These high order harmonics weaken rapidly, but when they land squarely on a GPS frequency there's trouble because GPS signals are extremely low level. This happens only for certain channels, where the multiplication is correct. If you hold down your transmit button, say, for 10 seconds, and watch your GPS signal strength page, you might see the degradation. There are several ways to fight the problem. Foremost, locate any com antenna three or more feet from the GPS antenna.
Next tactic is to install a notch filter in the com transmission line.
In some instances, the GPS and com antenna cannot be spaced sufficiently apart. Although considerably more expensive than the notch filter, it provides a solution Ina difficult installation. This area has proved very hazardous to homebuilders, but not because of the quality of the merchandise. The frequent problem is improper matching of circuit breaker and wiring. When it came time to lower the landing gear, they were greeted by a strong odor and the sudden appearance of smoke in the cabin.