How it came

Some 40 years ago, I was a student at that time, I got a mysterious aircraft navigation set from Dutch military surplus. The surplus guy said it was a “TACAN” set.  It had lots of cavities with  2C39A’s , so it seemed a good basis for a 23cm transmitter for the local ham radio station PAoTHT in Enschede, the Netherlands.


Before dismantling the unit, I was intrigued about its function, and I drew all circuit diagrams of all modules, trying to figure out how it would work. The unit seemed complete, but there were no crystals, no documentation, no nameplate.

The size of the cavities gave a rough idea about the frequency, some 1000 MHz . Text on the transformers said the supply had to be  115V, 400Hz, no way to run this thing., no way to see any waveforms.


In 2006, I found back my diagrams, and  I tried to find more information on the web. I figured out with Google and the help of a Spanish site that it had to be an AN/ARN-21.

The main unit was the RT220, and I discovered with Google that I still could buy one in Germany, which I did.  My daughter picked it up on  31 July 2006 , together with the original rack mount and control panel.

Unlike the one I analyzed 30 years ago, this was the ITT version, described by Sven Dodington. The one I once had was the British version made by STC.


So, once again I made all diagrams, and wondered if  I could bring the 50 year old black box  to life. To do so, I needed 300W of  115V, 400Hz power. I could borrow a California Instruments 3-phase generator.  This unit has 3 single-phase output transformers with 45V secondary windings which could be placed in series to get single phase power. After connecting the 400Hz  and 24Vdc, the control panel and a home brew biquad antenna, I started to look with an oscilloscope on the output of the IF amplifier to see if I could receive a nearby TACAN beacon.  After midnight, I saw the first glimpse of a signal, and after careful  placement of the antenna the signal was a TACAN beacon, no doubt !

So, the crystal oscillator, multipliers , mixer and IF strip all still worked—without readjustment !

Now it would be nice to see these signals on the indicators if I had these. Wondering how the servo loops would bring the pointer in the right position and how the range indicator would lock at the correct distance. I downloaded lots of patents and original specifications of the ARN-21 and found the indicators were the ID-307 and ID-310 and that they still were for sale.


In October 2006, I bought the cockpit instruments, and figured out how the connecting cables were wired and where to find the plugs.  For the azimuth, only the receiver is needed, and it worked almost immediately, with 2 degrees jitter. For distance, the transmitter would be  needed to interrogate the beacon and I am not licensed to do so. I realized that I needed a beacon simulator. It took me 2 months to build one from 15 CMOS circuits ( see here) and it works excellent. The simulator provides the bearing modulation as could be seen from any direction, and simulates the responses of 100 aircraft, of which I am one.  My ARN-21 searches its own reply, and then locks on the replies, at the same time showing  the distance. Marvelous how they designed this,  60 years ago with tubes , resolvers and motors, not a single transistor or digital circuit !

A tribute to its inventor, Sven H. Dodington.