[Milsurplus] Why DU

[Ralph Cameron]

My understanding is the loop developed from the early quest of  dirigible 
operators who discovered its property of exhibiting a null perpendicular to 
the plane of the loop. Airport location in foggy conditions was a feature 
discovered using audio frequencies and a ground located loop around an 
airfield. As the blimp approached When exactly over the center of the ground 
placed loop, the airborne monitor heard a null or so called "cone of 
silence".

The loop is aerodynamically suitable for an aircraft and many improvements 
since then to automate the loop to locate the null which was  coupled to a 
compass rose, enabled the pilot to home in on a particular radio signal 
source.

I don't know if 7500Khz had any particular signifcance  in terms of the DF 
ability. I would have thought the lower frequenceis which were more stable, 
would be more accurate for navigating. The MN-26 only covered to 1500Khz.

Ralph
VE3BBM