I edited this for a long time -- thanks for your patience.....
The people at 802-11.com saw my India-trip write up, liked it, and sent me two antennas to perform this test. Okay, it was my idea to test the 12db antenna and they offered me the other one too. Many thanks to them for making this test possible. I'm not comparing their antenna to anybody else's antenna, only their 12db antenna against their 9db antenna. I actually chose them for their 12db antenna (and the descriptiveness of their web site) about a year ago and was just about to buy it (literally) so this was a great boost.
I have very little connection with air802.com, having sent them about 3 emails, and recieved about 3. May last contact was over 8 months ago. I don't know anyone there, have never talked to or met any of them.
Here is the 12db antenna I used
Marine or RV WiFi Omni Directional Antenna, 1"-14 Thread Mount, 12 dBi Gain - Air802 LLC
Here is the 9db antenna I used
Marine and RV WiFi 2.4 GHz Omni Directional Wireless Antenna, 1"-14 Thread Mount, 9 dBi Gain - Air802 LLC
Some background on antennas and a thing called beamwidth
On a sailboat at anchor, it is very desirable to have an omnidirectional antenna. Omnidirectional antennas are the standard antennas that have a single wire or post that goes straight up. They exibit the same signal strength in all directions around the antenna. I you threaded a donut onto the antenna, the direction of the donut outward is the direction of the signal strength -- al lthe way around. Very little goes up, mostly all goes horizontal. The higher the gain, the flatter the direction of strength. Picture threading a pancake onto the antenna instead. Lots of strength straight out from the antenna and very little strenght even slightly above horizontal.
There are higher gain antennas that have more focused coverage over a specific direction -- like threading half a donut onto the antenna -- all the way up to parabolic ones that look like satellite dishes -- more like threading a carrot onto the *side* of the antenna. These require constant aiming toward shore and that's not what I wanted. Besides, I may put a webcam on the boat for when I'm not there, and that certainly requires continual, un-manned, 360-degree coverage for continual connectivity as the boat swings at anchor.
Other threads have wondered whether it makes sense to get a super high-gain antenna with a narrow vertical beamwidth. A 12db antenna has 7.5 degree beanwidth - that's a pancake. Or whether it makes sense to get a lower gain antenna to account for the boat rocking. A 9db has 12 degree beamwidth -- that's a thicker pancake. Really testing the limitations of beamwidth would require a little more testing than I've done here. You would need to take into account the internet's protocals for retransmitting dropped packets, in the event that your boat pitches up and down enough to temporarily thwart the connection to shore.
With our 50' boat it takes a bit of wind to cause the waves to increase enough to pitch the boat up and down 7.5 degrees (Just having the bow go up by 3.75 degree would start to reduce the signal strength). Instead we usually find that the waves roll by without affecting us too much. So I'm posting here to establish the baseline with some hard data. What do you get for that extra 3db of gain?
The Test Set-up
My test setup was pretty simple. It consisted of a halyard that pulled the bottom of each antenna up to a predetermined height above the deck. The antennas were directly connected to my Bullet2HP and then to 100 feet of CAT5 cable running down and into my laptop in the salon. The power for the Bullet2HP is carried on 4 otherwise-unused wires within the CAT5 cable. I hand-wired the correct wires to +12 and ground, but you'd probably be better off using a "POE injector" instead. This was, after all, my second Bullet2HP, having fried the other one somehow, perhaps by connnecting it backwards with all the testing and real connecting that I was doing with it.
Here are some shots of the test set-up. First, a picture of the Bullet2HP. This is the 1000 milliwatt version. (It has 1 watt of transmit power.) If you are going to go through the effort to put long range WiFi on your boat, it's worth the extra money to get the highest power one you can find.
Here's a shot of an antenna raised to a predetermined height above the deck.
Here's a shot of the 9db antenna about to go up.
Here's a shot of the 12db antenna about to go up.
Three measurements were taken by capturing screen shots from the software that came with the Bullet2HP: 1) The signal strength to my favorite access point 3/4 miles away, 2) the number of access points that I could see, and 3) the number of access points that I could reach with enough signal strength to reasonably expect to connect. I use -75db of signal strength as my definition of strong-enough-to-connect. If you want to choose another signal strength, the data will be seen below and you can see your expected results with that number.
The data with the 9db antenna.
With the 9db antenna aloft, the Bullet2hp software provided -42db
The number of access points that I could see was 57
The number of access points that I consider reachable (with a strength of -75db or higher, for instance -74 or -73 or -72) was 15
Here are 2 (overlapping) pages of screenshots showing the data,
The data with the 12db antenna.
With the 12db antenna aloft, the Bullet2hp software provided -38db
The number of access points that I could see was 34
The number of access points that I consider reachable (with a strength of -75db or higher, for instance -74 or -73 or -72) was 18
Here are 2 (overlapping) pages of screenshots showing the data.
.............................9db antenna......12db antenna
Signal Strength...............-42....................-38...... The 12db antenna is better. -38 is better than -42
Access Points Seen...........57.....................34....... The 9db antenna is better, not sure why I got this result.
Access Points Reachable....15.....................18....... The 12db antenna is better
Note that the 12db antenna resulted in a 4db stronger signal than the 9db antenna. Since a signal is almost exactly twice as strong every 3db, the longer 12db antenna was more than twice the signal strength. In fact, at 4db, it was 10^(4/10) stronger or 2.51 times as strong.
Since transmit power acts according to an inverse square law, the 12 db antenna lets you be "the square root of 2.51" farther from the acces point, or 58% farther away that the 9db antenna. And the 9db antenna is no slacker. It was pulling in extremely stong signals. Gone Fishing, Yacht Club and Snug Harbor are all 3 miles away, and judging by where their slips are located, their antennas are not necessarily pointing toward our boat.
Based on these tests, and assuming the pitch of your boat isn't too severe at anchor, I would recommend the 12db gain antenna. If you have a smaller boat in an anchorage with bigger waves action, and you are located close to a good access point(s), the 9db with serve you just fine.
If there are any questions on all of this, please let me know.