[Milsurplus] answer to Hue's DF question

[BOEING377@aol.com]

If i am flying over the Pacific, leaving Hawaii, for
example, and my LF/MF RDF is tuned to a Hawaii
10 kw broadcast station, in daylight, how far will
out will i have a good signal for DFing? Or strong
enough for good listening?

Any different at sea level? In a ship, how far out
could i hear this same broadcast station, around
10:00, well after sunrise, in daylight?

Assuming 1940s radio equipment ( which is not to
say poor equipment! )
Thanks- Hue Miller

Hue,

I have some relevant fishing boat experience using 1940s vintage surplus ARN 7 Radio Compass (ADF) gear.  The ARN 7 (similar to BC 433) was an outstanding LF and BC band dx receiver if you listened through a long wire rather than the loop or sense whip antenna. We tweaked the osc coils and related circuitry on band 4 so we could get DF shots on other fishing boats in the 2-3 Mc band. This caused a gap in the upper BC band coverage but it didnt matter as we still got full beacon band and plenty of BC stations. The ARN 7 earned its keep many times over getting good shots on boats that were catching fish but not disclosing their location. Some communicated on "secret" illegal channels using custom ground crystals. This was back in the days when marine MF HF voice was AM not SSB.

I could almost always get good DF shots on AM BC 10 KW stations from 100 miles, often considerably further. I frequently shot the now extinct Farallon Island NDB (located about 27 miles off San Francisco) on 318 Kc which I think was a 500 watt MCW signal . I always got good shots at distances up to 50 miles, sometimes up to 100 miles but the null gets pretty wide at those low signal levels. Just look for the average if you are getting big pointer swings.

Just cause you can hear em does NOT mean you can get good DF shots.  Keep that in mind at night when skip is active. DF shots on skip just tells you where it is bouncing from, which is not necessarily same direction as the bearing to the transmitter antenna.

I would imagine you'll do better from a plane than I did from a boat.  What is your 1940s DF gear? I have never used a better LF MF ADF than the good old ARN 7.  You could dial in compensation for distortion caused by your rigging (or aircraft structure) and get 100% accurate bearings on strong signals. This was done through an ingenious cam compensator located between the loop shaft and the bearing transmission selsyn.  You varied the shape of a circular steel strip cam with set screws located around its periphery. Once calibrated, your rigging's influence on bearings was eliminated.

Mark