"Am I missing something? From checking the various websites, it seems what you need is an omnidirectional antenna (8dBi or better), cable with the correct fittings and a USB adapter. "
Yes and no. A PCMCIA WiFi card can be bought with a power rating up to 400mW, versus about 50mW for a USB Wifi dongle. This means that the Wifi card starts out 12db more powerful than the dongle--and that's a huge difference. (Every 3db being a doubling of power.) Even with a less expensive PCMCIA card at 200mW, you're still gaining 6db of power over the USB dongle.
Now, you add the gain from the antenna. You can get external antennas in the 6-9db gain range very easily, with up to 22db being available in omnidirectional antennas.
But then you've got the "cons", because at microwave frequencies the antenna cable itself eats up the signal. So let's say you want the antenna at the masthead, and you need 75' of cable to get it from the computer to "up there". "Really good" cable will still lose about 6db of signal per 100 feet of cable, so your cable will lose about 4.5db of the signal.
If you use a 6db gain antenna, and the cable eats 4.5db of signal, you only gain 1.5db of signal! You can actually lose 3/4 of the signal by putting the antenna 75' away from the computer, whether you use USB or a PCMCIA card.
So if you use a PCMCIA card, with a 6db gain antenna plugged right into it, you may gain 16db of signal compared to using a USB dongle and putting the same antenna up the mast
A lot depends on how serious you want to get. I'd say that if you want to get serious, the best way is to use an Access Point (aka "AP") with the antenna attached to it, and hoist those both up the mast
as/if needed. The AP is essentially the same box as a "router" or "cable modem", except it is designed to connect to a Wifi signal on the input--instead of a wire. The AP has the same full power as a PCMCIA card, but since the antenna is connected right to it--there's no cable loss on the signal.
Ah, but your AP is now up the mast and your computer isn't...<G>. The two get connected by a standard ethernet cable, which can run over 200 feet without any signal losses, because it is ethernet not microwave signal cable.
Cost of the AP? $50-150. Cost of the antenna? Even a 6db gain for $30 should do, with it "up there". More as needed, easy to screw in.
The radiolabs solution (which really is a Wifi adapter mounted at the antenna base, so the two can be remoted) will still only give you the weaker USB Wifi power to begin with, ignoring the questions about unconventional driver support that may be needed. (Their web site is vague about it.)
I'd rather go with the AP and antenna aloft, and get a huge relative power gain out of it. Or, if it was legal
where I was using it, I'd splurge for a one-watt inline amplifier (about $200-250) which gives a tremendous kick to the signal. That gets installed in the antenna cable--and needs an antenna which is physically able to not burn out with a whole watt of power.
"I think you can get it all for under $200." Yup, there are options well inside that range, like a $100 AP, a built-in ethernet connection, a $25 ethernet cable, and a $50 antenna.
"This is for a laptop that has wifi. I think you have to disable the internal wifi but I'm new at this and still working it out." Probably. Unless your AP allows your internal card to use it as a bridge. (Pass the aspirin?<G>)
Whatever you do with an antenna--look for the type of cable supplied with it, and look for the loss per hundred feet on that cable. You want the antenna that has the highest omnidirectional gain--but also has the lowest cable loss. The net combination of the two is what matters.