The PowerAIM is able to make antenna measurements in the presence of strong external signals such as found on a commercial broadcast antenna. Stray signals up to 25Volts peak at the input can be tolerated while measurements are in progress. More information and the specs can be found here: arraysolutions.com/Products/poweraim_120.htm#top%20of%20page
By comparison, the AIM4170 can make accurate measurements with up to 100mv of stray signal on the input. When a broadcast station is operating nearby, the 4170 may not be able to measure a 160 meter antenna without using a high pass filter. The PowerAIM can make measurements within 50 KHz of strong interference without a filter.