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View Full Version : Direction-finder. How does it work?



potver
12-21-2004, 10:42 AM
I understand how to fly with the direction-finder.It works very well to find the waypoints.
But how does it works in reality?

mortoma
12-21-2004, 12:40 PM
Well they were simply radio receivers that use a highly directional antenna to home in on a radio signal. It could be used for any frequency used in the day, from longwave, up through the AM broadcast band and shortwave ( HF ) and also lower VHF frequencies that were being used by then. Any steady signal could be tuned and the directional needle would point to it. The system is still in use today but is rapidly loosing popularity. In modern days, the onboard avionic device in the aircraft is called an ADF ( automatic direction finder ) and the radio beacons set aside by government aviation authorities that a pilot would tune into are called NDBs ( non-directional beacons ). Also, non-government public AM broadcast stations can be tuned. And a pilot using such a system can also set the receiver to put the audio in his headphones, so you can listen to sports, music and news while flying. Not advised for instrument rules flying though, since you have to stay in constant contact with ATC. I have listened in to radio stations when flying by visual flight rules many times.

Unlike more modern navigation aids like GPS, LORAN and VORs/Vortacs, they don't allow for precise tracking due to the fact they don't automatically compensate for wind drift. For example if you are 100 miles straight west of one and fly towards it in a heavy side wind, say from the north, by the time you get closer to the beacon you may have drifted much to the south. So by the time you get really close you might actually end up going almost straight north!!! A pilot, knowing this, if he/she is smart and knowledgable, might steer to the northwest in this case to compensate to some degree himself/herself.

The implementation in IL2/FB/AEP/PF is rather unrealistic since your side is not going to be able to set up beacons behind enemy lines or out in the middle of an ocean!! Although there was nothing preventing say, the British from homing in on a Berlin AM broadcast station or whatever. Both the Axis and the Alied side used radio broadcasts to home in on enemy cites. The Germans even did this in the BoB. I'm both a Ham ( amatuer ) radio enthusiast and a pilot, so I should know about this stuff.

potver
12-21-2004, 02:04 PM
Thanks Mortoma , that explains a lot.
I,m not so ,,radio technical,,

Jungmann
12-21-2004, 07:40 PM
Good precis, Mortoma. Although the tricky thing about them back in the day was the false null (do I have that right?) where you'd peak your tuning of the compass on the wrong side of the antenna and get a heading 180 degrees off. How did they guard against that?

Cheers,

mortoma
12-21-2004, 09:53 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Jungmann:
Good precis, Mortoma. Although the tricky thing about them back in the day was the false null (do I have that right?) where you'd peak your tuning of the compass on the wrong side of the antenna and get a heading 180 degrees off. How did they guard against that?

Cheers, <HR></BLOCKQUOTE>I dunno, but that sounds like it is something that may have happened. The older antennas may have been bi-directional, like a ferrite core AM antenna like they commonly use in a portable radios for the AM band. Ever notice they pick up stations better when the radio is broadside to the radio station, either the front or back of the radio??? But if you point either end of the radio at the station the signal is weaker. I think modern ADF antennas are probably much more directional. They probably figured out a better antenna design and the newer ones have a superior front-to-back ratio, as we call it in radio circles.

Jungmann
12-21-2004, 10:35 PM
You see the ADF antennas on certain marks of the 109, on the turtledeck behind the cockpit. P-51s in the 14th AF in China had them, same place. On bombers, like the B -17 or B-24, they were enclosed in those football shaped things under the nose. Yes, they were like doughnuts--they could be mechanically rotated from the cockpit to get the bearing, but IIRC, you had to guard against getting a peak reading on your strength meter 180 degrees off the actual signal, and thus heading in exactly the wrong direction.

Jungmann
12-21-2004, 10:36 PM
By the way, I'm also an ex-ham. Former K6SWB/WA6BGZ.

Cheers,

STENKA_69.GIAP
12-22-2004, 04:59 AM
As a variation on the theme during the 1940/41 the Germans used a radio beam to guide bomber streams onto a target.

If I remember right they had two parralel beams one transmitting dots and the other dashes. So when you heared the dots you turned left and when dashes you turned right.

Countermeasures were applied by tranmitting a third signal to fill in the dots...

After which the hapless bombers flew round in circles....

HansKnappstick
12-22-2004, 05:18 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by STENKA_69.GIAP:
As a variation on the theme during the 1940/41 the Germans used a radio beam to guide bomber streams onto a target.

If I remember right they had two parralel beams one transmitting dots and the other dashes. So when you heared the dots you turned left and when dashes you turned right.

<HR></BLOCKQUOTE>

Yes and when you heard both dots and dashes meant you were roughly on route.

There was a third beam perpendicular to the two previous ones filling the gaps between dots and dashes to indicate you are over target. So, when you just hear a steady signal, just drop your load and hurry home.

I can imagine the Britons faking this signal for a premature bombdrop.

mortoma
12-22-2004, 02:17 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Jungmann:
By the way, I'm also an ex-ham. Former K6SWB/WA6BGZ.

Cheers, <HR></BLOCKQUOTE>If you don't mind me asking, why ex-ham?? The licenses are easy to renew. I really have not been active in the past several years but found it easy to go to the www.W5YI.com (http://www.W5YI.com) website and renew online for a samll fee. This way, if I want to, I can always get back into it later. But if you let the license lapse, you have to take all the tests over again!!! I have the old Advanced class license, which they did away with in the late ninties. I won't give out my call though. By the way, I hate the ARRL. But their website is awful handy.