Can I use a high gain antenna to extend the range of Linx RF modules?

Of course antenna gain implies directionality. Each Linx transmitter module is capable of achieving the maximum output power allowed for unlicensed operation under FCC Part 15. Concentrating the power in a particular direction would, in most cases, result in illegal power levels. At first, it might seem that a receiver could not pose any similar problem. It must be remembered, however, that most receivers have a local oscillator on-board that is acting as an unintentional transmitter. Depending on the strength of the LO feed-through it is quite possible for a gain antenna to concentrate the LO energy to an illegal level. In general, high gain antennas are best for applications requiring directionality or licensed or spread spectrum applications where additional power is legally allowed.