Wireless G antenna to R wireless router?

[McMafia]

I have two spare D-link ANT24-0700 7dbi antenna, and noticed the antenna on TPlink W8960N is 3dbi, so i swap it, surprisingly signal strength dropped
(just by looking at how many wireless signal bars)

But is the antenna G or N specific? or why does it drop in signal strength?

 

[stevech]

Antennas are frequency specific (say, 2.4GHz vs. other).
802.11g/n both use the 2.4GHz band. The g and n designate the form of the transmitted signal at that frequency.
There are 5.8GHz variants of 802.11n. But you'd know that.

As to why you see a drop - first: make sure you're seeing a consistent (statistically/average-wise) drop. Moving things inches can make big differences. Secondly, make sure of the obvious, e.g., the antenna connectors' gender match - WiFi products have these odd reversed polarity (trans-gender if you will) connectors in the US by FCC mandate (that's a long story). Some WiFi products have three antennas - two external and one internal. The device may arbitrarily choose the inferior internal antenna if the signal strength is "good enough". It's possible that the 7dBi antennas are just not 7dBi - there are liars in the WiFi market. Inside, these things are just rubber and wire - with a giant price markup over cost. And they're fragile internally.

The number of "bars" displayed is very course. Improving antenna gain from 3dBi to 5 or 6dBi will be hard to see in the "bars". The increase of 3dBi or so is tiny compared to the attenuation due to walls/floors and distance - these are many tens of dB. At close range, the 3dB improvement is even less visible.

Some routers have a web page display of the received signal strength FROM the clients. Some clients have the same for the FROM router/AP direction. This should be in dBm, not percent. With these, you'll see that the signal FROM the client TO the router/AP is much weaker - esp. for battery powered handhelds and some laptops. In this situation, the bi-directional link is "unbalanced" and that's bad for WiFi - because by nature it tries to run the same data rate in both directions for each data frame. So the from-client signal often is the constraint. Too often, people think a booming signal FROM the router/AP is a panacea.

A little Antenna Primer...
dB is Decibels - a log scale.
dBi is antenna gain relative to an ideal spherical (isotropic) radiator - like the sun is spherical, and a light bulb is less so. So a 3dBi antenna has 3dB of gain in relative terms. The isotropic spherical radiator is said to have no gain - it's the reference. Every antenna rated as some dBi gain has some directionality, i.e., it is not a spherical pattern. (another reference system is dBd, gain relative to a dipole, not spherical antenna. But dBi is used mostly in WiFi).

The rod antennas we see on WiFi routers and walkie-talkies is a form of a dipole antenna and these often have 2-3dBi of gain - because the pattern becomes like a dimpled sphere, where radiation is diminished off the tip of the rod, and off the other end of the rod (180deg.). The more gain a rod-like antenna has, the more the pattern becomes doughnut-shaped. The patch (panel) antennas have a pattern that's flashlight-beam-like - cone shaped. Parabolic dish antennas are even more so- with beam width down to 5 degrees or so, giving the high gain but with pointing accuracy being critical.

So it's like a focused light beam - brighter this way, dimmer that way.