You can do 3 things to help somewhat in reducing the probablility of interception. Using a high gain antenna for transmit, a high gain (read big aperture) receive antenna, and transmitting with just enough power to close the link and no more.
yup it is possible....i studied this posibility when i was chosing my Final Year Project at Undergrad studies......but somehow i didnt chose this as my FYP. Anyways, speaking loosely, Using smart AE's high directivity we can make a security system just as lasers are used in various security systems...
by decreasing the spacing between the elements u can decrease the beamwidth of ur transmitting antenna and as mention above it could act as a laser but problem aries that more sidebands of equal length of main beam are also created
i m designing a phase array antenna working on the freq of 418MHz and i used three mode of transmission one forward endfire mode and backward and also broadside mode the elements spacing is 3/8 lambda and elements are dipoles of 2.
the major purpose of my antenna is to direct in one desired direction and it can also be used for wireless security applications