The ARN-21D was introduced in 1961 and was the first with A/A mode. To handle the 4x higher average transmitter power, the AN/ARN-21D has a solid state modulator for 1050 pulses/sec. This modulator has two pulse forming networks, each PFN  is charged in 100 us, to guarantee the minimum pulse distance of 125 us.


The ARN-21D was introduced as an upgrade kit for the ARN-21B or –21C. The kit contains the solid state  modulator, a  relay, wire, and a new mode switch + bezel for the cockpit control panel



The kit is described in MIL-M-22965A(WEP) and in patent US3076190. These can be downloaded for free from the  Mil docs server  or a  patents server     I got a  picture from the ARN21D = ARN72 ,   photo courtesy Nick Vernon.


The  ARN21D modulator circuit diagram and some info is here.


The ARN21 series require an external indicator coupler like the   CU-395 /ARN   to drive the  Course Deviation Indicator ( ID-249 ) and the Radio Magnetic Indicator ( like ID-250). 


The original ARN21 was made by Hoffmann , Federal (ITT) and Stromberg-Carlson .

Collins also entered the ARN21 market in 1956 with the ARN21 (XN-10),  an ARN52-like box, soon followed by the  TCN-101 in 1957.  Their  brochures , including the indicators are on the RockwellCollinsMuseum  site.


In 1960, the ARN-21 series was followed up by the ARN-52, which supports the A-A mode,

and had built-in indicator couplers. Like the ARN21D, the ARN-52 has two solid state modules, the power supply and the modulator. All other modules have (lots of) tubes.


Control panels

From the beginning, control panels had two versions,  “NAV” and   “TACAN”.

They were fully compatible, with only minor differences in  the volume pot and bulb voltage

The following control panels were used for the AN/ARN21 series:


Top                     C 866   and  C 1763       Only REC and T/R modes)

Bottom                C 3844 and C 2010         with A/A mode 


When the C866 and C1763 were modified as an A/A upgrade, these control  panels became C866A or C 1763A.

A tribute to its inventor, Sven H. Dodington.


Other   ARN21  versions


TACAN as introduced in 1954 had 126 channels and used twin pulses, separated by 12us, both for uplink and downlink. Later on these were called the "X" channels.

The aircraft unit transmits on one of  126 channels 1025 … 1150 MHz, and receives for the first 63 channels on 63MHz below the transmit frequency, but for the upper 63 channels on  63 MHz above the transmit frequency.  The below/above selection is done by solenoids in the preselector cavities.  All "X" ground beacons are outside the 1025 … 1150 MHz band.

The ARN/21 A, B and C have these 126 transmit and 126 receive channels. This changed with the introduction of the ARN/21D, which could receive on all 252 channels, 126 from the ground, and 126 from other aircraft.


A/A mode

By swapping the preselector control wires, any ARN21 can also receive inside  the 1025-1150MHz band.  So another aircraft can be received that is 63 channels above or below its "own" channel.  This swap was introduced as A/A mode in the AN/ARN21D and provided for the “bilateral range function”. Each aircraft with this function is both an interrogator and a transponder.

As an interrogator, each aircraft transmits the usual  15 or 150  twin pulses /sec

As a transponder, each aircraft replies with a single pulse to let the interrogating aircraft  find their mutual distance.  The unit shall :

   a)  receive twin pulses, and reply each after 50us with a single pulse , except

   b)  the single or twin pulse that coincided with its range gate is not retransmitted,

         but used to track the displayed range.

With up to 5 aircraft nearby, all in "search" mode, 750 (single) pulses/sec must be transmitted on top of the usual 30 or 300 pulses/sec to interrogate the ground beacon. 


Y channels.

In 1965 , the number of channels was doubled  to 126 X and 126 Y channels. Also, ground beacons inside  1025 … 1150 MHz were allowed. To distinguish these Y-beacons from  X-channels or aircraft on the same frequency, specific twin pulse separations for the Y channels were chosen.   Seen from the aircraft, these are:


To beacon

(T/R mode)

To other aircraft

      (A/A mode)





X channel




single pulse

Y channel




single pulse



There are 4 American and 1 British versions of the ARN21 airborne navigation set.


Version   Year         Tubes     

AN/ARN-21A       1954        73            With Ferris detector.

  STARN-21           1955        106           British version of the ARN21A made by STC

AN/ARN-21B       1956        76            See this site

AN/ARN-21C       1960        56            All vacuum diodes or triodes used as diodes are replaced  by

                                                   si diodes. Turret got full 126 Xtals

AN/ARN-21D       1961        52            Has  Air-Air mode, (still only X channels). Upgrade from

      = ARN-72                                          21C by replacing the modulator for a solid-state one.

                                                   Also known as ARN-72 (thanks, Nick)


The ARN-21A was the first TACAN set . I have no circuit drawings of this unit, but it is probably close to the British STARN21. Both had a Ferris detector to reduce interference from first generation beacons on neighbor channels.


The ARN-21B is described on this site. See the diagrams with explanation and screenshots  on the second page of each pdf file.


The ARN-21C was introduced in 1960.  Mainly value engineering, the functionality is the same as the  B version. The C version has  a lower tube count, and less spurious emissions than the B version. Less tubes means less heat, less power consumption. The power supply unit is fully solid state.  The MTBF went from 20 to 200 h.

The turret got 126 crystals (!) , one for each channel. This eliminated the  42 MHz oscillator and mixer, and  the 42MHz sidebands. Further, a 1300 MHz lowpass filter was added in the antenna connection to filter the second harmonic. 


<—-Turret with 126 crystals in the ARN21C

The top view of the complete unit is here.

A wiring side view is here

Photos  courtesy  Bryan Seeds, vintage avionics site

The modulator of the

 ARN21B  (rear) and the

ARN21D ( in front)