How do I find the antenna gain (dBi) of a Lenovo X61t laptop?

Starting with a value that is 1dB greater than perfection is optimistic.

If an antenna of any kind is perfect and lossless then the average gain in all directions (and here think of a sphere rather than a circle because real objects exist in 3 dimensions) will be 0dBi. 0dBi means equal to or the same as an isotrope. After losses are taken into account, principally the tiny coax leads and absorption by the surrounding parts of the computer, your real world antenna may have an average gain of -6dBi (just my guess). Then if some direction from that antenna really does show that gain of 1dBi there must be other directions with gains correspondingly less than -6dBi.

Good luck.

Reality sucks when it gets in the way of progress.

Yep. Here's some typical PIFA antenna patterns:

They're anything but spherical.

(note the gain figures at the bottom of the page).

MIMO 2x2 antenna:

Click on "antenna pattern". Shows an average gain (whatever that means) of -4.5dBi.

Note that the gains specified do NOT include coax losses and losses due to crappy antenna location in the middle of the X61T laptop.

In all (i.e., both) my previous WiFi antenna experiences (setting up WISP reception for two different antennas and two different providers), I was used to antenna gains in the teens and twenties.

Since both you and Jeff (whom I admire and trust for reality figures) say I'd be lucky to get -6dBi out of that laptop antenna, that's the number I'll go with in my calculations.

I re-read Jeff's (prior) note and realize I have a lot more studying to do before posting more about the effective range at the slowest speeds (802.11b/g at

There is a lot of technical details I have missed.

That's why I just wanted to know the antenna gain in this thread.

While I'm shocked that it's -6dBi, I will certainly use that number in my calculations.

No wonder I can "see" a dozen or more access points with my 28dBm 630mW Bullet M2 with a 19dBi antenna while I can't see a single access point (other than my home broadband router) with my puny Lenovo X61t laptop

I'm surprised that I can't find the spec for the IBM FRU 93P4365 antenna on the net but, in lieu of a spec, -6dBi is what I'll use in my calculations!

I'm only interested in real-world speeds that I'm likely to get. In practice, that's the slowest spec (1Mbps to 6Mpbs 802.11b).

So, that's the spec I'm going to concentrate on.

Note that -6dBi [relative to isotropic radiator] != -6dBd [relative to a dipole].

Did you mean -9DBI?

No, but then -9dBi != -9dBd either.

However, a) dBi seems to be the standard for performance claims for antennae [so it's unlikely the OP was confusing the two], and b) the difference between dBd and dBi isn't as great as I'd expected anyway, so the point I was making is moot.

I didn't notice the bang before the equals. ;-)

Generally dipole versus the mythical perfect isotropic radiator is 3dB.

FCC rules use 0dBd = 2.15dbi.

The ITU uses dBd (gain referenced to a dipole) for field strength calcs, and dBi (gain referenced to an isotropic radiator), for most everything else. The FCC does much the same thing, adding dBd to exposure and SAR calcs. Some details (if anyone is interested):

(See first section).

I use dBi for everything, and pretend that dBd does not exist. I prefer the myth to the physical reality.

Thanks for the fair warning.

I looked it up just now.

Apparently: GdBi = GdBd + 2.15

Given that, am I right to asssume that -6dBi antenna would be -8.15dBd if it were listed relative to an ideal half-wave dipole (i.e., two perfect circles just touching by the edges at the antenna location) as opposed to an ideal isotropic radiator (i.e., one perfect circle with the antenna centered)?

I'm sorry if I wrote dBd instead of dBi by mistake.

For my WiFi-antenna gain calculations, I'm going to use dBi throughout as that appears to be microwave convention.

When I finally get to my TV-antenna gain calculations, I may be forced to use dBd simply because VHF/UHF manufacturers compare their antennas to a dipole, in practice (or so I've read).

Yup. I'm reading chapter 6 as we speak: wndw.net/pdf/wndw2-en/wndw2-ebook.pdf

Thanks. That 65MB handbook is downloading as I type!

BTW, this was interesting:

dBi always makes the numbers look better. ;-)

How fast can your car go? Well 200mph compared to a car doing 80mph in reverse.