External Versus Internal Antenna
USB Wi-Fi adapters can be categorized based on their antennas, which can be omnidirectional (transmitting and receiving in all directions) or directional (focusing the transmission in only one direction). The strength of an antenna gets expressed in decibels (dB) relative to a standard reference antenna. Because this standard reference antenna is isotropic, that metric gets expressed as dBi, meaning decibels relative to the isotropic antenna. While this spec is not consistently available for USB Wi-Fi adapters, in cases when it is, this allows a more direct comparison of the expected signal strength for the adapter.
One differentiating factor of USB adapters is an external antenna, or a slimmer profile with an internal antenna. Both have their advantages, disadvantages and applications.
In situations where the signal strength is less than optimal, whether due to router location, interfering obstacles like walls and/or floors, and when there are multiple Wi-Fi networks nearby, a USB-based Wi-Fi adapter with an external antenna is preferred. Also, in the situation where a 5GHz signal is chosen for the Wi-Fi connection, but it is not strong, an adapter with an external antenna can make it usable (keeping in mind that 5GHz penetrates obstacles like walls or floors less than 2.4GHz). An adapter with an external antenna that can be adjusted and positioned to optimize signal strength is particularly ideal for these situations.
While an adapter with an external antenna is ideal from the standpoint of signal strength, they also tend to be bulky and protrude from the USB port. While less of an issue with a desktop, this can be a particular problem in a notebook, especially when they can get hit, and the USB port potentially damaged. In situations where the signal strength is strong, a smaller USB Wi-Fi adapter with an internal antenna and slim profile becomes the preferred choice.
In the never-ending quest for tiny electronics, a class of USB Wi-Fi adapters has emerged that can be described as “nano,” with the adapter only slightly larger than the USB port itself. While devices like this can certainly offer performance in a smaller package, keep in mind that a strong signal from a router is necessary for such a device to maintain a fast connection.