[Milsurplus] ARN 7 and BC 433 performance

[boeing377 at aol.com]

  These were very accurate ADFs when properly set up and receiving a 
reasonably strong signal. The loop was coupled to the master bearing 
autosyn through a mechanical compensator that had a really ingenious 
design. You could set allen screws around the circumference of the 
compensator to shape a circular cam made of spring steel which set up a 
deviation curve so null azimuth distortion caused by aircraft structure 
was removed. The loop master autosyn was driven by a cam follower so 
that when properly calibrated, the slave autosyns on the I 81 and I 82 
indicators would read corrected directions, not distorted directions. I 
used this to eliminate the distortion caused by steel rigging cables on 
my dad's commercial fishing boat. We could get VERY accurate bearings 
on strong signals (narrow nulls). On weak sigs, the needle would wander 
a bit but you could see an avg in the middle. The ARN 7 outperformed 
all the commercial marine ADFs of the day because of this feature. 
Commercial DFs required you to prepare a deviation chart and refer to 
it for corrected bearings. Pretty useless in heavy seas when a boat is 
yawing a lot. The LP 21 loop had a mechanical compensator that did this 
automatically. The ARN 7 had a sealed case, no vent holes at all. It 
would get very hot. Some surplus ones I saw had holes drilled in the 
sides to allow cooling. Holes looked crude, like drilling was done in a 
radio repair shack not at the factory.

  Still looking for info on the BC 433s commercially converted post WW 2 
to cover 2-3Mc on highest band. Trying to find who made these. The 
converted control head had a very well done new dial showing the 2-3 Mc 
band, looked almost Bendix factory made but was not. One big difference 
between the BC 433 and the ARN 7 was the ratchet solenoid driven 
bandswitch motor on the ARN 7. It sounded like a machine gun where the 
BC 433 rotary gear motor was quiet. My Dad used to caution me not to 
change bands on the ARN 7 while he was sleeping. He could sleep with 
the noise of our CAT diesel but that "damned machine gun" would awaken 
him every time.