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Philips RT-3600
Wide-band analog VHF military radio

RT-3600 was an VHF wide-band FM military radio set, 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 were still in use in 2011. They are also popular with collectors and hams. The radio covers 26-70 MHz with 10 kHz deviation and a channel spacing of 50 kHz. Output power is 2W, but can be inreased to 30W by installing the (optional) AM-3600 power amplifier.
 
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 and voice encryption units.

The image on the right shows a typical setup, consisting of the RT-3600 radio itself (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) was placed aside the stack.
  
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, PTI was asked to develop a tactical VHF radio for the Army, based on a design by the Army's own R&D department Laboratorium Elektronische Ontwikkeling Krijgsmacht (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 [1].

In the early 1980s, following a series of reorganizations at Philips, PTI became part of Holland Signaal (HSA) 1 and was renamed Signaal Communications. In 1984, Signaal was commissioned by the DoD to develop the RT-4600 as the successor to the RT-3600 and in 1989, just before the company was taken over by Thomson (now: Thales), they developed the SPIDER manpack radio.
 
  1. After the reorganisation, the company name was officially Hollandse Signaal Apparaten (HSA), but was commonly abbreviated to Holland Signaal or Signaal Huizen.

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
During the Cold War, the Eastern Block countries constantly monitored the radio frequencies of the Western Block troups in West-Germany. Nevertheless, speech encryption was hardly ever used at a tactical level. During most, if not all, large-scale NATO exercises that took place in Germany in the 1970s and 1980s, all tactical radio traffic went through the air en clair (i.e. unencrypted).
 
It is little known that in the late 1960s, another Philips subsidary, Philips Usfa NV 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 released.

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 between the handset and the RT-3600 radio, 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.

Spendex 10 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.
 
When using voice encryption with the RT-3600, the Mode Selector (squelch) should be set to the rightmost position marked 'X' [A]. In the X-mode any filtering in the transmission and reception path is bypassed, in order to support wideband data signals. Furthermore, the squelch is always open and any noise cancelling should be done by the encryption-device. The Spendex-10 has a built-in automatic analog and digital squelch.

Apart from the Spendex 10, the RT-3600 was also used in combination with the much smaller American KY-57 voice and data encryption unit.
  
KY-57 voice encryption unit

Developed in the mid-1970s and used by the American armed forces and by NATO, the KY-57 featured the highly secret and advanced SAVILLE encryption algorithm. The KY-57 was succeeded in 1993 by the backwards compatible KY-99, which could also be used with the Philips RT-3600.

 More about Spendex 10
 More about the KY-57
 More about the KY-99
 
Power Amplifier   AM-3600
The RF output power of the RT-3600 is between 1 and 2 W, which should be sufficient for an operational rangen of approx. 3 km. The range can be extended by installing the (optional) AM-3600 into the rear end of the radio. Once installed, the AM-3600 offers the following settings:
 
  • Low
    8 km
    0.95 - 1.75 W
  • Medium
    15 km
    8 - 15 W
  • High
    30 km
    25 - 45 W
Microphone connection   U-229
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 (which were connected to the IC-3620).


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 [3].
 
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.
 
Technical specifications
  • Frequency
    26.000 - 69.950 MHz (range 1: 26-47 MHz, range 2: 47-70 MHz)
  • Modulation
    FM
  • Channels
    880
  • Spacing
    50 kHz
  • Deviation
    10 kHz
  • RF power
    1W/2W (or 8W/30W with AM-3600 amplifier)
  • Range
    8 km (or 30 km with AM-3600 amplifier)
  • Squelch
    CTCSS 150 Hz
  • Power
    24V DC (or 15V during portable use)
Options
  • RT-3600
    Transceiver (Receiver/Transmitter)
  • C-3621
    Switch box
  • MT-3621
    Mount
  • LS-3621
    Loudspeaker
  • IC-3620
    Intercom
  • AF-3620
    Amplifier/Loudspeaker unit
  • AM-3600
    30W HF amplifier (PA)
  • BX-3600
    Battery box (for portable use)
  • BX-3601
    Cell box (for portable use)
  • BG-3600
    Manpack harness
  • AT-272A/PRC
    Short antenna (for portable use)
  • AT-271A/PRC
    Long antenna (for portable use)
  • CX-3603
    Cable set (for portable use
Documentation
  1. Konklijke Landmacht, Technische Handleiding, RT-3600 (Dutch)
    Bediening en 1e echelons onderhoud. 28 October 1974.

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

Further information

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Crypto Museum. Created: Saturday 16 July 2011. Last changed: Friday, 22 July 2016 - 09:58 CET.
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