VDF Azimuth display teardown - Page 1
Forum section: electronics / rf-microwave

author: s1m0n3t
author_rank: Newbie
timestamp: July 24, 2018, 06:25:47 pm
content: Hi all, I found an old Azimuth Display, used in a VDF system (VHF Direction Finder) of an airport. I'd like to understand how it works and maybe use it in a creative way, like an odd clock or similar...I opened the main case and I took some photos. Starting from rear (vdf_display_rear.jpg): there is an AC input connector (220V, because it was used in Italy), a DB5 connector (called PL3), a DB25 connector (called SK4), a small 2pin circular connector (SK5) and a fuse.There is also a code for this product, 79-FAU.3C, but I don't found any document on internet. The manufactor is Fernau Avionics Ltd.On the front side there are a circular array of small leds, 3 digit luminescent 7 segment display (not led), 4 illuminated push buttons (QDM, QTF, STO, TX) and a knob with on-off switch.I tried to power on the display:- the 3 digit display (also a T indicator) and the first led (zero degrees) lights up for few seconds (vdf_display_on1.jpg)- immediately after, the led indicator "goes" to position 150 (with a "sliding" light of other leds) and the display shows 152 (vdf_display_on2.jpg)- after few seconds the 3 digits display and led indicator turns off (vdf_display_on3.jpg)- if I press "STO" button (maybe means "storage") the indicator turns on again, but with "000" value" (vdf_display_on4.jpg)- if I press TX button, nothing is happening- if I press QTF button, the QDM button turns off, and the other way around, such as two mode selectorIn the other photos you can find the inside electronics, with 3 main modules.Some of you know how to deepen the operation of this device, without removing all the boards and try to redesign all the schematics?I hope to arouse the interest of some of you....

author: Howardlong
author_rank: Super Contributor
timestamp: July 24, 2018, 07:56:00 pm
content: I haven’t worked in a tower so I don’t have first hand knowledge of these, but these are used by ground controllers to locate aircraft.During the general RT chit chat which is on VHF, the device uses the aircraft’s transmissions to locate it.QDM is the magnetic heading that the aircraft should use to reach the station, so in bad weather the tower can help the aircraft down.QTE is the true bearing of the aircraft from the station.From a brief description I’ve seen online, they figure out the direction by rapidly scanning an array of antennas in a circle, with reference to an antenna in the middle. It’s similar to a VOR but in reverse. The rapid scanning of the antennas in the circle produces modulation, and by demodulating this with respect to the received reference signal you can figure out the phase difference, ergo the bearing.At one stage in my career I knew all about VOR, and made VOR simulators to test avionics, but I’ve never worked with VDF, so I welcome any corrections and/or further information. Considering the ages of these technologies, avionics like these can be quite fascinating, it’s astonishing what could be achieved. VORs are now disappearing, they cost a lot to maintain, and everyone’s moving to GNSS. Although many VORs are physically gone, they are still very frequently used as virtual reference points in air navigation. Active VORs are still active for the four holding patterns around Heathrow for example, and although most aircraft in the holds will be using GNSS, the backup is still the VOR.