dBi (or superior gain) antennas? Is there a gain limit to the new antennas?

Antennas are usually designed to match the legal regulatory requirements which are in effect in your country. They are usually carefully adjusted to the signal power of the WiFi chip used in this router. For SOHO devices the manufacturer often keeps a ~3dBm reserve below the maximum allowed signal strength, so it is normally no problem to exchange the antennas by more powerful ones as long as you don't exceed the legal limit.

By using antennas with 15dBi antenna gain you will probably exceed this legal limit if you don't adjust TX power of the WiFi chip also according to the rules set forth by the regulatory authority in your country.

As for the danger of antennas: what danger do you have in mind? If you are concerned about radiation, then no, there is not much danger to expect from WiFi equipment except if you plan to continuously stick the 15dBi antennas in your ears for a very long time. If you live near a radio station emitting 150,000 Watts signal power, then yes, there might be a potential long-term risk to your health.

But why would you like to replace antennas of a router at all? What do you expect from this replacement?

plusmax1 wrote

I expect to get more coverage area in my LAN by replacing the original 3 dBi antennas with 5dBi antennas. More antenna gain, more coverage area. Am I wrong?

No, you are right. More powerful antennas will give you better coverage and also better signal quality in the existing area, but only at a certain extend - for example, replacing 3dBi standard antennas by 5dBi ones shouldn't be a problem regarding maximum allowed EIRP. But a 15dBi antenna could be troublesome for your neighbors and also to you if authorities do prosecute violations in your country. In my country, authorities have the right to confiscate all WiFi equipment of people violating regulatory provisions such as the EIRP (beside severe penalties).

Also, the range of the area covered even by fat antennas attached to the router is always limited by the antennas of the client devices. For example, the "antenna" of an iPhone is the metal strap around the enclosure and this antenna doesn't get any better by attaching bigger antennas to the router. Most often people don't pay attention to this fact - the client devices also need to send a strong signal back to the router and this can only be improved by better receiving sensitivity of your router's receiver/antenna combination. That's why I usually recommend to stay with the antennas provided by the manufacturer. They adapt the correct antennas to the WiFi chip used in the router.

However, I heard that more than 5dBi antennas may damage my TP-Link TL WR-941ND router. Is it true? By the way, thanks for your explanation.

That's a fairy tale. If impedance of the antennas fit to the WiFi adapter, they cannot cause any harm. Your router probably could be damaged if you use it for longer without any antennas attached, but not with antennas which have correct impedance.