From the folks who keep the standard, usb.org:
How long of a cable can I use to connect my device?
In practice, the USB specification limits the length of a cable between full speed devices to 5 meters (a little under 16 feet 5 inches). For a low speed device the limit is 3 meters (9 feet 10 inches).
Why can't I use a cable longer than 3 or 5m?
design doesn't allow it. When USB was designed, a decision was made to handle the propagation of electromagnetic fields on USB data lines in a way that limited the maximum length of a USB cable to something in the range of 4m. This method has a number of advantages and, since USB is intended for a desktop environment
, the range limitations were deemed acceptable. If you're familiar with transmission
line theory and want more detail on this topic, take a look at the USB signals section of the developers FAQ.
How far away from a PC can I put a USB device?
With the maximum of 5 hubs connected with 5m cables
and a 5m cable going to your full speed device, this will give you 30m of cable (see section 7.1.19 for details). With a low speed device, you will be able to get a range up to 27m, depending on how long the device's cable is. With a straightforward cable route
, you will probably be able to reach out 25m or so from the PC.
So, the alfa would be a very good deal IF you bear in mind that you will suffer signal loss by using antenna cable, and you'd want to locate the alfa box itself on a longer USB cord. Either a maximum of 5 meters away from your computer (enough to get up on deck) or a bit longer--if you start chaining repeating hubs, which could be a bad idea for several reasons including water
intrusion and ugliness.
Maybe put the antenna at the mast
base, or on the rail, and with five meters there's enough to run inside to the nav station? Or just use it on a pole, pop it out in the cockpit
when you are at anchor