I have the Alfa 500mW USB Wireless B/G Adapter. It comes with a 2 dbi antenna.
I have purchased a 15 dBi Omni 2.4Ghz WiFi WLAN Omni Antenna.
This antenna, which includes an adapter cable, has not improved the reception of wireless networks whatsoever.
I have tested it with Windows Xp, and with Linux using Kismet and Airodump-ng.
Because the results are identical with the standard 2dbi antenna or the new 15dbi, does the alfa wireless usb adapter have a limiter or something.
I live in Metro Vancouver and with the increased range I should be picking up a lot more networks.
Yes I did test it on another computer with the same result. I don't think it's a faulty antenna, I think what is telling is the fact that using either antenna, the 2dbi or the 15dbi, I'm getting the same amount of AP's showing up. Around 20 or so.
I am somewhat of a noob, but I would like to ask, does the 15dbi antenna need a more powerful wireless adapter than the 500mw Alfa AWUS036H?
It's supposed to be a very good one, but the guy who sold me the antenna said he had never heard of it.
I'm a little foggy on the correlation between the power of the adapter and the antenna...
Or it could be that there aren't many more AP's around within your antenna's range... which is always a possibility.
No, the antenna should function fine on your adapter. Your really not going to find anything much more powerful than the adapter you already have.
The only way that you might find more AP's is if you get above all the obstacles in your area (homes, foliage, etc...), and scan for some. So ultimately in order for you to pick up more AP's....you'll have to get up high somewhere, roof for example. I can guarantee you'll get more up there.
The walls of your home are one of the signal killers. So no matter how powerful your antenna or device is, you'll still loose a lot of its power when it has to go through your home's walls.
Most adapters you'll see on store shelves, regular G cards at least, will have a transmit power of 70 or 80 mW, which is very weak compared to your adapter's. 500 mW is about as good as your going to get. Your antenna is a 15 dBi. I don't understand dBi as well as I should, but i believe its the amount of beamwidth the antenna has. The higher it is, the more narrower the beamwidth gets (so higher is not always better). For example, a 5 to 10 dBi omni would be the perfect antenna for a home or office looking to cover many computers. The downside to an omni is that it doesn't cover devices or computers directly below or above the antenna, due to its vertical polarization...meaning that devices directly above or below the antenna may get poor signal quality.
You may try a high gain directional antenna. Focusing all 500 mW in one direction may pick up quite a lot of AP's.
Hi Gordon_1, in the case of trying to pick up signals, I think the receive sensitivity of your device is more important than the transmit power. If the new 15dBi antenna hasn't improved your reception, then either you're too far away from an AP, or your device's receive sensitivity is not very good. You might like to relocate your antenna to where it is more within line-of-sight of your nearest AP, or try a directional (grid) antenna for this purpose. You might also like to check if there is a driver or firmware upgrade for your device itself.
Actually, no trying to be nosey, but is there a reason you're trying to pick up more AP's? You mention that you can detect 20 or so. I think the important thing is not how many you can detect, but how stable is your connection to 1? Are you connecting to a municipal network, a WISP, or another private AP? What is the throughput like? What sort of bandwidth do you require? Those are the things that are probably more important to most users than how many AP's they can detect.
I have the Alfa 500mW USB Wireless B/G Adapter. It comes with a 2 dbi antenna.
I have purchased a 15 dBi Omni 2.4Ghz WiFi WLAN Omni Antenna.
This antenna, which includes an adapter cable, has not improved the reception of wireless networks whatsoever.
I have tested it with Windows Xp, and with Linux using Kismet and Airodump-ng.
Because the results are identical with the standard 2dbi antenna or the new 15dbi, does the alfa wireless usb adapter have a limiter or something.
I live in Metro Vancouver and with the increased range I should be picking up a lot more networks.
Dude I had this same problem, but was using a Hawking HSB2 at the time which I promptly returned because of the super annoying high pitched noise that it produced. I now use the Alfa AWUS036H like you do too, and it is AWESOME! When I first tried it, I could not BELIEVE how powerful that little thing was. So I bought 2 more! But yes, I noticed the same thing as you, and it was extremely frustrating to me for a while, but I've learned to accept it and just be happy with my little rubber duck. I had purchased a 15dBi omni, and a 24 dBi parabolic dish only to find out that their signal reception was approximately the same as a little 2 dBi rubber duck! And believe me I tried aiming them every which way, hanging them out the window and everything! It's weird but true, go figure! Hey in the end, the rubber duck is small, and unobtrusive, so it's better for us anyways.
Added 15dbi Hi Gain antenna = no improvement 
But yes, I noticed the same thing as you, and it was extremely frustrating to me for a while, but I've learned to accept it and just be happy with my little rubber duck. I had purchased a 15dBi omni, and a 24 dBi parabolic dish only to find out that their signal reception was approximately the same as a little 2 dBi rubber duck! And believe me I tried aiming them every which way, hanging them out the window and everything! It's weird but true, go figure! Hey in the end, the rubber duck is small, and unobtrusive, so it's better for us anyways. 
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