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Philips RT-3600
Wide-band analog VHF military radio

RT-3600 was an VHF wide-band analog military radio, developed for the Dutch Army by Philips Telecommunications Industry (PTI) in Hilversum (Netherlands) during the late 1960s and early 1970s. The RT-3600 was an extremely robust radio and, although it was phased-out in the late 1990s, many of them are still used today. They are also popular with collectors and radio hams.
The RT-3600 radio is often used in combination with other units, such as the IC-3620 intercom and the AF-6320 speaker unit. Furthermore, a wide variety of accessories are available for it, such as junction boxes, microphones, speakers, handsets, headsets, etc.

The image on the right shows a typical setup, consisting of an RT-3600 radio (bottom) and the IC-3620 intercom (top). The intercom was suitable for three radios. The second radio would be placed on top of the stack and the third one (if present) next to it.
RT-3600 radio (bottom) and IC-6320 intercom (top)

The contacts between PTI and the Dutch Department of Defense (DoD) date back to the early 1960s. In the late 1960s, they were asked to develop a robust HF radio for the Army, based on a design by the Army's own R&D department Laboratorium Elektronische Ontwikkeling Krijsmacht (LEOK) [1]. After a series of failures, this resulted in the extremely robust RT-3600 that can be dropped at any angle, from a height of 1.20 meter. It is rumoured that an RT-3600, that was dropped by accident out off a helicopter in Argentina, was still working after it was recovered.

In the early 1980s, following a series of reorganizations at Philips, PTI became part of Holland Signaal (HSA) and was renamed Signaal Communications. They were commissioned by the DoD to develop the successor: the RT-4600. In 1989, HSA was sold to Thomson (now: Thales).
RT-3600 radio Front view of the RT-3600 Rear view of the RT-3600 Front view of the IC-6320 intercom Rear view of the IC-6320 intercom RT-3600 radio (bottom) and IC-6320 intercom (top) RT-3600 radio (bottom) and IC-6320 intercom (top) RT-3600 radio (bottom) and IC-6320 intercom (top)

Voice Crypto
Although it was known that, during the Cold War, the enemy was monitoring our frequencies, speech encryption was hardly ever used at a tactical level. During most, if not all, large-scale NATO excercises in Germany, all tactical conversations went through the air en clair.
It is little known that in the late 1960s, another Philips subsidary, Philips Usfa in Eindhoven (Netherlands), started development of a tactical speech encryption system for the RT-3600. It was called Spendex-10 and in 1973, after a long range of experiments, the device was ready.

The image on the right shows the final version of the Spendex-10, seated on top of the IC-3620 intercom. The RT-3600 radio is at the bottom. The device is connected in between the handset and the RT-3600, by means of a 5-pin cable at the right. It is powered by the IC-3620.
Spendex-10 (with open door) mounted on top of an RT-3600 radio set.

It provided extremely good security, even by todays standards, and matched the design of the RT-3600 radio set. It could even transmit and receive digital data (at 600 baud) and could also be connected directly to the FM-200 line-of-sight radio link (LOS). Despite the good results, only a small quantity was ever built and the device was not taken into large scale production. More...

When using a voice encryption with the RT-3600, the Mode Selector (squelch) should be set to the rightmost position marked 'X' [3]. In this mode, the radio can use a larger bandwidth than when using normal speech. Any filtering in the transmission and reception path are bypassed this way. In this position, the squelch will always be open and any noise supression will have to be done by the encryption-device. The Spendex-10 has an automatic analog and digital squelch.
Microphone connection
There are several ways to connect a microphone or a handset to the RT-3600. In the Dutch Army, the most common way was to connect a handset with a 10-pin connector to either the IC-3620 intercom unit, or to any of the junction boxes in a vehicle.

It is also possible to connect a handset directly to the RT-3600 radio. At the front right of the radio are two standard 5-pin U-229 connectors. When using these connectors, please note that the wiring of these connectors is different the common NATO/USA standard. For the RT-3600 the lines for microphone (MIC) and speaker (SPK) need to be swapped. The table below shows both the RT-3600 wiring and the more common NATO wiring [4].
Pin RT-3600 NATO Description  
A GND GND Ground (common wire)  
B MIC SPK Microphone (RT-3600) or speaker  
C PTT PTT Push-to-Talk switch (connects to ground)  
D SPK MIC Speaker (RT-3600) or microphone  
E n.c. n.c. Generally not used  

When using the 5-pin audio connector on the RT-3600, this means that you either need to use the standard microphone/handset that was issued with this radio, or modify a standard NATO or USA handset according to the table above. At some point, a NATO convertor box was issued, to allow NATO handsets to be used with the RT-3600, but all it does is swap lines B and D.
  1. Raoul de Zoeten, Historie van de RT-3600 (Dutch)
    Website. RT-3600 history.

  2. Wammes Witkop, RT-3600 Parts
    Website 'Green Radios'. Overview of all RT-3600 parts and accessories.

  3. Konklijke Landmacht, Technische Handleiding, RT-3600 (Dutch)
    Bediening en 1e echelons onderhoud. 28 October 1974.

  4. Richard Lacroix, Audio Connectors U-229/U
    Website. MilSpec Communication Canada.

Further information

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Copyright 2009-2013, Paul Reuvers & Marc Simons. Last changed: Thursday, 13 December 2012 - 22:44 CET
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