Alfa Antenna Testing

Last year I purchased an Alfa 802.11G WIFI adapter to use in Baja where we were staying. There was a store about a 350 ft away where you could sign up for an account. It was a concrete building with the anenna sitting in about the middle. I used a can antenna on a pole outside my motorhome. Here is a picture of our camp at sunrise and you can see the pole sticking up from the motorhome with the can antenna mounted at the top.

van at sunrise

Several other people had purchased after market antenna and they were having issues so I decided to do some testing. I found a 12dbi antenna on eaby with the following specifications.

Description
12dbi Omni Directional Antenna
Max gained 12dbi, provide more signals
SMA male connector 2.4GHz 12dBi Wireless WLAN Antenna Aerial

Frequency: 2.4G VSWR: <1.5:1
Gain: 12dbi
Connector :RPSMA female connector
Impedance: 50 ohm
Power: 50W
Ambient temperature: -40~80 degrees
Color: black
Weight: 70g.

I decided to purchase one and give it a try. Here is a picture of the original antenna that comes with the Alfa along side the 12dbi antenna that I purchased.

Two antenna

I do all my testing with Netstumbler which works just fine on the Alfa. I setup the Alpha in a location where I can view a wifi signal at about 350 feet away and then start swapping antenna. You can stop Netstumbler while you change the antenna and it creates a small black space in the picture.

Netstumbler signal strength

You will notice that the 12dbi antenna did not do as well as either the can antenna or the original antenna. This suprises me a lot. It was a cheap antenna purchased off of Ebay but still I would think it would do better than the stock antenna. Here is a picture of the Alfa with the stock antenna.

Alfa with original antenna

Here is a picture with the 12dbi antenna installed

Alfa with 12bi antenna

Here is a picture of the Alfa connected to the can antenna.

Box

Just in case you think there might have been something strange in the measurements, I installed all three antenna again in the same order and took another screen shot. You can see that the signal strength came out almost exactly the same. The can antenna is far more directional than the other two so it is suprising that I got it pointing in the same direction.

Do the measurments again

You ask, how is this possible. Here are a couple of ideas.
1. The output on the Alfa might not be 50 ohms.
2. There might be an issue with the standing wave ratio.
3. Maybe this cheap antenna is not 12dbi at all.

I believe this answers at least partially why some of the people were having issues with this type of antenna last year.

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